Alleles Do Not Always Show Simple Dominance

In many introductory genetics problems, one allele is:

dominant

and another is:

recessive.

This is called:

complete dominance.

However, alleles do not always interact this way.

Two important alternative inheritance patterns are:

incomplete dominance

and:

codominance.

These patterns affect the phenotype of:

heterozygous individuals.

Understanding what happens in the heterozygote is the key to distinguishing the three inheritance patterns.

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Review: Complete Dominance

In complete dominance, one allele completely determines the phenotype of a heterozygous individual.

Suppose:

B = black fur

b = brown fur

and B is dominant.

Then:

BB → black

Bb → black

bb → brown

Notice that:

BB and Bb have the same phenotype.

The heterozygous individual shows the:

dominant phenotype.

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Complete Dominance

A simple way to remember complete dominance is:

One allele masks the phenotype associated with the other allele in the heterozygote.

For example:

AA → Trait A

Aa → Trait A

aa → Trait B

Therefore:

Aa resembles AA.

This is the pattern we have used in many basic Punnett-square problems.

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What Is Incomplete Dominance?

In incomplete dominance, neither allele completely dominates the:

other.

The heterozygous individual has a phenotype that is different from either homozygous phenotype and is often:

intermediate.

For example:

Red + White → Pink

The alleles themselves do not blend or disappear.

Instead, the heterozygous genotype produces an intermediate:

phenotype.

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Example: Snapdragon Flowers

Flower colour in snapdragons is a classic example used to illustrate:

incomplete dominance.

We can represent the alleles as:

Cᴿ = red allele

Cᵂ = white allele

Then:

CᴿCᴿ → red flowers

CᴿCᵂ → pink flowers

CᵂCᵂ → white flowers

The heterozygous plant is:

pink.

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Why Use Different Genetic Notation?

With complete dominance, we often use:

A and a.

However, this notation can be misleading for incomplete dominance because neither allele is completely:

dominant.

Instead, we can use a common gene symbol with different superscripts:

Cᴿ

and:

Cᵂ.

This makes it clear that both are alleles of the same gene without suggesting that one is:

dominant.

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Incomplete Does Not Mean the Alleles Blend

This is an important distinction.

Suppose:

CᴿCᵂ → pink.

The red and white alleles do not physically blend together to create a new pink:

allele.

The plant still possesses:

Cᴿ and Cᵂ.

During meiosis, these alleles separate normally.

The plant can produce gametes carrying:

Cᴿ

or:

Cᵂ.

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Crossing Two Pink Flowers

Suppose two heterozygous pink flowers are crossed:

CᴿCᵂ × CᴿCᵂ

Each parent produces two possible gametes:

Cᴿ

or:

Cᵂ.

We can predict the offspring using a Punnett:

square.

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Punnett Square: Pink × Pink

The possible genotypes are:

CᴿCᴿ

CᴿCᵂ

CᴿCᵂ

CᵂCᵂ.

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Predicting the Phenotypes

Remember:

CᴿCᴿ → red

CᴿCᵂ → pink

CᵂCᵂ → white

Therefore:

25% red

50% pink

25% white.

The phenotype ratio is:

1 red : 2 pink : 1 white.

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An Important Difference

In complete dominance, a heterozygous cross:

Aa × Aa

usually gives a phenotype ratio of:

3 : 1.

In incomplete dominance, a heterozygous cross such as:

CᴿCᵂ × CᴿCᵂ

gives a phenotype ratio of:

1 : 2 : 1.

Why?

Because all three genotypes produce distinguishable:

phenotypes.

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Genotype and Phenotype Ratios

For incomplete dominance:

CᴿCᴿ : CᴿCᵂ : CᵂCᵂ

gives a genotype ratio of:

1 : 2 : 1.

The phenotype ratio is also:

1 : 2 : 1.

This happens because each genotype produces a different:

phenotype.

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Crossing Red and White Flowers

Now consider:

CᴿCᴿ × CᵂCᵂ.

The red parent produces only:

Cᴿ gametes.

The white parent produces only:

Cᵂ gametes.

Every offspring receives:

CᴿCᵂ.

