What Is an Allele?

A gene is a section of DNA that contains information that can influence a particular characteristic or biological function.

Different versions of the same gene are called:

alleles.

For example, imagine a gene controlling flower colour in a plant.

One allele might contribute to:

purple flowers.

Another allele might contribute to:

white flowers.

Both are versions of the same gene, so they are:

alleles.

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Genes, DNA, and Chromosomes

To understand alleles, it helps to remember the relationship between:

DNA → genes → chromosomes

DNA is the molecule that stores genetic:

information.

A gene is a particular section of:

DNA.

Chromosomes are long DNA molecules packaged with proteins and contain many:

genes.

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Where Are Genes Located?

Genes occur at particular positions on:

chromosomes.

The specific position of a gene on a chromosome is called its:

locus.

Homologous chromosomes carry the same types of genes at corresponding:

loci.

However, they do not necessarily carry the same:

alleles.

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Why Do Genes Often Occur in Pairs?

Most human body cells are:

diploid.

Diploid means that the cells contain:

two sets of chromosomes.

Humans have:

23 pairs of chromosomes

for a total of:

46 chromosomes.

One chromosome of each homologous pair was inherited from the biological mother and the other from the biological:

father.

Therefore, for many genes, an individual has:

two copies of the gene.

One copy is located on each chromosome of a homologous:

pair.

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Homologous Chromosomes

A pair of homologous chromosomes contains the same types of genes at corresponding:

locations.

However, the alleles can be:

the same or different.

Suppose a gene has two alleles:

B

and:

b.

An individual could inherit:

B from one parent and B from the other

or:

B from one parent and b from the other

or:

b from one parent and b from the other.

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Allele Combinations

If a gene has two alleles represented by:

B and b

then an individual might have one of three combinations:

BB

Bb

bb

These combinations represent the individual's:

genotype for that gene.

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What Is a Genotype?

A genotype is the genetic makeup of an organism, or more specifically, the allele combination it possesses for a particular:

gene.

For example:

BB

is one genotype.

Bb

is another genotype.

bb

is another genotype.

The genotype can influence the organism's observable:

phenotype.

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What Is a Phenotype?

A phenotype is an observable characteristic of an:

organism.

Examples include:

• flower colour
• seed shape
• blood group
• certain coat colours
• some inherited conditions

The phenotype results from the expression of genetic information, often interacting with:

environmental factors.

A useful distinction is:

Genotype = allele combination

Phenotype = observable characteristic

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Homozygous Gene Pairs

If an individual has two identical alleles for a gene, the genotype is:

homozygous.

Examples include:

BB

and:

bb.

Both alleles are the:

same.

The word can be remembered as:

homo = same.

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Two Types of Homozygous Genotype

If we are using a simple dominant-recessive model, there are two possible homozygous:

conditions.

BB = homozygous dominant

bb = homozygous recessive

Both are homozygous because the two allele symbols are:

identical.

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Heterozygous Gene Pairs

If an individual has two different alleles for a gene, the genotype is:

heterozygous.

For example:

Bb

The two alleles are:

different.

A useful memory aid is:

hetero = different.

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Homozygous vs Heterozygous

Notice that:

Bb and bB represent the same genotype.

Usually, genetic notation writes the capital letter:

first.

Therefore, we normally write:

Bb.

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Dominant Alleles

In a simple Mendelian inheritance pattern, a dominant allele affects the phenotype when at least one copy is:

present.

Dominant alleles are usually represented using:

capital letters.

For example:

B

could represent a dominant allele.

If B is dominant, both:

BB

and:

Bb

show the dominant phenotype.

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Recessive Alleles

A recessive allele is expressed in the phenotype in a simple dominant-recessive relationship when the individual has two copies of the recessive:

allele.

Recessive alleles are usually represented using:

lowercase letters.

For example:

b

If B is dominant over b, the recessive phenotype occurs with:

bb.

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Dominant Does Not Mean More Common

A common misconception is that a dominant allele must be:

more common.

This is not true.

Dominant describes how an allele is expressed when paired with another allele.

It does not tell us how frequently the allele occurs in a:

population.

A dominant allele can be rare.

A recessive allele can be:

common.

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Dominant Does Not Mean Better

Another common misconception is that dominant means:

stronger, healthier, or better.

It does not.

Dominance simply describes the relationship between alleles in determining a:

phenotype.

A dominant allele is not automatically more useful or advantageous than a:

recessive allele.

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A Simple Flower Colour Example

Suppose a plant has a gene controlling flower colour.

Let:

P = purple flower allele

p = white flower allele

Suppose P is dominant over:

p.

