1. Traits and Variation

What Is a Trait?

A trait is a characteristic or feature of an organism.

Traits can include characteristics such as:

• eye colour
• blood type
• height
• hair characteristics
• flower colour
• leaf shape
• body size
• behaviour
• ability to perform particular activities

Some traits are strongly influenced by:

genes.

Others are strongly influenced by:

the environment.

Many traits are influenced by:

both genes and the environment.

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What Are Inherited Traits?

An inherited trait is a characteristic influenced by genetic information passed from parents to:

offspring.

Genes are sections of:

DNA.

During reproduction, genetic information is passed from one generation to the:

next.

This is called:

inheritance.

Examples of strongly inherited human characteristics include:

• blood group
• natural eye colour
• some aspects of hair colour
• some aspects of hair texture
• biological sex characteristics
• many inherited genetic conditions

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Genes and Traits

A gene is a section of DNA containing information that can influence a biological:

characteristic.

Different versions of the same gene are called:

alleles.

An individual inherits alleles from biological:

parents.

The particular combination of alleles an organism possesses is part of its:

genotype.

The observable characteristics of an organism are called its:

phenotype.

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Genotype and Phenotype

It is useful to distinguish between:

genotype

and:

phenotype.

Genotype refers to an organism's genetic makeup.

Phenotype refers to its observable characteristics.

A phenotype can be influenced by:

genotype, environment, or both.

A useful relationship is:

Genotype + Environment → Phenotype

This is a simplified model, but it helps explain why genetically similar individuals can sometimes look or behave:

differently.

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What Are Acquired Traits?

An acquired trait is a characteristic that develops during an organism's lifetime because of:

environment, experience, behaviour, or use.

Acquired traits are not usually produced by changes to the DNA sequence that can be passed through gametes.

Examples include:

• a scar from an injury
• increased muscle size from exercise
• a suntan
• learning to play an instrument
• learning a language
• improved athletic skill through training

These characteristics develop because of what happens during the individual's:

lifetime.

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Inherited vs Acquired Traits

The key question is:

Was the characteristic inherited genetically, or did it develop because of the individual's experiences or environment?

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An Important Misconception

Suppose a person develops very large muscles through:

exercise.

Their children will not automatically inherit those enlarged muscles.

The person's muscle development is an:

acquired characteristic.

The children may inherit genes that influence characteristics such as body structure or potential muscle development, but they do not inherit the parent's:

exercise-produced muscles.

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Genetic Influences

Some variation occurs because individuals have different:

genetic information.

Genetic differences can arise through:

• inheritance of different alleles
• meiosis
• crossing over
• independent assortment
• random fertilization
• mutation

These processes contribute to:

genetic variation.

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Environmental Influences

The environment can also affect an organism's:

phenotype.

Environmental influences include:

• nutrition
• temperature
• sunlight
• water availability
• physical activity
• disease
• education
• living conditions

These influences can cause differences between individuals even when their genetic information is:

similar.

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Example: Nutrition and Growth

Genes influence a person's potential:

height.

However, growth can also be affected by:

nutrition and health.

Two individuals with similar genetic potential may reach different adult heights if they experience very different conditions during:

development.

Therefore, height is influenced by:

both genes and environment.

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Example: Plant Growth

Consider two genetically similar plants.

Plant A receives:

• plenty of light
• sufficient water
• adequate minerals

Plant B receives:

• little light
• limited water
• poor mineral supply

The plants may develop very different:

phenotypes.

Plant A may become taller and healthier than Plant B even though their genetic information is very:

similar.

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Example: Skin Pigmentation and Sunlight

A person's genes influence their natural:

skin pigmentation.

Exposure to sunlight can also increase production of:

melanin,

causing tanning.

Therefore, observed skin pigmentation can be influenced by:

both genetic and environmental factors.

A suntan itself is an:

acquired characteristic.

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Traits Influenced by Both Genes and Environment

Many traits cannot be classified as purely genetic or purely:

environmental.

Examples can include:

• height
• body mass
• athletic performance
• some aspects of behaviour
• risk of many diseases
• plant growth
• milk production in cattle
• crop yield

These traits result from interactions between:

genes and environmental conditions.

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Continuous Variation

Some characteristics can take many values across a:

range.

This is called:

continuous variation.

Examples include:

• height
• body mass
• hand span
• foot length
• plant height

If we measured the heights of hundreds of people, we would find many values rather than only a few:

categories.

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Why Does Continuous Variation Occur?

Continuous variation is often influenced by:

many genes.

It can also be strongly influenced by:

environmental factors.

For example, human height is influenced by genetic factors, but also by conditions such as:

nutrition and health during development.

This produces a wide range of:

phenotypes.

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Discontinuous Variation

Some characteristics fall into clear:

categories.

This is called:

discontinuous variation.

A classic example is:

ABO blood group.

People belong to one of the categories:

A, B, AB, or O.

There are no intermediate ABO blood groups between:

A and B.

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Continuous vs Discontinuous Variation

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What Is Variation?

Variation means differences between individuals.

Look at members of almost any species and you will notice that they are not completely:

identical.

Humans vary in characteristics such as:

• height
• blood group
• facial features
• hair characteristics
• body proportions

Plants can vary in:

• height
• flower colour
• leaf size
• growth rate
• disease resistance

Variation occurs throughout:

populations.

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What Is a Population?

A population is a group of organisms of the same species living in the same area at the same:

time.

Individuals within a population usually show:

variation.

Some variation is genetic.

Some is environmental.

Much variation results from:

both.

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Why Aren't Individuals Genetically Identical?

In sexually reproducing organisms, offspring receive genetic information from:

two parents.

Meiosis creates different combinations of chromosomes and:

alleles.

During meiosis:

crossing over creates new combinations of alleles.

Independent assortment distributes chromosomes into gametes in different combinations.

Then:

random fertilization combines different gametes.

These processes create enormous genetic:

variation.

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Mutation and Variation

Another source of genetic variation is:

mutation.

A mutation is a change in:

DNA.

Mutations can produce new:

alleles.

Most genetic reshuffling during sexual reproduction rearranges existing alleles, while mutation can introduce:

new genetic variants.

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Are Mutations Always Harmful?

No.

A mutation may be:

• harmful
• neutral
• beneficial

Its effect can also depend on the:

environment.

A genetic variant that provides little advantage under one set of conditions might become useful if conditions:

change.

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Variation Within a Species

Members of the same species share many:

characteristics.

However, they are not necessarily identical.

For example, members of a plant species may differ in:

• height
• flower size
• resistance to disease
• tolerance to drought
• growth rate

These differences represent variation within the:

species.

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Why Is Variation Important?

Variation is important because environmental conditions can:

change.

Populations may experience:

• new diseases
• changing temperatures
• drought
• floods
• new predators
• new competitors
• changing food supplies

If every individual were genetically identical, they might all respond similarly to a new:

challenge.

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Variation and Survival

Suppose a population of plants experiences a severe:

drought.

Some plants may possess inherited characteristics that allow them to:

• conserve water more effectively
• develop deeper roots
• tolerate dehydration
• reduce water loss

These plants may be more likely to survive and:

reproduce.

Variation means that individuals do not all respond to environmental challenges in exactly the same:

way.

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Variation and Disease

Imagine a population exposed to a new:

pathogen.

If there is genetic variation, some individuals may be more resistant to the pathogen than:

others.

Those individuals may have a greater chance of surviving and:

reproducing.

Their alleles can then be passed to the next:

generation.

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Variation and Natural Selection

Natural selection occurs when inherited differences affect survival and reproductive success in a particular:

environment.

A simplified sequence is:

Variation exists in a population

↓

Environmental pressures affect individuals differently

↓

Some individuals survive and reproduce more successfully

↓

Their inherited alleles are passed to offspring

↓

Allele frequencies in the population can change over generations

Variation therefore provides the raw material for:

natural selection.

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Variation Does Not Occur Because Organisms Need It

A common misconception is that organisms develop useful inherited traits because they:

need them.

For example, a population does not encounter drought and then deliberately produce genes for:

drought resistance.

Instead:

variation already exists or arises through mutation.

Environmental conditions then influence which variants are more likely to be passed to future:

generations.

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Acquired Traits and Natural Selection

Natural selection depends on characteristics that can be:

inherited.

Suppose an animal becomes stronger through exercise.

That increased muscle size is largely an:

acquired characteristic.

The muscles themselves are not genetically passed to:

offspring.

Therefore, natural selection operates on inherited genetic variation rather than simply on changes acquired during an individual's:

lifetime.

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Identical Twins and Environment

Identical twins provide a useful example of genetic and environmental:

influences.

Identical twins begin with nearly identical genetic:

information.

However, they can develop differences because of:

• diet
• exercise
• illness
• experiences
• environmental exposures

This demonstrates that phenotype is not determined by genes:

alone.

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Genes Set Possibilities, Not Always Fixed Outcomes

For many traits, genes influence a range of possible:

outcomes.

The environment influences how those genetic tendencies are:

expressed.

For example, genes may contribute to the potential for tall growth.

However, severe nutritional deficiencies during development may prevent a person from reaching that:

potential.

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Environmental Variation Is Not Necessarily Inherited

Suppose two plants are genetically identical.

One receives plenty of water and grows:

large.

The other receives very little water and remains:

small.

The difference in size is largely caused by:

environmental conditions.

Seeds or offspring from the smaller plant do not automatically inherit its environmentally caused:

small size.

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Real-World Example: Crop Plants

Farmers often grow crop varieties with particular inherited:

traits.

Useful traits may include:

• high yield
• disease resistance
• drought tolerance
• large fruits
• rapid growth

However, crop performance also depends on:

environmental conditions.

Even a genetically high-yielding crop may perform poorly without sufficient:

water, minerals, or suitable temperature.

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Real-World Example: Animal Breeding

Humans have selected animals for inherited characteristics for:

thousands of years.

Examples include selecting animals for:

• milk production
• body size
• wool characteristics
• behaviour
• speed

This process is called:

selective breeding.

