Codominance Incomplete Dominance Practice Problems Answer Key

The world of genetics is filled with fascinating phenomena, and among the most intriguing are codominance and incomplete dominance. These concepts deviate from the simple dominant-recessive patterns we often first encounter in biology, adding layers of complexity and nuance to how traits are inherited. Understanding these inheritance patterns is essential for anyone studying genetics, whether you're a student, a researcher, or simply someone curious about the mechanisms of heredity. This comprehensive guide will delve into codominance and incomplete dominance, providing explanations, examples, and practice problems with detailed answer keys to solidify your understanding.

Unveiling Codominance: Where Both Alleles Shine

Codominance occurs when two different alleles of a gene are both expressed in a heterozygote. Unlike dominant-recessive inheritance, where one allele masks the expression of the other, in codominance, both alleles contribute to the phenotype. The result is a distinct phenotype that showcases both traits simultaneously.

Key Characteristics of Codominance:

Both alleles are expressed: Neither allele is dominant or recessive.
Heterozygotes display both traits: The phenotype of a heterozygote is not an intermediate blend, but a combination of both parental traits.
Distinct phenotypic expression: The traits associated with each allele are clearly visible.

Illustrative Examples of Codominance:

ABO Blood Group System: The human ABO blood group system is a classic example of codominance. The I gene has three alleles: Iᴬ, Iᴮ, and i. The Iᴬ allele codes for the A antigen, the Iᴮ allele codes for the B antigen, and the i allele is recessive and does not produce an antigen. Individuals with the genotype IᴬIᴮ express both the A and B antigens on their red blood cells, resulting in blood type AB. This is a clear demonstration of codominance, as both alleles are equally expressed.
Roan Coat Color in Horses and Cattle: In some breeds of horses and cattle, coat color is determined by codominance. For instance, in cattle, the R allele codes for red coat color, and the W allele codes for white coat color. A heterozygous individual (RW) will have a roan coat, which is a mixture of red and white hairs. The red and white hairs are interspersed, creating a distinct roan appearance where both colors are visible.
MN Blood Group System: The MN blood group system in humans is another example of codominance. The two alleles, M and N, code for different glycoproteins on the surface of red blood cells. Individuals with the MM genotype have only the M glycoprotein, those with the NN genotype have only the N glycoprotein, and heterozygotes (MN) have both M and N glycoproteins on their red blood cells.

Exploring Incomplete Dominance: A Blend of Alleles

Incomplete dominance is another non-Mendelian inheritance pattern where the heterozygote exhibits an intermediate phenotype between the two homozygous phenotypes. In this case, neither allele is completely dominant over the other, leading to a blending effect.

Key Characteristics of Incomplete Dominance:

Heterozygote exhibits an intermediate phenotype: The phenotype of the heterozygote is a mix or blend of the phenotypes of the homozygous parents.
Neither allele is fully dominant: No single allele masks the expression of the other.
Blending effect: The resulting phenotype is a compromise between the two homozygous traits.

Illustrative Examples of Incomplete Dominance:

Flower Color in Snapdragons: A classic example of incomplete dominance is flower color in snapdragons. The Cᴿ allele codes for red flowers, and the Cᵂ allele codes for white flowers. A heterozygous individual (CᴿCᵂ) will have pink flowers. The pink color is an intermediate phenotype, resulting from the blending of the red and white alleles.
Feather Color in Chickens: In certain breeds of chickens, feather color exhibits incomplete dominance. The B allele codes for black feathers, and the W allele codes for white feathers. Heterozygous chickens (BW) will have blue feathers. The blue color is a result of the blending of black and white pigments.
Human Hair Texture: Hair texture in humans can sometimes exhibit incomplete dominance. If one allele codes for curly hair and another codes for straight hair, a heterozygous individual may have wavy hair, which is an intermediate phenotype.

Distinguishing Codominance from Incomplete Dominance: Key Differences

While both codominance and incomplete dominance involve non-Mendelian inheritance patterns, it's crucial to understand their differences:

Phenotype of Heterozygote: In codominance, the heterozygote expresses both parental traits distinctly and simultaneously. In incomplete dominance, the heterozygote displays an intermediate phenotype that is a blend of the parental traits.
Expression of Alleles: In codominance, both alleles are fully expressed. In incomplete dominance, neither allele is fully expressed, leading to a blending effect.
Visual Representation: In codominance, you can see both traits distinctly (e.g., red and white hairs in roan cattle). In incomplete dominance, you see a blended trait (e.g., pink flowers in snapdragons).

Practice Problems: Codominance and Incomplete Dominance

To solidify your understanding of codominance and incomplete dominance, let's work through some practice problems. These problems will help you apply the concepts and develop your problem-solving skills.

Problem 1: Roan Cattle (Codominance)

In cattle, coat color is controlled by codominance. The R allele codes for red coat color, and the W allele codes for white coat color. A heterozygous individual (RW) has a roan coat (a mixture of red and white hairs).

a) What are the possible genotypes and phenotypes of the offspring if a roan bull is crossed with a white cow?

b) If you cross two roan cattle, what is the probability of getting a red calf?