Possible genotypes are:

PP

Pp

pp

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Genotype and Phenotype

Using our flower example:

The heterozygous plant is purple because:

P is dominant over p.

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Why Can Two Genotypes Produce the Same Phenotype?

Consider:

PP

and:

Pp.

These are different:

genotypes.

However, if P is dominant, both produce the:

purple phenotype.

Therefore:

Different genotypes can sometimes produce the same phenotype.

This distinction is extremely important in:

genetics.

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The Recessive Phenotype

For the white flower phenotype to appear, the plant must inherit:

two recessive alleles.

Therefore:

pp → white

A plant with:

Pp

carries the recessive p allele but does not show the recessive phenotype under this simple inheritance:

model.

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What Is a Carrier?

For some recessively inherited traits or conditions, a heterozygous individual may be described as a:

carrier.

A carrier possesses a recessive allele but does not show the recessive phenotype.

For example:

Aa

The individual has the recessive:

a allele

but also has the dominant:

A allele.

The recessive allele can still potentially be passed to:

offspring.

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Alleles Are Inherited

An offspring receives genetic information from its biological:

parents.

For a typical autosomal gene, one allele is inherited from one parent and the other allele from the:

other parent.

Suppose one parent contributes:

B

and the other contributes:

b.

The offspring's genotype is:

Bb.

This is a:

heterozygous genotype.

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Meiosis Separates Allele Pairs

Recall what happens during:

meiosis.

Homologous chromosomes are separated.

Because the two alleles of many genes are located on homologous chromosomes, allele pairs are also:

separated.

Suppose an individual has genotype:

Bb.

Its gametes do not normally receive both B and b for that gene.

Instead, a gamete receives:

B or b.

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Gametes Carry One Allele

Body cells are usually:

diploid.

Gametes are:

haploid.

Therefore, a body cell may contain:

two alleles

for a particular gene.

A gamete normally contains:

one allele

for that gene.

For a heterozygous individual:

Bb

the gametes can carry:

B

or:

b.

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Fertilization Restores the Pair

During fertilization, two haploid gametes:

fuse.

One allele comes from one gamete.

The second allele comes from the other:

gamete.

The offspring therefore receives a new pair of:

alleles.

For example:

B + b → Bb

The diploid condition is:

restored.

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From Parents to Offspring

Imagine two heterozygous parents:

Bb × Bb

Each parent can produce gametes containing:

B or b.

When gametes combine, possible offspring genotypes include:

BB

Bb

and:

bb.

This is the foundation of:

Punnett squares,

which are used to predict possible genetic combinations in:

offspring.

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Genetic Notation

Scientists use letters to represent:

alleles.

For simple dominant-recessive examples, we generally use:

capital letter = dominant allele

lowercase letter = recessive allele

Importantly, we normally use the same:

letter.

For example:

T and t

rather than:

T and s.

This makes it clear that they are different alleles of the same:

gene.

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Example: Plant Height

Suppose:

T = allele for tall plants

t = allele for short plants

and T is dominant.

Then:

TT = homozygous dominant

Tt = heterozygous

tt = homozygous recessive

Under this simplified model:

TT → tall

Tt → tall

tt → short

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Worked Example 1

A plant has the genotype:

TT

Are the alleles the same or different?

They are:

the same.

Therefore, the genotype is:

homozygous.

Because both alleles are dominant:

TT = homozygous dominant.

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Worked Example 2

A plant has the genotype:

Tt.

The alleles are:

different.

Therefore, the plant is:

heterozygous.

If T is dominant, the phenotype will show the characteristic associated with:

T.

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Worked Example 3

A plant has genotype:

tt.

Both alleles are:

recessive.

Therefore:

tt = homozygous recessive.

The recessive phenotype will be:

expressed.

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Worked Example 4

Suppose:

R = round seeds

r = wrinkled seeds

and R is dominant.

A plant has round seeds.

Can we determine its genotype from this information alone?

No.

The genotype could be:

RR

or:

Rr.

Both produce the dominant:

phenotype.

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Phenotype Does Not Always Reveal Genotype

This is an important genetic principle.

If an organism shows a dominant phenotype, its genotype might be:

homozygous dominant

or:

heterozygous.

For example:

AA → dominant phenotype

Aa → dominant phenotype

Looking at the phenotype alone may not tell us which genotype is:

present.

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A Recessive Phenotype Gives More Information

If a simple dominant-recessive trait shows the recessive phenotype, the genotype must normally be:

homozygous recessive.

For example:

aa.

Why?

Because if the dominant A allele were present, the dominant phenotype would be:

expressed.

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Not All Traits Follow Simple Dominance

The dominant-recessive model is useful for learning basic:

genetics.