It works because populations contain:

inherited genetic variation.

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Real-World Example: Antibiotic Resistance

Bacterial populations can contain genetic:

variation.

Some bacteria may possess mutations that provide resistance to a particular:

antibiotic.

When the antibiotic is used, susceptible bacteria are more likely to die.

Resistant bacteria are more likely to:

survive and reproduce.

The antibiotic does not create resistance because the bacteria:

need it.

Instead, it creates a selective environment in which resistant variants have an:

advantage.

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Comparing Genetic and Environmental Variation

Many characteristics involve:

both.

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Worked Example: Blood Group

A person's ABO blood group is determined primarily by:

inherited alleles.

Changing diet, exercising, or spending more time in sunlight will not change someone's ABO:

blood group.

Therefore, ABO blood group is an example of strongly:

genetic variation.

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Worked Example: Scar

A person develops a scar after:

injury.

The scar was not inherited from their:

parents.

It developed because of an environmental event during the person's:

lifetime.

Therefore, the scar is an:

acquired trait.

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Worked Example: Height

Two people differ in:

height.

Can we immediately say the difference is entirely genetic?

No.

Height is influenced by genetic factors as well as environmental conditions such as:

nutrition and health.

Therefore, height demonstrates an interaction between:

genes and environment.

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Worked Example: Plant Colour

Two genetically identical plants are grown under different light conditions.

One becomes pale while the other remains:

green.

If the difference results from the growing conditions, it represents:

environmental variation.

The plants can have similar genotypes but different:

phenotypes.

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Inherited Does Not Mean Unchangeable

A trait can have a strong genetic component without being completely:

fixed.

For example, genes influence body size, but nutrition and activity can influence the final:

phenotype.

Therefore:

inherited does not necessarily mean unaffected by environment.

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Environmental Does Not Mean Unimportant

Environmental influences can produce major differences in:

phenotype.

Nutrition can affect growth.

Sunlight can affect plant development.

Exercise can affect muscle size.

Temperature can affect growth and development in many:

organisms.

Genes and environment frequently:

interact.

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Common Mistake: Every Trait Is Either Genetic or Environmental

Many traits are influenced by:

both.

Height is a good example.

Genes influence height, but so do:

nutrition, health, and developmental conditions.

It is often better to think about the relative contributions of genes and environment rather than forcing every trait into one:

category.

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Common Mistake: All Inherited Traits Come from One Gene

Some characteristics are influenced primarily by:

one or a few genes.

Others are influenced by:

many genes.

Traits influenced by many genes are called:

polygenic traits.

Human height is an example of a highly:

polygenic characteristic.

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Common Mistake: Variation Means Mutation

Mutation is one source of:

variation.

But genetic variation also comes from:

• crossing over
• independent assortment
• random fertilization

Environmental conditions can create phenotypic variation without changing:

DNA sequences.

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Common Mistake: Acquired Characteristics Are Passed to Offspring

A characteristic developed during an individual's lifetime is not normally inherited genetically by:

offspring.

For example:

A scar is not inherited.

A suntan is not inherited.

Muscles developed through exercise are not inherited as enlarged muscles.

Inheritance depends on genetic information passed through:

reproduction.

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Common Mistake: Variation Is Always Beneficial

Variation is not automatically:

beneficial.

A particular variant may be:

• advantageous
• disadvantageous
• neutral

Its effect depends partly on the:

environment.

Variation is important because it creates differences on which natural selection can:

act.

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Check Your Understanding

1. Define a trait.

2. Define an inherited trait.

3. Define an acquired trait.

4. Give three examples of inherited traits.

5. Give three examples of acquired traits.

6. What is a gene?

7. What is an allele?

8. Define genotype.

9. Define phenotype.

10. Explain how genotype and environment can both influence phenotype.

11. Why is a scar considered an acquired characteristic?

12. Why is ABO blood group considered an inherited characteristic?

13. Explain why increased muscle size caused by exercise is not normally inherited.

14. Define variation.

15. What is a population?

16. Give three sources of genetic variation.

17. Explain how meiosis contributes to variation.

18. Explain how mutation contributes to variation.

19. Give three environmental factors that can affect phenotype.

20. Explain why height is influenced by both genes and environment.

21. Distinguish between continuous and discontinuous variation.

22. Give two examples of continuous variation.

23. Give one example of discontinuous variation.

24. Explain why variation is important when environmental conditions change.

25. Explain how genetic variation can affect a population exposed to disease.

26. Explain the relationship between variation and natural selection.

27. Why doesn't natural selection simply cause organisms to develop traits because they need them?

28. Two genetically similar plants are grown with different amounts of water. Explain why they may have different phenotypes.

29. Explain why inherited does not always mean completely determined by genes.

30. Explain why genetic variation is important for the long-term survival and evolution of populations.

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Key Terms

• Trait: Characteristic or feature of an organism.
• Inherited trait: Characteristic influenced by genetic information passed from parents to offspring.
• Acquired trait: Characteristic developed during an organism's lifetime because of environment, experience, or behaviour.
• Inheritance: Transmission of genetic information between generations.
• Gene: Section of DNA that influences a biological characteristic or function.
• Allele: Alternative version of a gene.
• Genotype: Genetic makeup of an organism.
• Phenotype: Observable characteristics of an organism.
• Variation: Differences among individuals.
• Genetic variation: Variation caused by differences in genetic information.
• Environmental variation: Variation caused by differences in environmental conditions.
• Population: Group of organisms of the same species living in the same area at the same time.
• Continuous variation: Variation showing a range of possible values.
• Discontinuous variation: Variation consisting of distinct categories.
• Polygenic trait: Characteristic influenced by many genes.
• Mutation: Change in DNA.
• Crossing over: Exchange of DNA between homologous chromosomes during meiosis.
• Independent assortment: Distribution of chromosomes into different combinations during meiosis.
• Natural selection: Process in which inherited differences influence survival and reproductive success.
• Selective breeding: Human selection of organisms with desired inherited traits for reproduction.

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Key Takeaways

• A trait is a characteristic of an organism.
• Inherited traits are influenced by genetic information passed from parents to offspring.
• Acquired traits develop during an individual's lifetime.
• Genes are sections of DNA that influence characteristics.
• Different versions of genes are called alleles.
• Genotype describes genetic makeup.
• Phenotype describes observable characteristics.
• Phenotype can be influenced by both genotype and environment.
• ABO blood group is an example of a strongly inherited trait.
• Scars, suntans, and learned skills are examples of acquired characteristics.
• Many traits cannot be described as entirely genetic or entirely environmental.
• Height is influenced by both genes and environmental conditions.
• Variation refers to differences among individuals.
• Variation can be genetic, environmental, or a combination of both.
• Meiosis contributes to genetic variation through crossing over and independent assortment.
• Random fertilization creates additional genetic combinations.
• Mutations can produce new genetic variants.
• Continuous variation produces a range of values.
• Height and body mass are examples of continuous variation.
• Discontinuous variation produces distinct categories.
• ABO blood group is an example of discontinuous variation.
• Genetic variation is important because individuals may respond differently to environmental change.
• Variation can help populations persist when diseases or environmental conditions change.
• Genetic variation provides the raw material for natural selection.
• Natural selection acts on existing inherited variation; organisms do not develop useful inherited traits simply because they need them.
• Acquired characteristics are not normally passed genetically to offspring.
• Variation is not automatically beneficial; a trait's effect depends on the environment.
• Genes and environment frequently interact to produce an organism's phenotype.
• Understanding traits and variation provides an important foundation for studying inheritance, natural selection, and evolution.

2. Alleles and Gene Pairs

3. Dominant and Recessive Inheritance

What Is Dominant and Recessive Inheritance?

Organisms inherit different versions of genes called:

alleles.

For many introductory genetics examples, two alleles interact through a pattern called:

complete dominance.

In complete dominance, one allele is:

dominant

and the other is:

recessive.

The dominant allele can determine the phenotype when only one copy is present.

The recessive phenotype normally appears only when an individual inherits:

two recessive alleles.

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A Simple Example

Imagine a plant species in which flower colour is controlled by one gene.

Suppose:

P = purple flowers

p = white flowers

and P is dominant over p.

The possible genotypes are:

PP

Pp

pp

But there are only two phenotypes:

purple flowers

and:

white flowers.

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Dominant Alleles

A dominant allele is an allele that is expressed in the phenotype when at least one copy is present under complete dominance.

Dominant alleles are usually represented by:

capital letters.

For example:

P

could represent the dominant allele for purple flowers.

An organism could therefore have:

PP

or:

Pp

and still have purple flowers.

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Recessive Alleles

A recessive allele is an allele whose phenotype is normally expressed only when two copies are present under complete dominance.

Recessive alleles are usually represented by:

lowercase letters.

For example:

p

could represent the recessive allele for white flowers.

The white phenotype would occur with:

pp.

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Genetic Notation

Scientists use symbols to represent alleles.

For a simple dominant-recessive trait:

Capital letter = dominant allele

Lowercase letter = recessive allele

The same letter should normally be used for both alleles.

For example:

B and b

rather than:

B and w.

This shows that B and b are alternative versions of the same:

gene.

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Genotype

An organism's genotype describes the alleles it possesses.

For example:

BB

Bb

bb

are three possible genotypes for a gene with two alleles.

Genotype refers to:

genetic information.

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Phenotype

An organism's phenotype is an observable characteristic resulting from its genotype and, for many traits, interactions with the:

environment.

For our simple flower example:

PP → purple

Pp → purple

pp → white

The letters describe the:

genotype.

The flower colour describes the:

phenotype.

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Genotype vs Phenotype

This distinction is one of the most important ideas in:

genetics.

Genotype = the allele combination

Phenotype = the observable characteristic

For example:

Tt = genotype

Tall = phenotype

An organism can therefore have a particular genetic combination that influences the characteristic we:

observe.

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Homozygous Dominant

An organism with two copies of the dominant allele is:

homozygous dominant.

For example:

BB

Both alleles are identical, so the genotype is:

homozygous.

