AP Biology

## Learning Outcomes

- Distinguish incomplete dominance from codominance using phenotype descriptions and genotype notation.
- Predict offspring genotypes and phenotypes for monohybrid crosses involving non-Mendelian dominance patterns.
- Explain why incomplete dominance and codominance still follow allele segregation during meiosis.
- Interpret phenotypic ratios such as 1:2:1 and connect them to genotype ratios.
- Use AP-style evidence and reasoning to justify inheritance predictions.
- Avoid common errors involving “blended” traits, dominant alleles, and unfamiliar allele symbols.

## AP Biology Syllabus

In AP Biology Unit 5, heredity is tested through both content knowledge and data analysis. Incomplete dominance and codominance are part of non-Mendelian genetics because the heterozygous phenotype is not the same as the phenotype of either homozygote in the simple complete-dominance pattern. You should be able to apply meiosis, allele segregation, Punnett squares, probability, and evidence-based reasoning to unfamiliar organisms or traits described in a question.

- Explain how alleles segregate during meiosis and recombine during fertilization.
- Identify how genotype affects phenotype when neither allele is completely dominant.
- Predict offspring ratios in crosses involving incomplete dominance or codominance.
- Interpret symbols such as $$C^R$$, $$C^W$$, $$I^A$$, and $$I^B$$ when given in a question.
- Compare complete dominance, incomplete dominance, and codominance.
- Use data from offspring counts to support or reject a proposed inheritance pattern.
- Justify genetic predictions using evidence from crosses, not memorized examples alone.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. In a flower species, red crossed with white produces all pink offspring. What type of dominance pattern is most likely?
2. What phenotypic ratio is expected when two heterozygous pink flowers are crossed in an incomplete dominance system?
3. How is codominance different from incomplete dominance?
4. In human ABO blood groups, why does a person with genotype $$I^AI^B$$ have type AB blood?
5. Why do incomplete dominance and codominance not violate Mendel’s law of segregation?

## Introduction

Mendel’s classic pea plant experiments introduced the idea that alleles segregate during gamete formation and that one allele can mask another in a heterozygote. For example, if purple flower color is completely dominant over white, a heterozygous plant has purple flowers. However, not all traits fit that simple complete-dominance pattern. In incomplete dominance and codominance, the heterozygote has a distinct phenotype that reveals something about both alleles.

These patterns are called non-Mendelian because their phenotypic ratios differ from the classic 3:1 ratio often seen in a monohybrid cross with complete dominance. They are not exceptions to meiosis. Alleles still separate into gametes during meiosis, and offspring still inherit one allele from each parent. The difference is how the alleles affect the phenotype when they occur together in a heterozygote.

> **Key Term: allele**
> A version of a gene. A diploid organism usually has two alleles for a gene, one inherited from each parent.
>
> **Key Term: genotype**
> The allele combination an organism has for a gene or set of genes, such as $$C^RC^W$$.
>
> **Key Term: phenotype**
> The observable trait or measurable characteristic produced by the genotype and, in many cases, environmental effects.

### How non-Mendelian dominance changes phenotypic ratios

In complete dominance, one allele masks the effect of another allele in the heterozygote. If $$A$$ is completely dominant over $$a$$, then $$AA$$ and $$Aa$$ have the same phenotype, while $$aa$$ has a different phenotype. A cross between two heterozygotes gives a genotype ratio of 1 $$AA$$ : 2 $$Aa$$ : 1 $$aa$$, but the phenotype ratio is 3 dominant : 1 recessive.

Incomplete dominance and codominance change that outcome because the heterozygote has its own phenotype. Since each genotype produces a different phenotype, the phenotypic ratio in a heterozygote-by-heterozygote cross is often the same as the genotype ratio: 1:2:1.

> **Key Term: incomplete dominance**
> A pattern of inheritance in which the heterozygous phenotype is intermediate between the two homozygous phenotypes.