Therefore:

100% pink offspring.

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Crossing Pink and White Flowers

Consider:

CᴿCᵂ × CᵂCᵂ.

Therefore:

50% CᴿCᵂ → pink

50% CᵂCᵂ → white.

The phenotype ratio is:

1 pink : 1 white.

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What Is Codominance?

In codominance, both alleles are fully expressed in the phenotype of a:

heterozygous individual.

Neither allele masks the other.

Neither produces an intermediate blend.

Instead:

both characteristics appear.

This is the key idea behind:

codominance.

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Example: Roan Cattle

Coat colour in some cattle provides a commonly used example of:

codominance.

Suppose:

Cᴿ = red hair allele

Cᵂ = white hair allele

Then:

CᴿCᴿ → red

CᵂCᵂ → white

CᴿCᵂ → roan

A roan animal has both:

red hairs and white hairs.

The hairs do not become an intermediate pink colour.

Both characteristics are:

expressed.

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Codominance Is Not Blending

Suppose a red-haired animal and a white-haired animal produce a roan:

offspring.

The offspring does not have hairs that are halfway between red and:

white.

Instead, it has:

red hairs and white hairs.

Both alleles contribute visibly to the:

phenotype.

This is codominance.

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Codominant Genetic Notation

Because neither allele is recessive, capital and lowercase notation is usually not:

appropriate.

Instead, alleles can be written using superscripts.

For example:

Cᴿ

and:

Cᵂ.

Then:

CᴿCᴿ → red

CᴿCᵂ → roan

CᵂCᵂ → white.

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Crossing Two Roan Cattle

Suppose:

CᴿCᵂ × CᴿCᵂ.

Each parent produces:

Cᴿ

or:

Cᵂ.

The Punnett square is:

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Predicting the Offspring

The genotype probabilities are:

25% CᴿCᴿ

50% CᴿCᵂ

25% CᵂCᵂ.

The phenotype probabilities are:

25% red

50% roan

25% white.

Again, the phenotype ratio is:

1 : 2 : 1.

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Incomplete Dominance vs Codominance

These inheritance patterns can look similar in Punnett squares because both may produce:

1 : 2 : 1 phenotype ratios.

The difference is what happens in the:

heterozygote.

In incomplete dominance:

the heterozygote has an intermediate phenotype.

In codominance:

both phenotypes are expressed together.

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A Useful Visual Comparison

Imagine two alleles produce:

red

and:

white.

Complete dominance

RR → red

RW → red

WW → white

The heterozygote resembles one homozygote.

Incomplete dominance

RR → red

RW → pink

WW → white

The heterozygote is intermediate.

Codominance

RR → red

RW → red AND white

WW → white

The heterozygote expresses both.

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The Three Patterns Compared

This is the most important comparison to:

remember.

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Complete Dominance Example

Suppose:

B = black

b = white

with complete dominance.

Then:

BB → black

Bb → black

bb → white.

The heterozygous phenotype is:

black.

It matches one of the homozygous phenotypes.

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Incomplete Dominance Example

Suppose:

Cᴿ = red

Cᵂ = white.

Then:

CᴿCᴿ → red

CᴿCᵂ → pink

CᵂCᵂ → white.

The heterozygous phenotype is:

intermediate.

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Codominance Example

Suppose:

Cᴿ = red hair

Cᵂ = white hair.

Then:

CᴿCᴿ → red

CᴿCᵂ → red AND white hairs

CᵂCᵂ → white.

The heterozygous phenotype displays:

both characteristics.

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Human ABO Blood Groups

The human ABO blood group system provides an important real-world example of:

codominance.

There are three commonly discussed alleles:

Iᴬ

Iᴮ

and:

i.

Iᴬ and Iᴮ are:

codominant.

Both Iᴬ and Iᴮ are dominant over:

i.

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ABO Genotypes and Phenotypes

The genotype:

IᴬIᴮ

produces blood group:

AB.

Both A and B antigens are expressed on red blood:

cells.

This is an example of:

codominance.

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Predicting ABO Inheritance

Suppose one parent has genotype:

Iᴬi

and another has:

Iᴮi.