Both alleles are dominant, so it is:

homozygous dominant.

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Heterozygous

An organism with two different alleles is:

heterozygous.

For example:

Bb

If B is dominant over b, the phenotype associated with B will normally be:

expressed.

Therefore:

Bb → dominant phenotype

under complete dominance.

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Homozygous Recessive

An organism with two recessive alleles is:

homozygous recessive.

For example:

bb.

Because no dominant allele is present, the recessive phenotype is:

expressed.

Therefore:

bb → recessive phenotype.

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The Basic Pattern

For a simple trait where A is dominant over a:

This pattern is worth remembering.

AA → dominant

Aa → dominant

aa → recessive

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Why Does the Heterozygous Individual Show the Dominant Trait?

Consider:

Aa.

The individual possesses both:

A and a.

Under complete dominance, one copy of A is sufficient to produce the dominant:

phenotype.

The recessive allele is still:

present.

It has not disappeared.

It simply does not produce the recessive phenotype in this:

heterozygous combination.

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The Recessive Allele Is Still Inherited

An organism with genotype:

Aa

can pass either:

A

or:

a

to its offspring.

This is important because a recessive allele can remain in a population even when it is not visible in an individual's:

phenotype.

A heterozygous individual can therefore pass a recessive allele to the next:

generation.

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What Is a Carrier?

For certain recessively inherited traits or genetic conditions, a heterozygous individual can be called a:

carrier.

Suppose:

A = dominant allele

a = recessive allele

A person with:

Aa

has the dominant phenotype but carries the recessive:

allele.

That allele may be passed to:

offspring.

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Predicting Phenotype from Genotype

Suppose:

T = tall

t = short

and T is dominant.

What phenotype would each genotype produce?

TT → tall

Tt → tall

tt → short

This allows us to predict phenotype when we know the:

genotype.

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Predicting Genotype from Phenotype

Working in the opposite direction can be more:

difficult.

Suppose a plant is:

tall.

Its genotype could be:

TT

or:

Tt.

We cannot determine which genotype it has from its phenotype:

alone.

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A Recessive Phenotype Reveals More

Now suppose the plant is:

short.

If short is recessive, its genotype must be:

tt.

Why?

Because:

TT → tall

and:

Tt → tall.

Only:

tt → short.

Therefore, a recessive phenotype often allows us to identify the genotype directly in a simple dominant-recessive:

system.

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Worked Example: Seed Shape

Suppose:

R = round seeds

r = wrinkled seeds

and R is dominant.

Determine the phenotype of:

RR

Because R is dominant:

RR → round seeds.

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Worked Example: Heterozygous Seeds

Now consider:

Rr.

The plant has one dominant allele and one recessive:

allele.

Because R is dominant:

Rr → round seeds.

The r allele is still present and can potentially be passed to:

offspring.

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Worked Example: Recessive Seeds

Now consider:

rr.

No dominant R allele is:

present.

Therefore:

rr → wrinkled seeds.

This is the:

recessive phenotype.

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Mendel and Pea Plants

Much of our basic understanding of dominant and recessive inheritance comes from the experiments of:

Gregor Mendel.

Mendel studied inheritance in:

pea plants.

He investigated characteristics such as:

• seed shape
• seed colour
• flower colour
• pod characteristics
• plant height

His experiments revealed predictable patterns of:

inheritance.

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Mendel's Tall and Short Plants

Consider a simplified Mendelian example:

T = tall

t = short

with T dominant.

Possible genotypes are:

TT → tall

Tt → tall

tt → short

Two different genotypes therefore produce the same:

phenotype.

This demonstrates why genotype and phenotype must not be treated as the same:

thing.

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Alleles Separate During Meiosis

An individual may possess two alleles in its body:

cells.

However, gametes are:

haploid.

During meiosis, homologous chromosomes separate.

The two alleles of a gene are therefore separated into different:

gametes.

For an individual with:

Aa

a gamete receives either:

A

or:

a.

---

Fertilization Combines Alleles

During fertilization, two gametes:

fuse.

One allele comes from one biological parent.

The other allele comes from the other biological:

parent.

For example:

A + a → Aa

or:

a + a → aa.

This produces the offspring's:

genotype.

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Predicting Inheritance

Suppose one parent has genotype:

Aa

and the other also has:

Aa.

Each parent can produce gametes carrying:

A or a.

Possible combinations in offspring are:

AA

Aa

Aa

aa.

These combinations can be organized using a:

Punnett square.

---

A Simple Punnett Square

For:

Aa × Aa

we can organize the possibilities like this:

Possible genotypes are therefore:

AA

Aa

Aa

aa.

---

Genotype Probabilities

From the Punnett square:

1 out of 4 = AA

2 out of 4 = Aa

1 out of 4 = aa

Therefore:

25% AA

50% Aa

25% aa

This produces a genotype ratio of:

1 : 2 : 1.

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Phenotype Probabilities

If A is completely dominant over a:

AA → dominant phenotype

Aa → dominant phenotype

Aa → dominant phenotype

aa → recessive phenotype

Therefore:

75% dominant phenotype

25% recessive phenotype

The phenotype ratio is:

3 : 1.

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Probability Does Not Guarantee Results

A Punnett square predicts:

probabilities.

It does not guarantee exactly what will happen in a small number of:

offspring.

For example, a 25% probability of a recessive phenotype does not mean that exactly one of every four children or offspring must have that:

phenotype.

Each fertilization event is a separate:

event.

---

Another Cross: AA × aa

Suppose:

AA × aa.

The AA parent can produce only:

A gametes.

The aa parent can produce only:

a gametes.

Therefore every offspring receives:

Aa.

All offspring are:

heterozygous.

If A is dominant, all show the:

dominant phenotype.

---

Another Cross: Aa × aa

Consider:

Aa × aa.

The heterozygous parent can produce:

A or a.

The homozygous recessive parent can produce only:

a.

Possible offspring are:

Aa

and:

aa.

Under complete dominance, this gives:

50% dominant phenotype

and:

50% recessive phenotype.

---

Using Genetic Notation Correctly

When representing a dominant-recessive gene:

A = dominant allele

a = recessive allele

Use the same letter because the symbols represent two versions of the same:

gene.

Good notation:

B and b

T and t

R and r

Poor notation for alleles of one gene:

B and W

Using different letters can incorrectly suggest that they are different:

genes.

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Choosing a Letter

It is usually best to choose a letter that makes the two alleles easy to:

distinguish.

For example:

T and t

are easy to tell apart.

Letters such as:

C and c

can also work.

Letters where uppercase and lowercase forms look very similar can sometimes cause:

confusion.

---

Real Human Traits Are Often More Complex

Simple classroom examples are extremely useful for learning the principles of:

inheritance.

However, many human characteristics do not follow a simple one-gene dominant-recessive:

pattern.

Characteristics such as height, skin pigmentation, and many aspects of appearance are influenced by:

many genes and environmental factors.

Therefore, they should not be treated as simple dominant-recessive traits.

---

Examples of Simple Dominant-Recessive Inheritance

Clear examples can be found in many organisms.

In Mendel's pea plants, traits he studied included simplified dominant-recessive relationships involving:

• round versus wrinkled seeds
• yellow versus green seeds
• purple versus white flowers
• tall versus dwarf plants

Some inherited human genetic conditions also follow relatively simple dominant or recessive inheritance patterns.

---

Example: Cystic Fibrosis

Cystic fibrosis is commonly inherited as an:

autosomal recessive condition.

A person generally needs to inherit two disease-associated variants in the relevant gene to be affected.

A person with one such variant may be an unaffected:

carrier.

This provides a real-world example of the importance of:

recessive inheritance.

---

Example: Huntington Disease

Huntington disease is commonly inherited in an:

autosomal dominant pattern.

One disease-associated copy of the relevant gene can be sufficient for the condition to:

develop.

This illustrates how dominant inheritance can operate in a human genetic:

condition.

---

Dominant Does Not Mean Healthy

The examples above reveal something very important:

dominant does not mean healthy.

An allele associated with a genetic disorder can be:

dominant.

Likewise, a recessive allele is not automatically harmful.

Dominant and recessive describe:

inheritance relationships,

not whether an allele is good or bad.

---

Dominant Does Not Mean Common

A dominant allele does not have to be:

common.

A recessive allele does not have to be:

rare.

Dominance describes what happens in a:

heterozygous individual.

Frequency describes how common an allele is in a:

population.

These are different concepts.

---

Dominant Does Not Mean Stronger

Alleles do not compete physically with each:

other.

The dominant allele is not:

stronger.

Instead, the molecular effects of one functional copy may be sufficient to produce a particular:

phenotype.

The word dominant describes the observed inheritance:

pattern.

---

Why Can an Allele Be Recessive?

Imagine that a gene contains instructions for producing a functional:

protein.

One allele produces a functional protein.

Another allele produces little or no functional:

protein.

Sometimes one functioning copy of the gene produces enough protein for the dominant:

phenotype.

The effect of the other allele may then appear recessive.

This is one common molecular explanation for complete:

dominance.

---

Not All Alleles Show Complete Dominance

Dominant-recessive inheritance is only one inheritance:

pattern.

Other relationships include:

incomplete dominance

and:

codominance.

Some genes also have:

multiple alleles.

Therefore, we should not assume that every pair of alleles follows complete:

dominance.

---

Incomplete Dominance

In incomplete dominance, the heterozygous phenotype is different from either homozygous phenotype and is often:

intermediate.

For example, in some flowers:

RR → red

WW → white

RW → pink

Neither allele completely dominates the:

other.

---

Codominance

In codominance, both alleles contribute visibly to the heterozygous:

phenotype.

The ABO blood group system provides an important:

example.

The alleles:

Iᴬ

and:

Iᴮ

are codominant.

Therefore:

IᴬIᴮ → blood group AB.

Both are:

expressed.

---

Complete Dominance vs Other Patterns

The dominant-recessive model is therefore an important starting point, but not the entire story of:

genetics.