A common example is flower color in snapdragons or similar classroom genetics examples. Suppose the allele $$C^R$$ contributes to red pigment and the allele $$C^W$$ contributes to white coloration.

- $$C^RC^R$$ plants have red flowers.
- $$C^WC^W$$ plants have white flowers.
- $$C^RC^W$$ plants have pink flowers.

The pink heterozygote is not evidence that the alleles physically blend or permanently change. The alleles remain separate, and they will segregate into gametes. A pink plant can still pass on either $$C^R$$ or $$C^W$$.

> **Exam Warning:**
> Do not describe incomplete dominance as “blending inheritance” unless you clearly mean only that the heterozygous phenotype appears intermediate. The alleles themselves do not blend. They remain distinct and segregate during meiosis.

Codominance also gives the heterozygote a phenotype that differs from either homozygote, but it does so in a different way.

> **Key Term: codominance**
> A pattern of inheritance in which both alleles in a heterozygote are fully and separately expressed in the phenotype.

In codominance, the heterozygote does not look intermediate. Instead, both allele products are detectable. A classic example is the human ABO blood group. The $$I^A$$ allele produces A antigen on red blood cells, while the $$I^B$$ allele produces B antigen. A person with genotype $$I^AI^B$$ has type AB blood because both A and B antigens are present.

Another common example is roan coat color in cattle, where red hairs and white hairs are both present. The animal may look mixed from a distance, but the phenotype is not an intermediate hair color produced by blending pigment in every hair. Instead, both red and white hairs are expressed.

### Solving incomplete dominance crosses

The best way to solve incomplete dominance problems is to begin with genotype, not phenotype. AP questions may use unfamiliar organisms, so focus on the inheritance pattern described in the question.

> **Key Term: homozygous**
> Having two identical alleles for a gene, such as $$C^RC^R$$ or $$C^WC^W$$.
>
> **Key Term: heterozygous**
> Having two different alleles for a gene, such as $$C^RC^W$$.

Consider this cross:

> **Question:**
> In a plant species, red flowers are produced by genotype $$C^RC^R$$, white flowers by $$C^WC^W$$, and pink flowers by $$C^RC^W$$. What are the expected offspring from a cross between a red plant and a white plant?

The red parent is $$C^RC^R$$, so it can produce only $$C^R$$ gametes. The white parent is $$C^WC^W$$, so it can produce only $$C^W$$ gametes. Every offspring receives one $$C^R$$ allele and one $$C^W$$ allele.

Expected offspring:

- 100% $$C^RC^W$$
- 100% pink flowers

Now consider the F1 cross:

> **Question:**
> Two pink plants are crossed. What genotypic and phenotypic ratios are expected?

Each pink parent is $$C^RC^W$$. Each parent can produce two gamete types: $$C^R$$ and $$C^W$$.

The possible offspring are:

- $$C^RC^R$$: red
- $$C^RC^W$$: pink
- $$C^WC^R$$: pink
- $$C^WC^W$$: white

So the genotype ratio is:

$$
1\ C^RC^R : 2\ C^RC^W : 1\ C^WC^W
$$

The phenotype ratio is:

1\ red : 2\ pink : 1\ white
This 1:2:1 phenotypic ratio is a major clue for incomplete dominance in monohybrid crosses.

> **Key Term: Punnett square**
> A diagram used to predict possible offspring genotypes by combining the gametes from two parents.
>
> **Test Tip:**
> If a monohybrid cross between two heterozygotes produces three phenotypes in a 1:2:1 ratio, consider incomplete dominance or codominance. A 3:1 phenotype ratio usually suggests complete dominance.

A cross between a pink plant and a white plant gives a different ratio. The pink parent $$C^RC^W$$ produces $$C^R$$ and $$C^W$$ gametes. The white parent $$C^WC^W$$ produces only $$C^W$$ gametes. The offspring are:

- 50% $$C^RC^W$$, pink
- 50% $$C^WC^W$$, white

The expected phenotype ratio is 1 pink : 1 white.