However, real inheritance can be more:

complex.

Some genes show:

• incomplete dominance
• codominance
• multiple alleles
• sex-linked inheritance

Many characteristics are also influenced by:

multiple genes.

Therefore, the simple capital-letter/lowercase-letter model does not explain every inherited:

trait.

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Incomplete Dominance

In incomplete dominance, neither allele completely masks the:

other.

The heterozygous phenotype may be intermediate between the two homozygous:

phenotypes.

For example, in some plants:

RR → red flowers

WW → white flowers

RW → pink flowers

Here, the heterozygous phenotype is:

intermediate.

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Codominance

In codominance, both alleles are expressed in a heterozygous:

individual.

Neither allele completely masks the:

other.

The human ABO blood group system includes an important example.

The alleles:

Iᴬ and Iᴮ

are codominant.

An individual with genotype:

IᴬIᴮ

has blood group:

AB.

Both A and B characteristics are:

expressed.

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Multiple Alleles

A gene can have more than two possible alleles in a:

population.

The ABO blood group gene has three commonly discussed alleles:

Iᴬ

Iᴮ

and:

i.

However, an individual diploid person normally possesses only:

two alleles

for this gene because they have two homologous copies of the relevant:

chromosome.

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Two Alleles per Individual Does Not Mean Only Two Alleles Exist

This distinction is important.

An individual may have:

two alleles for a gene.

But the population may contain:

many different alleles of that gene.

Imagine a gene with five possible alleles in a population.

One individual still usually inherits only:

two of them.

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Alleles Influence Traits Through Proteins

How can different alleles affect:

traits?

Genes contain information used to produce functional products, often:

proteins.

Different alleles may contain slightly different DNA:

sequences.

These differences can affect:

• whether a protein is produced
• how much protein is produced
• the structure of the protein
• how effectively the protein functions

These molecular differences can influence the organism's:

phenotype.

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From Allele to Trait

A simplified pathway is:

DNA

↓

Gene

↓

Allele

↓

Protein or functional product

↓

Cell function

↓

Phenotype

Different alleles can therefore contribute to different:

phenotypes.

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Alleles and Variation

Individuals within a population may possess different:

alleles.

This creates:

genetic variation.

For example, some individuals may have:

AA

others:

Aa

and others:

aa.

Different allele combinations can contribute to different characteristics within the:

population.

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Alleles Come from Mutation

Ultimately, new alleles arise through changes in DNA called:

mutations.

Once a new allele exists, it can potentially be passed from one generation to the:

next.

Sexual reproduction then reshuffles alleles into different combinations through:

meiosis and fertilization.

Therefore:

Mutation creates new alleles.

Sexual reproduction creates new combinations of alleles.

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Alleles and Genetic Variation

Consider a population in which every individual had exactly the same:

alleles.

There would be relatively little genetic variation for those:

genes.

If multiple alleles are present, individuals can inherit different:

combinations.

This increases genetic variation within the:

population.

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Real-World Example: ABO Blood Group

Human ABO blood group provides a useful example of:

alleles.

The system involves three major alleles:

Iᴬ

Iᴮ

and:

i.

Possible genotypes include:

IᴬIᴬ

Iᴬi

IᴮIᴮ

Iᴮi

IᴬIᴮ

ii

These combinations produce different:

blood groups.

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ABO Genotypes and Phenotypes

This example shows that relationships between alleles can be more complex than simple:

dominant versus recessive.

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Why Allele Pairs Matter

Allele pairs help determine an organism's:

genotype.

The interaction between those alleles can influence:

phenotype.

For a simple gene:

AA

may produce one phenotype.

Aa

may produce the same phenotype if A is dominant.

aa

may produce a different phenotype.

Understanding allele pairs therefore helps us predict patterns of:

inheritance.

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Common Mistake: A Gene and an Allele Are the Same Thing

A gene is a section of DNA associated with a particular biological function or:

characteristic.

An allele is a particular version of that:

gene.

Think of:

Gene = the general instruction

Allele = a particular version of the instruction

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Common Mistake: Dominant Means Most Common

Dominance does not describe how frequently an allele occurs in a:

population.

A dominant allele can be:

rare.

A recessive allele can be:

common.

Dominance describes how alleles interact in a:

heterozygous genotype.

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Common Mistake: Dominant Means Stronger

Alleles do not physically fight one another.

A dominant allele does not overpower a recessive allele because it is:

stronger.

Dominance describes the phenotype produced by a particular allele:

combination.

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Common Mistake: Heterozygous Means Recessive

Heterozygous simply means:

two different alleles.

For example:

Aa.