---

Worked Problem 1

Suppose:

B = black fur

b = brown fur

and B is dominant.

An animal has genotype:

Bb.

What is its phenotype?

Because B is dominant:

black fur.

Is the animal homozygous or heterozygous?

Heterozygous.

---

Worked Problem 2

Using the same gene, an animal has:

bb.

What is its phenotype?

Because there is no dominant B allele:

brown fur.

Its genotype is:

homozygous recessive.

---

Worked Problem 3

An animal has black fur.

Can we determine its exact genotype?

No.

It could be:

BB

or:

Bb.

Both produce the dominant:

phenotype.

---

Worked Problem 4

An animal has brown fur.

If brown is recessive, what must its genotype be?

bb.

The recessive phenotype requires two recessive alleles under complete:

dominance.

---

Worked Problem 5

Suppose:

R = red fruit

r = yellow fruit

and R is dominant.

Cross:

Rr × rr

Possible offspring are:

Rr, rr, Rr, rr.

Therefore:

50% Rr

50% rr

and the expected phenotype probabilities are:

50% red

50% yellow.

---

From Genotype to Phenotype

When given a genotype, use this process:

Step 1: Identify the alleles.

Step 2: Determine which allele is dominant.

Step 3: Determine whether the genotype is homozygous or heterozygous.

Step 4: Apply the dominance relationship.

For example:

Gg

If G is dominant:

Gg → dominant phenotype.

---

From Phenotype to Genotype

When given a phenotype:

Step 1: Determine whether the phenotype is dominant or recessive.

Step 2: If it is recessive, the genotype is usually homozygous recessive.

Step 3: If it is dominant, there may be two possible genotypes.

For example:

Dominant phenotype:

GG or Gg

Recessive phenotype:

gg.

---

Common Mistake: Genotype and Phenotype Are the Same

They are not.

Genotype = genetic combination

Phenotype = observable characteristic

For example:

Bb = genotype

Black fur = phenotype.

---

Common Mistake: A Recessive Allele Disappears

A recessive allele can remain present even when it is not visible in the:

phenotype.

A heterozygous individual:

Aa

still possesses the:

a allele.

It can therefore pass a to its:

offspring.

---

Common Mistake: A Dominant Phenotype Must Be Homozygous

A dominant phenotype can result from:

AA

or:

Aa.

Therefore, seeing the dominant phenotype does not necessarily reveal the exact:

genotype.

---

Common Mistake: A Recessive Phenotype Means the Allele Is Weak

Recessive does not mean:

weak.

It simply means that under complete dominance its phenotype is not expressed in the:

heterozygous condition.

---

Common Mistake: 25% Means Exactly One in Every Four

Genetic probabilities describe the chance for each:

offspring.

If a cross predicts a 25% probability, four offspring do not necessarily include exactly one with that:

genotype or phenotype.

Real results can differ because fertilization involves:

chance.

---

Check Your Understanding

1. Define a dominant allele.

2. Define a recessive allele.

3. Explain the difference between genotype and phenotype.

4. What type of letter normally represents a dominant allele?

5. What type of letter normally represents a recessive allele?

6. If B is dominant over b, what phenotype would BB show?

7. What phenotype would Bb show?

8. What phenotype would bb show?

9. Which genotype is homozygous dominant?

10. Which genotype is heterozygous?

11. Which genotype is homozygous recessive?

12. Explain why BB and Bb can produce the same phenotype.

13. Why does a recessive phenotype usually reveal the genotype?

14. Why might a dominant phenotype not reveal the exact genotype?

15. If T represents tall and t represents short, with T dominant, what phenotype does Tt produce?

16. What genotype must a short plant have?

17. What alleles can a Tt individual place into its gametes?

18. Explain what happens to allele pairs during meiosis.

19. Explain how fertilization creates a new allele pair.

20. What is a carrier?

21. Why can a recessive allele pass through generations without appearing in the phenotype?

22. Cross Aa × Aa. List the four possible genotype outcomes in a Punnett square.

23. What is the expected genotype ratio for Aa × Aa?

24. What is the expected phenotype ratio if A is completely dominant?

25. Cross Aa × aa. What proportion of offspring are expected to show the recessive phenotype?

26. Explain why genetic probabilities do not guarantee the exact number of offspring with each phenotype.

27. Why does dominant not mean more common?

28. Why does dominant not mean better or healthier?

29. Give one example of an inheritance pattern that does not follow complete dominance.

30. Explain how genetic notation can be used to predict patterns of inheritance.

---

Key Terms

• Allele: Alternative version of a gene.
• Dominant allele: Allele expressed in a heterozygous genotype under complete dominance.
• Recessive allele: Allele whose phenotype under complete dominance is normally expressed when two recessive copies are present.
• Genotype: Allele combination possessed by an organism.
• Phenotype: Observable characteristic resulting from genetic information and, for many traits, environmental influences.
• Homozygous: Having two identical alleles.
• Heterozygous: Having two different alleles.
• Homozygous dominant: Having two dominant alleles.
• Homozygous recessive: Having two recessive alleles.
• Carrier: Heterozygous individual carrying a recessive allele associated with a particular trait or condition.
• Complete dominance: Inheritance pattern in which the heterozygote shows the same phenotype as the dominant homozygote.
• Punnett square: Diagram used to predict possible allele combinations in offspring.
• Probability: Measure of how likely an outcome is.
• Incomplete dominance: Inheritance pattern in which the heterozygous phenotype differs from both homozygous phenotypes and is often intermediate.
• Codominance: Inheritance pattern in which both alleles are expressed in the heterozygous phenotype.

---

Key Takeaways

• Different versions of a gene are called alleles.
• In complete dominance, one allele is dominant and another is recessive.
• Dominant alleles are usually represented with capital letters.
• Recessive alleles are usually represented with lowercase letters.
• Genotype describes an organism's allele combination.
• Phenotype describes an observable characteristic.
• AA is homozygous dominant.
• Aa is heterozygous.
• aa is homozygous recessive.
• Under complete dominance, AA and Aa show the dominant phenotype.
• The recessive phenotype normally appears with aa.
• A dominant phenotype can therefore have more than one possible genotype.
• A recessive phenotype usually identifies the genotype more directly.
• Recessive alleles do not disappear when they are not expressed.
• Heterozygous individuals can pass recessive alleles to their offspring.
• Some heterozygous individuals are described as carriers.
• During meiosis, allele pairs separate into gametes.
• Gametes normally carry one allele for each gene.
• Fertilization combines alleles from two gametes.
• Punnett squares can be used to predict possible offspring genotypes and phenotypes.
• Punnett-square results represent probabilities, not guaranteed numbers of offspring.
• Dominant does not mean stronger.
• Dominant does not mean healthier.
• Dominant does not mean more common.
• Recessive does not mean weak or harmful.
• Many real traits do not follow simple dominant-recessive inheritance.
• Incomplete dominance and codominance are examples of other inheritance patterns.
• Understanding dominant and recessive inheritance prepares us to solve monohybrid crosses and Punnett-square problems.

4. Punnett Squares

What Is a Punnett Square?

A Punnett square is a diagram used to predict the possible allele combinations that offspring could inherit from their:

parents.

Punnett squares allow us to predict:

• possible offspring genotypes
• possible offspring phenotypes
• probability of each genotype
• probability of each phenotype

They are one of the most useful tools for studying basic:

genetics.

However, Punnett squares predict:

probabilities.

They do not tell us exactly what offspring will be produced.

---

Before Using a Punnett Square

To construct a Punnett square, we first need to know the:

genotypes of the parents.

Suppose a gene has two alleles:

T = tall

t = short

and T is dominant over:

t.

An individual could therefore have one of three genotypes:

TT

Tt

tt.

---

Review: Genotype and Phenotype

Remember:

Genotype = allele combination

Phenotype = observable characteristic

For our plant example:

Because T is dominant, both TT and Tt produce the:

tall phenotype.

---

Where Do the Alleles in a Punnett Square Come From?

Most body cells of a diploid organism contain two alleles for a particular:

gene.

During meiosis, the allele pair:

separates.

Each gamete normally receives only:

one allele.

For example, an individual with genotype:

Tt

can produce gametes containing:

T

or:

t.

---

The Basic Structure of a Punnett Square

A simple Punnett square for one gene usually contains:

four boxes.

One parent's possible gametes are written across the:

top.

The other parent's possible gametes are written down the:

side.

The alleles are then combined inside each:

box.

For example:

Each box represents one possible allele combination in an:

offspring.

---

The Five Steps for Constructing a Punnett Square

A reliable method is:

Step 1: Identify the alleles.

Step 2: Write the genotypes of the parents.

Step 3: Determine the alleles each parent can place in its gametes.

Step 4: Place the gametes around the Punnett square and fill the boxes.

Step 5: Determine genotype and phenotype probabilities.

Let's examine each step.

---

Step 1: Identify the Alleles

Suppose tall is dominant over short.

We can write:

T = tall allele

t = short allele

Always make it clear which allele is:

dominant.

Using the same letter helps show that T and t are versions of the same:

gene.

---

Step 2: Write the Parent Genotypes

Suppose both parents are:

heterozygous.

Their genotypes are:

Tt × Tt.

This is called a genetic:

cross.

The × symbol means that we are examining the possible offspring produced by these two:

genotypes.

---

Step 3: Determine the Gametes

Each gamete receives one allele.

The first parent is:

Tt.

Therefore, its gametes can contain:

T or t.

The second parent is also:

Tt.

Its gametes can also contain:

T or t.

---

Step 4: Fill the Punnett Square

Place one parent's alleles across the top and the other parent's alleles down the side.

Now we have four possible:

offspring genotypes.

They are:

TT

Tt

Tt

tt.

---

Step 5: Calculate Genotype Probabilities

There are four boxes.

One contains:

TT.

Two contain:

Tt.

One contains:

tt.

Therefore:

TT = 1/4 = 25%

Tt = 2/4 = 50%

tt = 1/4 = 25%

The expected genotype ratio is:

1 TT : 2 Tt : 1 tt

or simply:

1 : 2 : 1.