This type of prediction may appear in AP multiple-choice questions, grid-style data questions, or free-response prompts where you must justify the expected ratio.

### Recognizing codominance in genotype and phenotype

Codominance problems often describe a heterozygote that expresses both traits at the same time. The key is to ask: does the heterozygote show an intermediate phenotype, or does it show both phenotypes separately?

In the ABO blood group system:

- $$I^AI^A$$ or $$I^Ai$$ gives type A blood.
- $$I^BI^B$$ or $$I^Bi$$ gives type B blood.
- $$I^AI^B$$ gives type AB blood.
- $$ii$$ gives type O blood.

The $$I^A$$ and $$I^B$$ alleles are codominant with each other, while both are dominant to $$i$$. This example also involves multiple alleles because there are three alleles in the population: $$I^A$$, $$I^B$$, and $$i$$. However, each individual still carries only two alleles for the gene.

> **Key Term: multiple alleles**
> More than two allele versions of a gene exist in a population, although a diploid individual has only two alleles for that gene.

Suppose a person with type AB blood has children with a person who has type O blood. The AB parent has genotype $$I^AI^B$$. The O parent has genotype $$ii$$. The AB parent can pass on $$I^A$$ or $$I^B$$. The O parent can pass on only $$i$$.

Expected offspring:

- 50% $$I^Ai$$, type A
- 50% $$I^Bi$$, type B

No offspring are expected to have type AB or type O blood from this cross.

Now consider a cross between a type A heterozygote and a type B heterozygote:

- Parent 1: $$I^Ai$$
- Parent 2: $$I^Bi$$

Possible offspring:

- $$I^AI^B$$: type AB
- $$I^Ai$$: type A
- $$I^Bi$$: type B
- $$ii$$: type O

The expected phenotype ratio is:

1\ type\ AB : 1\ type\ A : 1\ type\ B : 1\ type\ O
This is a good AP-style example because it tests codominance, multiple alleles, genotype-to-phenotype reasoning, and probability at the same time.

> **Exam Warning:**
> Do not assume that every uppercase allele is “dominant” in the simple Mendelian sense. In ABO blood groups, $$I^A$$ and $$I^B$$ are codominant with each other, and both are dominant to $$i$$. Always use the relationships given in the question.

Codominance can also be tested with unfamiliar examples. A question might describe a gene where allele $$M^1$$ produces one cell-surface protein and allele $$M^2$$ produces a different cell-surface protein. If heterozygotes have both proteins on their cells, that is codominance.

### Applying probability, evidence, and AP-style reasoning

Incomplete dominance and codominance questions are not only about filling in Punnett squares. AP Biology often asks you to analyze a scenario, make a prediction, justify a claim, or evaluate offspring data.

A strong response connects three ideas:

1. The parents’ genotypes determine the gametes they can produce.
2. Fertilization combines one allele from each parent.
3. The dominance pattern determines how genotype becomes phenotype.

For example:

> **Question:**
> A student crosses two pink-flowered plants and observes 24 red, 49 pink, and 27 white offspring. The student claims that flower color shows incomplete dominance. Support the student’s claim.

A strong answer would say that the observed offspring numbers are close to a 1 red : 2 pink : 1 white ratio, which is expected from a cross between two heterozygotes in incomplete dominance. The pink heterozygotes produce both $$C^R$$ and $$C^W$$ gametes, and fertilization gives $$C^RC^R$$, $$C^RC^W$$, and $$C^WC^W$$ offspring in an expected 1:2:1 genotype ratio. Because each genotype has a distinct phenotype, the phenotype ratio is also 1:2:1.

> **Key Term: monohybrid cross**
> A genetic cross that tracks inheritance of one gene or one trait.