The first parent can produce gametes containing:

Iᴬ or i.

The second can produce:

Iᴮ or i.

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ABO Punnett Square

Possible blood groups are:

AB

B

A

O.

Each has a probability of:

25%.

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What Does the ABO Example Teach Us?

The ABO system demonstrates that a single gene can involve more than:

two alleles in a population.

It also demonstrates that allele relationships can include both:

codominance

and:

dominance.

Iᴬ and Iᴮ are codominant with each other.

Both are dominant over:

i.

Genetic inheritance can therefore be more complex than a simple dominant-recessive:

model.

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Worked Example 1: Identify the Pattern

A red-flowered plant is crossed with a white-flowered plant.

All offspring are:

pink.

What inheritance pattern is suggested?

The heterozygous offspring have an intermediate phenotype.

Therefore, the pattern is:

incomplete dominance.

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Worked Example 2: Identify the Pattern

A black-feathered bird is crossed with a white-feathered bird.

The heterozygous offspring have both black and white:

feathers.

What inheritance pattern is suggested?

Both characteristics are expressed.

Therefore:

codominance.

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Worked Example 3: Identify the Pattern

A black animal crossed with a white animal produces heterozygous offspring that are all:

black.

The heterozygote resembles one homozygous parent.

Therefore:

complete dominance.

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Worked Example 4: Incomplete Dominance

Suppose flower colour shows incomplete dominance:

CᴿCᴿ = red

CᴿCᵂ = pink

CᵂCᵂ = white.

Cross:

CᴿCᵂ × CᵂCᵂ.

The offspring are:

50% CᴿCᵂ

50% CᵂCᵂ.

Therefore:

50% pink

50% white.

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Worked Example 5: Codominance

Suppose feather colour shows codominance:

FᴮFᴮ = black

FᵂFᵂ = white

FᴮFᵂ = black and white.

Cross:

FᴮFᵂ × FᴮFᵂ.

Expected offspring:

25% black

50% black and white

25% white.

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How to Solve an Incomplete Dominance Problem

Use this process:

Step 1: Identify the two alleles.

Step 2: Determine the phenotype of each homozygous genotype.

Step 3: Determine the intermediate heterozygous phenotype.

Step 4: Write the parent genotypes.

Step 5: Identify possible gametes.

Step 6: Construct the Punnett square.

Step 7: Calculate genotype probabilities.

Step 8: Convert the genotypes into phenotypes.

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How to Solve a Codominance Problem

The process is almost the same.

Step 1: Identify the alleles.

Step 2: Determine the phenotype associated with each allele.

Step 3: Remember that the heterozygote expresses both.

Step 4: Write the parent genotypes.

Step 5: Identify the gametes.

Step 6: Construct the Punnett square.

Step 7: Calculate genotype probabilities.

Step 8: Determine phenotype probabilities.

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How Can You Tell Which Pattern Is Present?

Focus on the:

heterozygote.

Ask:

Does it look like one homozygous phenotype?

Then it may show:

complete dominance.

Is it intermediate between the two homozygous phenotypes?

Then it may show:

incomplete dominance.

Does it clearly express both allele-associated characteristics?

Then it may show:

codominance.

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Why the Heterozygote Is So Important

Suppose we observe only:

AA

and:

BB.

We know the two homozygous phenotypes, but we do not yet know how the alleles interact.

We need to observe:

AB.

The phenotype of the heterozygote provides evidence about the inheritance:

pattern.

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Predicting from Offspring Evidence

Suppose red and white parents produce offspring.

If the offspring are:

red

this could indicate complete dominance of the red-associated allele.

If they are:

pink

this could indicate incomplete dominance.

If they show:

red and white simultaneously

this could indicate codominance.

The offspring phenotype therefore provides evidence about how the alleles:

interact.

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Ratios Can Provide Evidence

Suppose two heterozygous organisms are crossed.

The offspring appear in three phenotypes in approximately:

1 : 2 : 1 proportions.

This pattern may suggest:

incomplete dominance or codominance.

However, the ratio alone cannot tell us which one.

We must examine what the heterozygous phenotype actually:

looks like.