---

Predicting Phenotypes

Now translate the genotypes into:

phenotypes.

Remember:

TT → tall

Tt → tall

tt → short

Our Punnett square contains:

3 tall possibilities

and:

1 short possibility.

Therefore:

Tall = 3/4 = 75%

Short = 1/4 = 25%

The expected phenotype ratio is:

3 : 1.

---

Genotype Ratio vs Phenotype Ratio

Do not confuse:

genotype ratio

with:

phenotype ratio.

For:

Tt × Tt

the genotype ratio is:

1 TT : 2 Tt : 1 tt

The phenotype ratio is:

3 tall : 1 short.

The ratios are different because TT and Tt produce the same:

phenotype.

---

Probability

A probability describes how likely an outcome is.

Probability can be written as:

• a fraction
• a decimal
• a percentage

For example:

1/4 = 0.25 = 25%

1/2 = 0.50 = 50%

3/4 = 0.75 = 75%

Punnett squares are therefore closely connected to:

probability.

---

Cross 1: Homozygous Dominant × Homozygous Recessive

Consider:

TT × tt

The TT parent can produce only:

T gametes.

The tt parent can produce only:

t gametes.

The Punnett square is:

Every offspring genotype is:

Tt.

---

Interpreting TT × tt

The genotype probability is:

100% Tt

If T is dominant, the phenotype probability is:

100% tall.

Notice something important:

The offspring are all:

heterozygous.

They all carry the recessive t allele even though they show the:

dominant phenotype.

---

Cross 2: Heterozygous × Homozygous Recessive

Now consider:

Tt × tt.

The Tt parent can produce:

T or t.

The tt parent can produce only:

t.

Possible genotypes are:

Tt, Tt, tt, tt.

---

Interpreting Tt × tt

The genotype probabilities are:

50% Tt

50% tt

The phenotype probabilities are:

50% tall

50% short.

The phenotype ratio is:

1 : 1.

---

Cross 3: Homozygous Dominant × Heterozygous

Consider:

TT × Tt.

The TT parent produces only:

T.

The Tt parent produces:

T or t.

Therefore:

50% TT

50% Tt

0% tt.

---

What About the Phenotypes?

Both:

TT

and:

Tt

produce the dominant phenotype.

Therefore:

100% tall

0% short.

This is a good example of why genotype probabilities and phenotype probabilities are not always the:

same.

---

Cross 4: Homozygous Recessive × Homozygous Recessive

Consider:

tt × tt.

Both parents can produce only:

t gametes.

Therefore:

100% tt

and:

100% recessive phenotype.

---

Cross 5: Homozygous Dominant × Homozygous Dominant

Consider:

TT × TT.

Both parents can produce only:

T gametes.

Every possible offspring is:

TT.

Therefore:

100% TT

and:

100% dominant phenotype.

---

Comparing Common Crosses

Understanding these patterns makes Punnett squares much easier to:

interpret.

---

Example: Fur Colour

Suppose fur colour in an imaginary animal follows simple complete dominance.

Let:

B = black fur

b = brown fur

Cross:

Bb × Bb.

Each parent can produce:

B or b.

---

Fur Colour Results

The genotype probabilities are:

25% BB

50% Bb

25% bb

Because B is dominant:

BB → black

Bb → black

bb → brown

Therefore:

75% black

25% brown.

---

Example: Seed Shape

Suppose:

R = round seeds

r = wrinkled seeds

and R is dominant.

Cross:

Rr × rr.

The genotype probabilities are:

50% Rr

50% rr.

The phenotype probabilities are:

50% round

50% wrinkled.

---

Example: Flower Colour

Suppose:

P = purple flowers

p = white flowers

and P is dominant.

A homozygous purple plant is crossed with a white plant.

First determine the genotypes:

PP × pp.

The Punnett square is:

Therefore:

100% Pp

and:

100% purple flowers.

---

Working Backward from Phenotype

Sometimes we know the phenotype but not the:

genotype.

Suppose black fur is dominant.

An animal with brown fur must have:

bb.

But an animal with black fur could be:

BB

or:

Bb.

This means a dominant phenotype does not always reveal the exact:

genotype.

---

Test Crosses

A test cross can help determine the unknown genotype of an organism showing a dominant:

phenotype.

Suppose a tall plant could be:

TT

or:

Tt.

It can be crossed with a homozygous recessive plant:

tt.

Why use tt?

Because the recessive parent always contributes:

t.

The offspring can therefore provide information about the unknown parent's:

genotype.

---

Test Cross: If the Unknown Plant Is TT

Suppose:

TT × tt.

All offspring are:

Tt.

Therefore all show the:

dominant phenotype.

---

Test Cross: If the Unknown Plant Is Tt

Suppose:

Tt × tt.

Possible offspring are:

Tt

and:

tt.

Some offspring can therefore show the:

recessive phenotype.

Observing recessive offspring demonstrates that the dominant-phenotype parent contributed a recessive:

allele.

---

Punnett Squares Show Possible Outcomes

Each box in a Punnett square represents a possible:

genotype.

The boxes do not represent four actual children, plants, or:

animals.

They represent possible combinations of parental:

alleles.

This distinction is extremely important.

---

Probability Does Not Mean Certainty

Suppose a Punnett square predicts:

25% recessive phenotype.

If four offspring are produced, does exactly one have to show the recessive phenotype?

No.

Possible real outcomes could include:

• no recessive offspring
• one recessive offspring
• two recessive offspring
• three recessive offspring
• four recessive offspring

Some outcomes are more likely than others, but chance determines which gametes actually combine.

---

Think of a Coin Toss

A fair coin has:

50% probability of heads

and:

50% probability of tails.

If you toss the coin twice, you are not guaranteed to obtain exactly one head and one:

tail.

You could obtain:

heads, heads.

The same principle applies to genetic:

probability.

---

Larger Numbers Approach Expected Ratios

If only a few offspring are produced, observed results can differ greatly from expected:

probabilities.

With many offspring, the observed proportions often become closer to the predicted:

probabilities.

For example, a 3:1 expected phenotype ratio may not appear clearly among four offspring.

Among hundreds or thousands of offspring, the overall proportion is more likely to approach:

3:1.

---

Predicting Expected Numbers

Punnett-square probabilities can also be used to predict expected numbers.

Suppose:

Tt × Tt

produces an expected 25% short offspring.

If there are 200 offspring:

Expected short = 25% of 200

= 0.25 × 200

= 50

We would expect approximately:

50 short offspring.

This is an expectation, not a guarantee.

---

Another Expected Number Example

Suppose a cross predicts:

75% dominant phenotype.

There are 80 offspring.

Expected number showing the dominant phenotype:

0.75 × 80 = 60

So approximately:

60 offspring

would be expected to show the dominant phenotype.

---

Probability as a Fraction

If one of four Punnett-square boxes contains a genotype:

Probability = 1/4

If two boxes contain it:

Probability = 2/4 = 1/2

If three boxes contain it:

Probability = 3/4

If all four contain it:

Probability = 4/4 = 1.

---

Probability as a Percentage

Convert fractions to percentages by multiplying by:

100%.

For example:

1/4 × 100% = 25%

2/4 × 100% = 50%

3/4 × 100% = 75%

4/4 × 100% = 100%

These percentages are extremely common in simple Punnett-square:

problems.

---

A Complete Worked Problem

In a species of plant, smooth seeds are dominant over wrinkled seeds.

Let:

S = smooth

s = wrinkled

Two heterozygous plants are crossed.

Step 1: Parent genotypes

Ss × Ss

Step 2: Gametes

Parent 1:

S or s

Parent 2:

S or s

Step 3: Punnett square

Step 4: Genotype probabilities

25% SS

50% Ss

25% ss

Step 5: Phenotype probabilities

75% smooth

25% wrinkled.

---

Interpreting the Result

What does:

25% ss

mean?

It means each offspring has a:

25% probability

of inheriting genotype ss under the assumptions of this simple genetic model.

It does not mean that every group of four offspring will contain exactly one:

ss offspring.

---

Finding the Probability of a Carrier

Suppose a recessive condition is represented by:

a.

Consider:

Aa × Aa.

The offspring genotypes are:

AA, Aa, Aa, aa.

If heterozygous individuals are carriers, then:

2 of 4 are carriers.

Therefore:

Probability of a carrier = 2/4 = 50%.

---

Finding the Probability of Homozygous Offspring

Consider again:

Aa × Aa.

The homozygous offspring are:

AA

and:

aa.

Two of the four boxes are homozygous.

Therefore:

Probability of homozygous offspring = 2/4 = 50%.

---

Finding the Probability of Heterozygous Offspring

For:

Aa × Aa

two boxes contain:

Aa.

Therefore:

Probability of heterozygous offspring = 2/4 = 50%.

Punnett squares can therefore answer questions about more than just:

phenotype.

---

Interpreting an Unknown Cross

Suppose a cross produces approximately:

50% dominant phenotype

and:

50% recessive phenotype.

A likely simple monohybrid cross is:

Aa × aa.

Why?

The heterozygous parent produces:

A and a gametes.

The homozygous recessive parent produces only:

a gametes.

This produces an expected:

1 : 1 phenotype ratio.

---

Interpreting a 3:1 Pattern

Suppose a large number of offspring show approximately:

75% dominant phenotype

and:

25% recessive phenotype.

Under a simple complete-dominance model, this is consistent with:

Aa × Aa.

The recessive offspring must receive:

a from both parents.

Therefore, both parents must be capable of contributing the recessive:

allele.

---

Punnett Squares and Inheritance Patterns

Punnett squares help us connect:

parent genotypes

to:

gametes

to:

offspring genotypes

to:

offspring phenotypes.

A useful chain is:

Parent genotypes

↓

Possible gametes

↓

Punnett square

↓

Offspring genotypes

↓

Offspring phenotypes

↓

Probabilities

This is the central logic of a Punnett-square problem.

