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.

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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.

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A Simple Punnett Square

For:

Aa × Aa

we can organize the possibilities like this:

Possible genotypes are therefore:

AA

Aa

Aa

aa.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Complete Dominance vs Other Patterns

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

genetics.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Common Mistake: Genotype and Phenotype Are the Same

They are not.

Genotype = genetic combination

Phenotype = observable characteristic

For example:

Bb = genotype

Black fur = phenotype.

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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.

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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.

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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.

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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.

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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.

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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.

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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.