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Why a 1:2:1 Ratio Appears

Consider:

A¹A² × A¹A².

The genotypes are:

A¹A¹

A¹A²

A¹A²

A²A².

Therefore, the genotype ratio is:

1 : 2 : 1.

If all three genotypes have distinguishable phenotypes, the phenotype ratio is also:

1 : 2 : 1.

This occurs in many simple examples of incomplete dominance and:

codominance.

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Complete Dominance Produces a Different Phenotype Ratio

For:

Aa × Aa

the genotype ratio is still:

1 AA : 2 Aa : 1 aa.

But AA and Aa have the same:

phenotype.

Therefore, the phenotype ratio becomes:

3 dominant : 1 recessive.

This shows how allele interactions change phenotype:

predictions.

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Genotype Ratios Can Stay the Same

This is an important insight.

A heterozygous cross can produce the same:

1 : 2 : 1 genotype ratio

under:

• complete dominance
• incomplete dominance
• codominance

What changes is the relationship between genotype and:

phenotype.

Therefore, inheritance pattern affects the:

phenotype ratio.

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Common Mistake: Incomplete Dominance Means the Alleles Mix

The alleles themselves do not:

blend.

A pink flower with genotype:

CᴿCᵂ

still has two distinct alleles:

Cᴿ and Cᵂ.

It can pass either allele to its:

offspring.

This is why red and white phenotypes can reappear in later generations.

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Common Mistake: Codominance Produces an Intermediate

Codominance does not normally produce a blended intermediate:

phenotype.

Instead:

both allele-associated characteristics are expressed.

For example:

Red + white hairs → red AND white hairs

not:

Red + white hairs → pink hairs.

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Common Mistake: Incomplete Dominance Means One Allele Is Recessive

In incomplete dominance, neither allele completely masks the:

other.

It is therefore usually inappropriate to describe one allele as simply dominant and the other as:

recessive.

The heterozygous phenotype is distinct from both homozygous phenotypes.

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Common Mistake: Codominant Means Both Alleles Are Dominant Over Each Other

Codominance means both alleles are:

expressed in the heterozygote.

It does not mean that the alleles somehow defeat or overpower each:

other.

Dominance terminology describes phenotype expression, not physical competition between:

alleles.

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Common Mistake: Every Trait Is Complete Dominance

Simple dominant-recessive inheritance is useful for introducing:

genetics.

But real inheritance includes many other:

patterns.

These include:

• incomplete dominance
• codominance
• multiple alleles
• polygenic inheritance
• sex-linked inheritance
• gene interactions

Inheritance is therefore much richer than:

dominant versus recessive.

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Common Mistake: Phenotype Ratios Are Guaranteed

A cross predicting:

25% red

50% pink

25% white

does not guarantee that four offspring will include exactly:

one red, two pink, and one white.

These values represent:

probabilities.

Actual offspring numbers can differ because fertilization involves:

chance.

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Comparing the Three Patterns

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Real-World Importance

Understanding different allele relationships helps scientists explain why inheritance does not always follow simple:

dominant-recessive patterns.

These concepts are important in:

• medicine
• blood typing
• plant breeding
• animal breeding
• genetics research
• conservation
• agriculture

They also demonstrate that phenotype depends not only on which alleles are present, but on:

how those alleles interact.

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Check Your Understanding

1. Define complete dominance.

2. Define incomplete dominance.

3. Define codominance.

4. What happens to the heterozygous phenotype in complete dominance?

5. What happens to the heterozygous phenotype in incomplete dominance?

6. What happens to the heterozygous phenotype in codominance?

7. Why is the heterozygote useful for identifying an inheritance pattern?

8. A red flower crossed with a white flower produces pink offspring. Which inheritance pattern is suggested?

9. A red-haired animal crossed with a white-haired animal produces offspring with both red and white hairs. Which pattern is suggested?