---

Punnett Squares Are Models

A Punnett square is a:

model.

Like all scientific models, it simplifies:

reality.

A basic 2 × 2 Punnett square assumes a relatively simple inheritance pattern involving one gene with two alleles.

Real genetics can involve:

• multiple genes
• multiple alleles
• codominance
• incomplete dominance
• sex-linked inheritance
• gene interactions
• environmental influences

Therefore, simple Punnett squares are a starting point for understanding inheritance, not a complete description of all:

genetics.

---

Common Mistake: Putting Both Alleles in Each Gamete

Suppose a parent is:

Tt.

A gamete should contain:

T or t,

not:

Tt.

Gametes are haploid and normally contain only one allele for each:

gene.

---

Common Mistake: Changing the Allele Symbols

If the alleles are:

B and b,

continue using:

B and b.

Do not suddenly change them to different letters inside the Punnett:

square.

Consistent notation makes inheritance much easier to:

follow.

---

Common Mistake: Confusing Genotype and Phenotype

Remember:

BB, Bb, bb = genotypes

while:

black, brown, tall, short = phenotypes.

A genotype is an allele:

combination.

A phenotype is an observable:

characteristic.

---

Common Mistake: Counting Genotypes Incorrectly

For:

Aa × Aa

the boxes are:

AA, Aa, Aa, aa.

There are three different genotype types, but there are:

four possible boxes.

Therefore:

AA = 25%

Aa = 50%

aa = 25%.

Do not give each of the three genotype types:

33.3%.

---

Common Mistake: Treating Each Box as an Actual Offspring

Four boxes do not mean the parents will have:

four offspring.

The boxes represent possible allele combinations and their relative:

probabilities.

A couple could have one child, five children, or no children; the Punnett square still describes the probability for each genetic:

outcome under the model.

---

Common Mistake: Assuming Dominant Means More Likely

A dominant allele is not automatically more likely to be:

inherited.

For example, in:

Aa × aa

the dominant A allele has a 50% chance of being inherited from the heterozygous:

parent.

Dominance affects:

phenotype expression,

not the probability that an allele enters a gamete.

---

Common Mistake: Assuming 75% Means Exactly 3 out of 4

A predicted probability of 75% means each offspring has a:

75% probability

of that outcome.

It does not guarantee exactly three affected offspring in every group of:

four.

---

Check Your Understanding

1. Define a Punnett square.

2. What information can a Punnett square predict?

3. What is the difference between genotype and phenotype?

4. Why does each gamete normally contain only one allele for a particular gene?

5. What gametes can a TT parent produce?

6. What gametes can a Tt parent produce?

7. What gametes can a tt parent produce?

8. Construct a Punnett square for TT × tt.

9. What percentage of offspring from TT × tt are Tt?

10. If T is dominant, what percentage show the dominant phenotype?

11. Construct a Punnett square for Tt × Tt.

12. What percentage of offspring are TT?

13. What percentage are Tt?

14. What percentage are tt?

15. What is the genotype ratio for Tt × Tt?

16. What is the phenotype ratio for Tt × Tt?

17. Construct a Punnett square for Tt × tt.

18. What percentage of offspring from Tt × tt show the recessive phenotype?

19. Why are genotype and phenotype ratios sometimes different?

20. Convert 1/4 to a percentage.

21. Convert 3/4 to a percentage.

22. If a cross predicts a 25% probability of a phenotype and produces 120 offspring, how many would you expect to show the phenotype?

23. Why might the actual number differ from the expected number?

24. What does a 1:1 phenotype ratio mean?

25. What simple cross can produce a 1:1 dominant-to-recessive phenotype ratio?

26. Explain how a test cross can help determine an unknown genotype.

27. Why does a dominant phenotype not always reveal the genotype?

28. Why does a recessive phenotype often reveal the genotype?

29. Explain why Punnett squares predict probability rather than certainty.

30. Describe the complete process for solving a Punnett-square problem from parent genotypes to offspring phenotype probabilities.

---

Key Terms

• Punnett square: Diagram used to predict possible allele combinations in offspring.
• Allele: Alternative version of a gene.
• Genotype: Allele combination possessed by an organism.
• Phenotype: Observable characteristic of an organism.
• Dominant allele: Allele expressed in a heterozygote under complete dominance.
• Recessive allele: Allele whose phenotype under complete dominance is normally expressed when two copies are present.
• Homozygous: Having two identical alleles.
• Heterozygous: Having two different alleles.
• Gamete: Haploid reproductive cell containing one allele for each gene.
• Genetic cross: Comparison of two parent genotypes to predict possible offspring.
• Probability: Measure of the likelihood of an outcome.
• Genotype ratio: Relative proportions of different offspring genotypes.
• Phenotype ratio: Relative proportions of different offspring phenotypes.
• Test cross: Cross involving a homozygous recessive individual used to investigate an unknown genotype.
• Monohybrid cross: Genetic cross involving one gene or characteristic.

---

Key Takeaways

• A Punnett square predicts possible genetic outcomes in offspring.
• Punnett squares predict probabilities, not guaranteed results.
• Begin by identifying the alleles and parent genotypes.
• During meiosis, allele pairs separate.
• Each gamete normally carries one allele for a particular gene.
• Parent gametes are placed around the outside of the Punnett square.
• Alleles are combined inside the boxes.
• The boxes represent possible offspring genotypes.
• Genotype refers to allele combinations.
• Phenotype refers to observable characteristics.
• Genotype probabilities and phenotype probabilities are not always the same.
• For Tt × Tt, the expected genotype ratio is 1 : 2 : 1.
• For Tt × Tt under complete dominance, the expected phenotype ratio is 3 : 1.
• 1/4 = 25%.
• 1/2 = 50%.
• 3/4 = 75%.
• 4/4 = 100%.
• TT × tt produces 100% heterozygous Tt offspring.
• Tt × tt produces an expected 1:1 phenotype ratio.
• A dominant phenotype can correspond to more than one genotype.
• A recessive phenotype usually corresponds to the homozygous recessive genotype under complete dominance.
• A test cross can help investigate an unknown dominant-phenotype genotype.
• Larger numbers of offspring are more likely to approach predicted ratios.
• Actual results may differ from predicted ratios because fertilization involves chance.
• Dominance affects expression of a trait, not the probability that an allele is inherited.
• Punnett squares are models and do not describe every type of inheritance.
• Simple Punnett squares provide the foundation for studying more complex patterns of genetic inheritance.

5. Predicting Inheritance

How Can We Predict Inheritance?

Parents pass genetic information to their offspring through:

gametes.

Each gamete carries one allele for each gene, and fertilization combines alleles from:

two parents.

Because we understand how alleles are separated during meiosis and combined during fertilization, we can predict the:

probability of different genetic outcomes.

We can use:

• parent genotypes
• Punnett squares
• probabilities
• offspring phenotypes
• family information

to make predictions about:

inheritance.

These are predictions of probability, not guarantees about individual offspring.

---

From Parents to Offspring

A useful way to think about inheritance is:

Parent genotypes

↓

Meiosis

↓

Gametes containing individual alleles

↓

Random fertilization

↓

Offspring genotype

↓

Offspring phenotype

Understanding this sequence allows us to predict how traits can pass from one generation to the:

next.

---

A Simple Inheritance Example

Suppose fur colour in an imaginary animal is controlled by one gene.

Let:

B = black fur

b = brown fur

Suppose B is dominant over:

b.

The possible genotypes are:

BB

Bb

bb.

The corresponding phenotypes are:

BB → black

Bb → black

bb → brown.

---

Genotype and Phenotype

Remember the important difference:

Genotype = allele combination

Phenotype = observable characteristic

For example:

Bb = genotype

Black fur = phenotype

Inheritance predictions often begin with genotypes and then use the dominance relationship to predict:

phenotypes.

---

Predicting from Parent Genotypes

Suppose two heterozygous black animals reproduce.

Their genotypes are:

Bb × Bb.

Each parent can produce two types of gametes:

B

or:

b.

We can use a Punnett square to predict the possible:

offspring genotypes.

---

Constructing the Cross

Possible offspring genotypes are:

BB

Bb

Bb

bb.

---

Calculating Genotype Probabilities

From the cross:

Bb × Bb

we obtain:

1 BB

2 Bb

1 bb.

Therefore:

BB = 1/4 = 25%

Bb = 2/4 = 50%

bb = 1/4 = 25%

The expected genotype ratio is:

1 : 2 : 1.

---

Calculating Phenotype Probabilities

Because B is dominant:

BB → black

Bb → black

bb → brown

Therefore:

Black = 3/4 = 75%

Brown = 1/4 = 25%

The expected phenotype ratio is:

3 : 1.

---

Probability

Probability describes how likely an event is to occur.

Probability can be expressed as:

fraction

decimal

or:

percentage.

For example:

1/4 = 0.25 = 25%

1/2 = 0.50 = 50%

3/4 = 0.75 = 75%

1 = 100%

---

Probability Formula

A simple probability can be calculated using:

Probability = Number of desired outcomes ÷ Total number of possible outcomes

For example, suppose one of four equally likely Punnett-square outcomes is:

bb.

Then:

Probability of bb = 1 ÷ 4

= 1/4

= 25%.

---

Why Is Inheritance Based on Chance?

During meiosis, alleles are separated into:

gametes.

Which sperm or pollen cell participates in fertilization is generally not determined by what trait would be useful to the:

offspring.

Similarly, which egg is fertilized is not selected to produce a particular:

genotype.

Fertilization therefore contains an important element of:

chance.

---

Random Fertilization

Consider a heterozygous parent:

Aa.

Its gametes can carry:

A

or:

a.

Another Aa parent can also produce:

A

or:

a.

When gametes combine randomly, several combinations are possible:

AA

Aa

aa.

We can calculate the probability of each combination, but we cannot know with certainty which combination a particular fertilization will:

produce.

---

Probability Does Not Mean Certainty

Suppose a genetic cross predicts:

25% probability of aa.

Does this mean exactly one of every four offspring must be:

aa?