The AP exam may also connect these patterns to chi-square analysis. If the expected ratio is 1:2:1 and the total number of offspring is 100, the expected counts are 25, 50, and 25. If the observed counts are close to those values, the data may support the proposed inheritance pattern. If the observed counts are very different, the model may be rejected, depending on the chi-square result and significance level.

You do not need to force every data set into a perfect ratio. Real offspring counts vary by chance, especially in small samples. The key question is whether the variation is small enough to be explained by random chance or large enough to challenge the proposed model.

> **Test Tip:**
> For AP free-response questions, write the reasoning, not just the ratio. A prediction such as “1:2:1” is stronger when you explain that each heterozygous parent produces two gamete types and the heterozygote has its own phenotype.

These inheritance patterns can also be combined with other genetics concepts. For example, an AP question might describe flower color with incomplete dominance and plant height with complete dominance. If the genes are on different chromosomes or assort independently, you can multiply probabilities.

Suppose flower color shows incomplete dominance:

- Red: $$C^RC^R$$
- Pink: $$C^RC^W$$
- White: $$C^WC^W$$

Plant height shows complete dominance:

- Tall: $$T_$$
- Short: $$tt$$

If a pink heterozygous tall plant $$C^RC^W Tt$$ is crossed with another $$C^RC^W Tt$$ plant, you can treat the two genes separately if the question states or implies independent assortment.

For flower color:

$$
C^RC^W \times C^RC^W \rightarrow \frac{1}{4}\ red,\ \frac{1}{2}\ pink,\ \frac{1}{4}\ white
$$

For height:

$$
Tt \times Tt \rightarrow \frac{3}{4}\ tall,\ \frac{1}{4}\ short
$$

The probability of a pink, tall offspring is:

$$
\frac{1}{2} \times \frac{3}{4} = \frac{3}{8}
$$

This kind of problem tests whether you understand both allele expression and the probability rules linked to meiosis.

> **Key Term: independent assortment**
> The separation of alleles for different genes into gametes independently of one another, usually when genes are on different chromosomes or far apart on the same chromosome.

A final exam skill is interpreting wording carefully. If a stem says “heterozygotes have a lighter red color than homozygous red individuals,” that suggests incomplete dominance. If it says “heterozygotes produce both red pigment and white pigment in different patches,” that suggests codominance. If it says “heterozygotes look exactly like one homozygote,” that suggests complete dominance.

AP Biology questions often introduce organisms, genes, or symbols you have not seen before. That is normal. The stem should give enough information to solve the problem. Read the scenario, identify the allele relationships, and then apply meiosis and probability.

## Key Point Checklist

This article has covered the following key knowledge points:

- Incomplete dominance and codominance are non-Mendelian patterns of allele expression.
- These patterns do not break Mendel’s law of segregation; alleles still separate during meiosis.
- In incomplete dominance, the heterozygote has an intermediate phenotype.
- In codominance, both alleles are fully and separately expressed in the heterozygote.
- A heterozygote-by-heterozygote cross often gives a 1:2:1 genotype ratio.
- In incomplete dominance and codominance, the phenotype ratio may also be 1:2:1.
- Pink flowers from red and white parents are a common incomplete dominance example.
- Type AB blood is a common codominance example because both A and B antigens are expressed.
- ABO blood groups also show multiple alleles because $$I^A$$, $$I^B$$, and $$i$$ exist in the population.
- AP questions may use unfamiliar allele symbols, so rely on the relationships described in the prompt.
- Free-response answers should justify predictions using gamete formation, fertilization, and genotype-to-phenotype reasoning.
- Expected ratios can be used in data analysis, including chi-square-style evaluation of inheritance models.

## Key Terms and Concepts

- allele
- genotype
- phenotype
- incomplete dominance
- codominance
- homozygous
- heterozygous
- Punnett square
- multiple alleles
- monohybrid cross
- independent assortment


Non-Mendelian genetics - Incomplete dominance and codominance