10. Explain the difference between blending and incomplete dominance.

11. Why do alleles remain distinct during incomplete dominance?

12. If CᴿCᴿ is red, CᴿCᵂ is pink, and CᵂCᵂ is white, what pattern is shown?

13. Cross CᴿCᵂ × CᴿCᵂ.

14. What percentage of the offspring are expected to be red?

15. What percentage are expected to be pink?

16. What percentage are expected to be white?

17. What is the phenotype ratio?

18. Cross CᴿCᴿ × CᵂCᵂ. What phenotype is expected in all offspring?

19. Cross CᴿCᵂ × CᵂCᵂ. What phenotype ratio is expected?

20. Explain why codominance does not produce an intermediate phenotype.

21. Give an example of codominance.

22. Explain why blood group AB demonstrates codominance.

23. What are the three commonly discussed alleles of the ABO blood group gene?

24. What blood group results from IᴬIᴮ?

25. Cross Iᴬi × Iᴮi. What four blood groups are possible?

26. Why can incomplete dominance and codominance both produce a 1:2:1 phenotype ratio?

27. Why does complete dominance usually produce a 3:1 phenotype ratio from a heterozygous cross?

28. Can a 1:2:1 ratio alone distinguish incomplete dominance from codominance? Explain.

29. How would you use offspring phenotypes to distinguish among complete dominance, incomplete dominance, and codominance?

30. Explain why understanding allele interactions improves our ability to predict inheritance.

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Key Terms

• Complete dominance: Inheritance pattern in which the heterozygote has the same phenotype as the dominant homozygote.
• Incomplete dominance: Inheritance pattern in which the heterozygote has a phenotype different from either homozygote and often intermediate between them.
• Codominance: Inheritance pattern in which both alleles are expressed in the heterozygous phenotype.
• Allele: Alternative version of a gene.
• Homozygous: Having two identical alleles for a gene.
• Heterozygous: Having two different alleles for a gene.
• Genotype: Allele combination possessed by an organism.
• Phenotype: Observable characteristic of an organism.
• Punnett square: Diagram used to predict possible offspring genotypes.
• Probability: Measure of the likelihood of an outcome.
• Phenotype ratio: Relative proportion of different phenotypes expected among offspring.
• Genotype ratio: Relative proportion of different genotypes expected among offspring.
• Multiple alleles: More than two forms of a gene existing within a population.
• ABO blood group: Human blood-group system involving the Iᴬ, Iᴮ, and i alleles.

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Key Takeaways

• Complete dominance, incomplete dominance, and codominance describe different relationships between alleles.
• The heterozygous phenotype is the key to distinguishing the three patterns.
• In complete dominance, the heterozygote resembles the dominant homozygote.
• In incomplete dominance, the heterozygote has a distinct phenotype that is often intermediate.
• In codominance, both allele-associated characteristics are expressed.
• Incomplete dominance does not mean that the alleles themselves blend together.
• Codominance does not produce an intermediate or blended phenotype.
• Genetic notation should reflect the inheritance pattern being studied.
• Superscripts are useful when neither allele is simply dominant or recessive.
• A heterozygous cross often produces a 1:2:1 genotype ratio.
• Under complete dominance, that genotype ratio typically produces a 3:1 phenotype ratio.
• Under incomplete dominance, the same cross can produce a 1:2:1 phenotype ratio.
• Under codominance, the same cross can also produce a 1:2:1 phenotype ratio.
• Therefore, genotype ratios alone do not necessarily identify the inheritance pattern.
• The phenotype of the heterozygote provides essential evidence.
• Red, pink, and white snapdragon flowers are a classic example used to explain incomplete dominance.
• Roan cattle provide a common example used to explain codominance.
• Human AB blood group demonstrates codominance between Iᴬ and Iᴮ.
• The ABO blood group system also demonstrates multiple alleles.
• Iᴬ and Iᴮ are codominant with each other, while both are dominant over i.
• Punnett squares can predict inheritance under incomplete dominance and codominance just as they can under complete dominance.
• The meaning assigned to each genotype determines the phenotype prediction.
• Predicted ratios are probabilities rather than guaranteed numbers of offspring.
• Different allele interactions help explain the diversity of inheritance patterns found in living organisms.
• Understanding these patterns prepares us to investigate multiple alleles, blood groups, pedigrees, and more complex genetic inheritance.