No.

Each offspring represents another genetic:

event.

Four offspring could theoretically include:

• zero aa offspring
• one aa offspring
• two aa offspring
• three aa offspring
• four aa offspring

The Punnett square tells us what is:

expected over many events,

not what must occur in a small family.

---

Think About Coin Tosses

A fair coin has a:

50% chance of heads

and a:

50% chance of tails.

If you toss it four times, you are not guaranteed:

two heads and two tails.

You might obtain:

four heads.

The probability is still 50% on each independent:

toss.

Genetic probability works in a similar way.

---

Each Offspring Is a New Event

Suppose two parents have a:

25% probability

of producing an offspring with a particular genotype.

Their first child has that genotype.

Does this mean their next child cannot have it?

No.

For the same parental genotypes, the probability for the next independent conception remains:

25%.

Previous outcomes do not use up genetic:

possibilities.

---

The Gambler's Fallacy in Genetics

A common mistake is thinking:

"The first three children did not inherit the trait, so the fourth one must."

This reasoning is:

incorrect.

If the probability for each conception is 25%, the probability remains:

25% for each independent conception.

Previous outcomes do not force the next:

outcome.

---

Larger Samples and Expected Ratios

Although small families may differ greatly from predicted ratios, larger numbers of offspring tend to produce proportions closer to expected:

probabilities.

Suppose:

Aa × Aa

predicts 25% aa.

Among four offspring, the actual proportion could vary widely.

Among 1,000 offspring, the proportion would generally be expected to be much closer to:

25%.

This is one reason Mendel studied large numbers of:

pea plants.

---

Calculating Expected Numbers

Suppose a genetic cross predicts:

25% recessive phenotype.

There are 200 offspring.

Expected number:

0.25 × 200 = 50

So we would expect approximately:

50 offspring

to show the recessive phenotype.

---

Another Example

Suppose a cross predicts:

75% dominant phenotype.

A population produces 400 offspring.

Expected number:

0.75 × 400 = 300

Therefore, approximately:

300 offspring

would be expected to show the dominant phenotype.

The actual number may not be exactly:

300.

---

Predicting from a Recessive Phenotype

Phenotypes can provide clues about:

genotypes.

Suppose:

B = black

b = brown

with B dominant.

If an animal has brown fur, its genotype must be:

bb.

Why?

Because:

BB → black

Bb → black

bb → brown.

The recessive phenotype therefore provides strong evidence about the:

genotype.

---

Predicting from a Dominant Phenotype

Suppose an animal has black fur.

Can we determine its exact genotype?

No.

It could be:

BB

or:

Bb.

Therefore, a dominant phenotype often provides less information about genotype than a:

recessive phenotype.

---

Using Offspring as Genetic Evidence

Sometimes offspring phenotypes can reveal information about their parents':

genotypes.

Suppose two black animals produce a brown offspring.

Brown is recessive.

Therefore, the brown offspring must be:

bb.

Where did the two b alleles come from?

One came from:

each parent.

Therefore, both black parents must carry:

b.

Their genotypes must be:

Bb × Bb.

This is an example of using genetic evidence to make an inheritance:

prediction.

---

Evidence-Based Genetic Reasoning

A strong genetic explanation should connect:

evidence → allele information → genotype → conclusion.

For example:

Evidence: Two dominant-phenotype parents produced a recessive-phenotype offspring.

Reasoning: The recessive offspring must have received one recessive allele from each parent.

Conclusion: Both parents must possess at least one recessive allele.

This type of reasoning is extremely useful when analyzing:

family inheritance patterns.

---

Family Inheritance Patterns

Genetic traits can sometimes be followed through several generations of a:

family.

Scientists and genetic counselors can examine which individuals show a particular phenotype and use that information to investigate possible:

inheritance patterns.

A diagram used to represent genetic relationships within a family is called a:

pedigree.

---

What Is a Pedigree?

A pedigree is a diagram showing biological relationships and the occurrence of a particular trait across:

generations.

Pedigrees can help scientists investigate whether a trait might follow patterns such as:

• dominant inheritance
• recessive inheritance
• sex-linked inheritance

At this level, we will focus mainly on simple:

dominant and recessive patterns.

---

Reading a Simple Pedigree

Pedigrees use standardized:

symbols.

Traditionally:

Square = male

Circle = female

A filled symbol commonly indicates an individual showing the trait being:

studied.

An unfilled symbol commonly indicates an individual who does not show the:

trait.

Horizontal lines connect reproductive partners, while vertical lines connect parents to:

offspring.

---

Generations in a Pedigree

Generations are commonly labeled using Roman numerals:

I

II

III

Individuals within a generation are numbered from:

left to right.

For example:

II-3

means:

the third individual in generation II.

This allows scientists to discuss individuals in a pedigree precisely.

---

Recognizing a Recessive Pattern

Suppose two parents do not show a trait but produce an offspring who:

does.

Under a simple autosomal dominant-recessive model, this can provide evidence that the trait is:

recessive.

If the affected offspring is:

aa,

then each parent must have contributed:

a.

If the parents themselves do not show the recessive phenotype, they could be:

Aa.

Therefore:

Aa × Aa

can produce:

aa offspring.

---

Recognizing a Dominant Pattern

For a simple dominant trait, an individual showing the trait usually has at least one:

dominant allele.

The genotype could be:

AA

or:

Aa.

If an affected heterozygous individual reproduces with an unaffected aa individual:

Aa × aa

the expected phenotype probabilities are:

50% dominant phenotype

50% recessive phenotype.

However, actual family sizes are often small, so real pedigrees may not show these ratios exactly.

---

Worked Family Example 1

Suppose attached earlobe type in an imaginary species follows simple inheritance.

Let:

E = dominant phenotype

e = recessive phenotype.

Two individuals show the dominant phenotype.

They produce an offspring showing the recessive phenotype.

What can we conclude?

The offspring must be:

ee.

Therefore, each parent contributed:

e.

Because the parents show the dominant phenotype, each must also possess E.

Therefore:

Parent 1 = Ee

Parent 2 = Ee.

---

Worked Family Example 2

Suppose a recessive condition is represented by:

r.

An individual showing the condition has genotype:

rr.

This individual has a child with someone who is:

RR.

Cross:

rr × RR.

All offspring receive:

r from one parent

and:

R from the other.

Therefore:

100% Rr.

Under complete dominance, none would show the recessive phenotype, but all would carry the recessive:

allele.

---

Worked Family Example 3

Now suppose:

rr × Rr.

The rr parent produces only:

r gametes.

The Rr parent produces:

R or r.

Therefore:

50% Rr

50% rr.

The probability of the recessive phenotype is:

50%.

---

Using Unknown Genotypes

Inheritance problems sometimes include an unknown:

genotype.

Suppose a dominant-phenotype organism could be:

AA

or:

Aa.

How might we determine which genotype is more likely?

We can examine:

offspring.

If the organism produces an aa offspring, then it must have contributed an:

a allele.

Therefore, it cannot be:

AA.

It must be:

Aa.

---

Test Crosses as Evidence

A test cross can be used to investigate an unknown dominant-phenotype genotype.

The unknown individual is crossed with a:

homozygous recessive individual.

Suppose the unknown genotype is:

A_

where the underscore means the second allele is not yet known.

It could be:

AA

or:

Aa.

---

If the Unknown Parent Is AA

Cross:

AA × aa

All offspring are:

Aa.

All show the:

dominant phenotype.

---

If the Unknown Parent Is Aa

Cross:

Aa × aa

Possible offspring are:

Aa

and:

aa.

Therefore, recessive-phenotype offspring are:

possible.

If recessive offspring appear, this provides evidence that the unknown parent was:

heterozygous.

---

Genetic Evidence Can Rule Out Possibilities

Good genetic reasoning does not always tell us exactly what a genotype:

is.

Sometimes it tells us what the genotype:

cannot be.

For example, if an individual produces a recessive offspring, that parent cannot be homozygous dominant under the simple inheritance model.

Why?

Because a homozygous dominant parent has no recessive allele to:

contribute.

---

Genetic Evidence and Certainty

It is important to distinguish between:

possible

probable

and:

certain.

For example, an organism showing a dominant phenotype could be:

AA or Aa.

Phenotype alone does not tell us which.

However, if that organism produces an aa offspring, then we know it contributed:

a.

That provides stronger evidence that it is:

Aa.

---

Multiplication Rule for Independent Events

Sometimes we want to calculate the probability of several independent genetic events occurring:

together.

Suppose each child has a 25% probability of genotype aa.

What is the probability that two consecutive children are both aa?

Multiply the probabilities:

1/4 × 1/4 = 1/16

Therefore:

Probability = 1/16 = 6.25%.

---

Another Probability Example

Suppose each offspring has a:

50% probability

of a particular phenotype.

What is the probability that two consecutive offspring both show that phenotype?

1/2 × 1/2 = 1/4

Therefore:

25%.

This works because we are considering independent:

events.

---

Three Consecutive Outcomes

Suppose the probability of a recessive phenotype is:

1/4.

What is the probability that three consecutive offspring all show the recessive phenotype?

1/4 × 1/4 × 1/4

= 1/64

≈ 1.56%

This outcome is unlikely, but it is still:

possible.

---

Probability of Either Outcome

Sometimes several outcomes satisfy a question.

For:

Aa × Aa

suppose we want the probability of a homozygous offspring.

The possible homozygous genotypes are:

AA

and:

aa.

Probability of AA:

1/4

Probability of aa:

1/4

Add them:

1/4 + 1/4 = 1/2

Therefore:

50% of offspring are expected to be homozygous.

---

Probability of a Heterozygous Offspring

For:

Aa × Aa

the Punnett square contains:

AA, Aa, Aa, aa.

Two of the four outcomes are:

Aa.

Therefore:

P(Aa) = 2/4

= 1/2

= 50%.

---

Probability of the Dominant Phenotype

Again consider:

Aa × Aa.

Dominant phenotype genotypes are:

AA

and:

Aa.