Under complete dominance, the heterozygous individual normally shows the:

dominant phenotype.

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Common Mistake: Homozygous Always Means Dominant

Homozygous means the two alleles are:

the same.

Therefore:

AA = homozygous dominant

but:

aa = homozygous recessive.

Both are:

homozygous.

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Common Mistake: Every Trait Has Only Two Alleles

Introductory genetics often uses two alleles because it makes inheritance patterns easier to:

understand.

However, many genes have more than two alleles within a:

population.

An individual diploid organism still generally carries only two alleles for a particular autosomal gene.

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Check Your Understanding

1. Define an allele.

2. Explain the difference between a gene and an allele.

3. Where are genes located?

4. What is a gene locus?

5. Why do diploid organisms usually have two alleles for a particular autosomal gene?

6. What are homologous chromosomes?

7. Define genotype.

8. Define phenotype.

9. What does homozygous mean?

10. What does heterozygous mean?

11. Classify AA as homozygous or heterozygous.

12. Classify Aa as homozygous or heterozygous.

13. Classify aa as homozygous or heterozygous.

14. What does homozygous dominant mean?

15. What does homozygous recessive mean?

16. Explain what a dominant allele is.

17. Explain what a recessive allele is.

18. Why does dominant not mean better or more common?

19. If B is dominant over b, which phenotype would genotype Bb show?

20. If B is dominant over b, which genotype is required for the recessive phenotype?

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

22. Why can a dominant phenotype sometimes have two possible genotypes?

23. What happens to allele pairs during meiosis?

24. How many alleles for a particular gene does a typical haploid gamete carry?

25. An organism has genotype Tt. Which alleles could its gametes receive?

26. Explain how fertilization restores allele pairs.

27. How can different alleles influence phenotype?

28. Explain how alleles contribute to genetic variation.

29. Where do new alleles ultimately come from?

30. Explain why understanding alleles is important for predicting inheritance.

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Key Terms

• Gene: Section of DNA containing information that influences a biological characteristic or function.
• Allele: Alternative version of a gene.
• Locus: Specific location of a gene on a chromosome.
• Homologous chromosomes: Chromosome pair carrying the same types of genes at corresponding loci.
• Diploid: Having two sets of chromosomes.
• Haploid: Having one set of chromosomes.
• Genotype: Allele combination or genetic makeup of an organism.
• Phenotype: Observable characteristics of an organism.
• Homozygous: Having two identical alleles for a gene.
• Heterozygous: Having two different alleles for a gene.
• Homozygous dominant: Having two dominant alleles.
• Homozygous recessive: Having two recessive alleles.
• Dominant allele: Allele expressed in the phenotype when present in a heterozygous genotype under complete dominance.
• Recessive allele: Allele whose phenotype under complete dominance is normally expressed when two recessive copies are present.
• Carrier: Heterozygous individual carrying a recessive allele associated with a particular trait or condition.
• Incomplete dominance: Inheritance pattern in which the heterozygous phenotype is intermediate.
• Codominance: Inheritance pattern in which both alleles are expressed in a heterozygote.
• Mutation: Change in DNA that can create a new allele.
• Genetic variation: Genetic differences among individuals in a population.

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Key Takeaways

• A gene is a section of DNA.
• An allele is a version of a gene.
• Genes occur at specific locations called loci on chromosomes.
• Diploid organisms generally possess two copies of each autosomal gene because chromosomes occur in homologous pairs.
• One chromosome of each homologous pair comes from each biological parent.
• The two alleles can be the same or different.
• Homozygous means two identical alleles.
• Heterozygous means two different alleles.
• AA is homozygous dominant.
• Aa is heterozygous.
• aa is homozygous recessive.
• An organism's allele combination is its genotype.
• Its observable characteristics form its phenotype.
• Under complete dominance, a dominant allele can affect phenotype when one copy is present.
• A recessive phenotype usually requires two recessive alleles.
• Different genotypes can sometimes produce the same phenotype.
• Dominant does not mean stronger, better, or more common.
• During meiosis, allele pairs separate as homologous chromosomes separate.
• A haploid gamete normally carries one allele for each gene.
• Fertilization combines alleles from two gametes and restores allele pairs.
• Different alleles can produce differences in proteins or other functional products and therefore influence phenotype.
• New alleles ultimately arise through mutation.
• Meiosis and fertilization rearrange existing alleles into new combinations.
• Alleles contribute to genetic variation within populations.
• Not every gene follows simple dominant-recessive inheritance.
• Incomplete dominance, codominance, and multiple alleles are important examples of more complex inheritance.
• Understanding alleles and gene pairs provides the foundation for using Punnett squares and predicting patterns of inheritance.