Therefore:

P(dominant phenotype)

= P(AA) + P(Aa)

= 1/4 + 2/4

= 3/4

= 75%.

---

Probability of the Recessive Phenotype

The recessive phenotype requires:

aa.

Therefore:

P(recessive phenotype)

= 1/4

= 25%.

---

Experimental Results vs Predicted Results

Imagine a cross predicts:

75% purple flowers

and:

25% white flowers.

A researcher grows 100 offspring and observes:

72 purple

28 white.

Does this mean the Punnett-square prediction was wrong?

Not necessarily.

The predicted values are:

probabilities.

Random variation can cause observed results to differ somewhat from the expected:

ratio.

---

Using Evidence from Larger Samples

Suppose another researcher grows:

10,000 offspring.

If the inheritance model is correct, the observed proportion would generally be expected to lie relatively close to the predicted:

3 : 1 ratio.

Large samples provide stronger evidence for identifying inheritance patterns because random fluctuations tend to have a smaller proportional:

effect.

---

When Evidence Does Not Fit the Prediction

Suppose a simple model predicts:

75% dominant phenotype

and:

25% recessive phenotype.

But a very large experiment consistently produces a very different:

pattern.

Scientists should not simply ignore the:

evidence.

Instead, they might ask:

• Is the assumed genotype correct?
• Is the trait actually controlled by one gene?
• Is dominance complete?
• Are environmental factors involved?
• Is another inheritance pattern operating?

Scientific predictions should always be tested against:

evidence.

---

Limitations of Simple Inheritance Predictions

Punnett squares are powerful, but simple crosses make several:

assumptions.

Real traits may involve:

• multiple genes
• multiple alleles
• codominance
• incomplete dominance
• sex-linked inheritance
• gene interactions
• environmental effects

Therefore, not every characteristic can be accurately predicted using a simple:

2 × 2 Punnett square.

---

Real Human Traits

Many familiar human characteristics are sometimes incorrectly taught as simple dominant-recessive:

traits.

Characteristics such as:

• eye colour
• hair colour
• height
• skin pigmentation

are generally more genetically complex than a simple:

A/a model.

When learning Punnett squares, it is often better to use clearly defined model traits or well-established single-gene examples.

---

Evidence-Based Prediction

A strong inheritance prediction should include:

1. The allele definitions

Example:

B = dominant allele

b = recessive allele

2. The parent genotypes

Example:

Bb × bb

3. The possible gametes

Example:

B or b × b

4. The Punnett square

5. The genotype probabilities

6. The phenotype probabilities

7. A conclusion supported by the genetic evidence

This makes the reasoning clear and:

testable.

---

Worked Investigation

Suppose a plant species has:

R = red flowers

r = white flowers.

R is dominant.

A red plant is crossed with a white plant.

Their offspring include:

48 red plants

and:

52 white plants.

What can we infer?

---

Step 1: Determine the White Parent

White is recessive.

Therefore, the white parent must be:

rr.

---

Step 2: Consider the Red Parent

The red parent could initially be:

RR

or:

Rr.

We need evidence to decide between these possibilities.

---

Step 3: Use the Offspring

Some offspring are:

white.

White offspring must be:

rr.

The white parent contributes one r.

The red parent must therefore also contribute:

r.

Therefore, the red parent cannot be:

RR.

It must be:

Rr.

---

Step 4: Test the Prediction

The predicted cross is:

Rr × rr.

This predicts:

50% Rr → red

50% rr → white.

The observed results:

48 red : 52 white

are reasonably close to the predicted:

1 : 1 ratio.

The evidence therefore supports the proposed simple inheritance:

model.

---

Why Genetic Predictions Matter

Understanding inheritance probabilities has applications in:

• plant breeding
• animal breeding
• conservation biology
• genetic research
• medicine
• agriculture

Genetic predictions help scientists understand how alleles can move through:

populations and generations.

---

Common Mistake: Probability Means Guarantee

A probability of:

25%

does not mean exactly one of every four offspring will show the:

trait.

It means each independent offspring has the predicted chance under the assumptions of the:

model.

---

Common Mistake: Previous Children Change the Next Probability

Suppose a couple's genetic cross gives a 25% probability for a particular genotype.

They have three children without that genotype.

The probability for the fourth child does not automatically become:

higher.

Each conception is a new:

event.

---

Common Mistake: Dominant Means More Likely to Be Inherited

A dominant allele is not automatically more likely to enter a:

gamete.

A heterozygous:

Aa

individual normally has a 50% probability of passing A and a 50% probability of passing:

a.

Dominance affects expression, not allele:

segregation.

---

Common Mistake: Phenotype Always Reveals Genotype

A recessive phenotype can often identify genotype under simple complete dominance.

A dominant phenotype usually:

cannot.

For example:

A_

could mean:

AA or Aa.

Additional genetic evidence may be required.

---

Common Mistake: A Pedigree Proves an Inheritance Pattern

A small pedigree can suggest an inheritance:

pattern.

However, small families provide limited data, and chance can produce patterns that resemble other inheritance:

models.

Scientists therefore consider the amount of evidence and alternative explanations before drawing strong:

conclusions.

---

Common Mistake: Every Human Trait Is Mendelian

Many human traits are influenced by multiple genes and environmental:

factors.

Simple Punnett-square models should only be used when the underlying inheritance pattern supports those:

assumptions.

---

Check Your Understanding

1. What information is needed to predict a simple genetic cross?

2. Explain the difference between genotype and phenotype.

3. Why does each gamete normally carry only one allele for each gene?

4. Why is fertilization described as involving chance?

5. Cross Bb × Bb using a Punnett square.

6. What is the probability of BB?

7. What is the probability of Bb?

8. What is the probability of bb?

9. If B is dominant, what is the probability of the dominant phenotype?

10. What is the probability of the recessive phenotype?

11. Why does a 25% probability not mean exactly one of four offspring must show the trait?

12. If the first offspring has genotype bb, what is the probability that the next offspring from Bb × Bb will also be bb?

13. Explain why previous offspring do not change the genetic probability for the next independent conception.

14. A cross predicts 75% tall offspring. If 200 offspring are produced, approximately how many would you expect to be tall?

15. Why might the actual number differ from the expected number?

16. What genotype must an individual with a recessive phenotype have under complete dominance?

17. Why can an individual showing the dominant phenotype have two possible genotypes?

18. Two dominant-phenotype parents produce a recessive-phenotype offspring. What does this tell you about their genotypes?

19. What is a pedigree?

20. What information can a pedigree provide?

21. How might unaffected parents produce an offspring with a recessive trait?

22. Explain how offspring can provide evidence about an unknown parent genotype.

23. What is a test cross?

24. Why is a homozygous recessive individual useful in a test cross?

25. If each offspring has a 50% probability of a trait, what is the probability that two consecutive offspring both show the trait?

26. If each offspring has a 25% probability of genotype aa, what is the probability that two consecutive offspring are both aa?

27. Explain why larger samples often match predicted genetic ratios more closely than small samples.

28. What should scientists do if experimental results repeatedly disagree with a genetic prediction?

29. Give two reasons why a simple Punnett square might not accurately model a real trait.

30. Explain how genetic evidence can be used to support an inheritance prediction.

---

Key Terms

• Inheritance: Transmission of genetic information from parents to offspring.
• Allele: Alternative version of a gene.
• Genotype: Allele combination possessed by an organism.
• Phenotype: Observable characteristic of an organism.
• Gamete: Haploid reproductive cell carrying one allele for each gene.
• Fertilization: Fusion of gametes to produce a new genetic combination.
• Probability: Measure of the likelihood of an outcome.
• Genetic cross: Analysis of parental genotypes to predict offspring.
• Punnett square: Diagram used to predict possible offspring genotypes.
• Dominant allele: Allele expressed in a heterozygote under complete dominance.
• Recessive allele: Allele whose phenotype is normally expressed when two recessive copies are present.
• Carrier: Heterozygous individual carrying a recessive allele associated with a trait or condition.
• Pedigree: Diagram showing biological relationships and inheritance of a trait across generations.
• Test cross: Cross involving a homozygous recessive individual used to investigate an unknown genotype.
• Expected ratio: Proportion predicted by a genetic model.
• Observed ratio: Proportion actually found in experimental or family data.

---

Key Takeaways

• Inheritance can be predicted using knowledge of alleles, meiosis, fertilization, and probability.
• Parents pass alleles to offspring through gametes.
• Each gamete normally carries one allele for each gene.
• Fertilization combines alleles from two parents.
• Punnett squares predict possible offspring genotypes and phenotypes.
• Genetic predictions describe probabilities, not certainties.
• 1/4 = 25%, 1/2 = 50%, and 3/4 = 75%.
• For Bb × Bb, the expected genotype ratio is 1 BB : 2 Bb : 1 bb.
• Under complete dominance, Bb × Bb produces an expected 3:1 phenotype ratio.
• Each independent conception represents a new genetic event.
• Previous offspring do not change the probability for the next offspring when the parental genotypes remain the same.
• Small families may differ greatly from predicted ratios because of chance.
• Larger samples generally provide stronger evidence for expected inheritance patterns.
• A recessive phenotype often provides strong information about genotype.
• A dominant phenotype may correspond to more than one genotype.
• Offspring phenotypes can provide evidence about parental genotypes.
• Two dominant-phenotype parents can produce a recessive offspring if both carry the recessive allele.
• Pedigrees can be used to investigate inheritance across generations.
• A test cross can provide evidence about an unknown genotype.
• Genetic evidence can sometimes rule out possible genotypes.
• Good genetic reasoning connects evidence, alleles, genotypes, probabilities, and conclusions.
• Expected results and observed results do not have to be exactly identical.
• Large, consistent differences between prediction and evidence may indicate that the genetic model needs to be reconsidered.
• Not every trait follows simple Mendelian inheritance.
• Many real traits involve multiple genes, different allele relationships, or environmental influences.
• Inheritance predictions are strongest when they are supported by clear genetic evidence and appropriate probability reasoning.