Problem 2: Snapdragon Flowers (Incomplete Dominance)

In snapdragons, flower color is determined by incomplete dominance. The Cᴿ allele codes for red flowers, and the Cᵂ allele codes for white flowers. A heterozygous individual (CᴿCᵂ) has pink flowers.

a) What are the possible genotypes and phenotypes of the offspring if you cross a pink snapdragon with a white snapdragon?

b) If you want to produce only pink snapdragons, what cross would you perform?

Problem 3: MN Blood Group (Codominance)

The MN blood group system in humans is determined by codominance. The M allele codes for the M glycoprotein, and the N allele codes for the N glycoprotein.

a) A woman with blood type M marries a man with blood type MN. What are the possible blood types of their children?

b) If both parents have blood type MN, what is the probability of having a child with blood type N?

Problem 4: Feather Color in Chickens (Incomplete Dominance)

In chickens, feather color is determined by incomplete dominance. The B allele codes for black feathers, and the W allele codes for white feathers. A heterozygous individual (BW) has blue feathers.

a) If you cross a blue chicken with a white chicken, what are the possible feather colors of their offspring?

b) What cross would you perform to get a 1:2:1 ratio of black, blue, and white feathered chickens?

Problem 5: Hypothetical Flower Color (Codominance)

In a certain flower species, petal color is determined by codominance. The P¹ allele codes for purple petals, and the P² allele codes for yellow petals. Heterozygous individuals (P¹P²) have petals that are both purple and yellow.

a) If you cross a purple flower with a flower that has both purple and yellow petals, what are the possible genotypes and phenotypes of their offspring?

b) What cross would result in 50% of the offspring having purple and yellow petals, and 50% having yellow petals?

Answer Key: Detailed Solutions to Practice Problems

Here are the detailed solutions to the practice problems, complete with explanations to enhance your understanding.

Solution 1: Roan Cattle (Codominance)

a) Cross: Roan bull (RW) x White cow (WW)

Punnett Square:

|       | R  | W  |
| :---- | :- | :- |
| **W** | RW | WW |
| **W** | RW | WW |

Genotypes: 50% RW (Roan), 50% WW (White)
Phenotypes: 50% Roan, 50% White

b) Cross: Roan cattle (RW) x Roan cattle (RW)

Punnett Square:

|       | R  | W  |
| :---- | :- | :- |
| **R** | RR | RW |
| **W** | RW | WW |

Genotypes: 25% RR (Red), 50% RW (Roan), 25% WW (White)
Phenotypes: 25% Red, 50% Roan, 25% White
Probability of a red calf: 25%

Solution 2: Snapdragon Flowers (Incomplete Dominance)

a) Cross: Pink snapdragon (CᴿCᵂ) x White snapdragon (CᵂCᵂ)

Punnett Square:

|         | Cᴿ  | Cᵂ  |
| :------ | :-- | :-- |
| **Cᵂ** | CᴿCᵂ | CᵂCᵂ |
| **Cᵂ** | CᴿCᵂ | CᵂCᵂ |

Genotypes: 50% CᴿCᵂ (Pink), 50% CᵂCᵂ (White)
Phenotypes: 50% Pink, 50% White

b) To produce only pink snapdragons, you would cross two pink snapdragons (CᴿCᵂ x CᴿCᵂ). This is because the resulting offspring genotypes would be 25% CᴿCᴿ (Red), 50% CᴿCᵂ (Pink), and 25% CᵂCᵂ (White). Removing the red and white offspring would leave you with only pink snapdragons. However, a more practical approach to ensure mostly pink offspring would be to continuously cross pink snapdragons with each other and select for pink offspring in each generation.

Solution 3: MN Blood Group (Codominance)

a) Cross: Woman (Blood type M, MM) x Man (Blood type MN, MN)

Punnett Square:

|       | M  | M  |
| :---- | :- | :- |
| **M** | MM | MM |
| **N** | MN | MN |

Genotypes: 50% MM (Blood type M), 50% MN (Blood type MN)
Phenotypes: 50% Blood type M, 50% Blood type MN

b) Cross: Both parents have blood type MN (MN x MN)

Punnett Square:

|       | M  | N  |
| :---- | :- | :- |
| **M** | MM | MN |
| **N** | MN | NN |

Genotypes: 25% MM (Blood type M), 50% MN (Blood type MN), 25% NN (Blood type N)
Phenotypes: 25% Blood type M, 50% Blood type MN, 25% Blood type N
Probability of a child with blood type N: 25%

Solution 4: Feather Color in Chickens (Incomplete Dominance)

a) Cross: Blue chicken (BW) x White chicken (WW)

Punnett Square:

|       | B  | W  |
| :---- | :- | :- |
| **W** | BW | WW |
| **W** | BW | WW |

Genotypes: 50% BW (Blue), 50% WW (White)
Phenotypes: 50% Blue, 50% White

b) To get a 1:2:1 ratio of black, blue, and white feathered chickens, you would cross two blue chickens (BW x BW).

Punnett Square:

|       | B  | W  |
| :---- | :- | :- |
| **B** | BB | BW |
| **W** | BW | WW |

Genotypes: 25% BB (Black), 50% BW (Blue), 25% WW (White)
Phenotypes: 25% Black, 50% Blue, 25% White
Ratio: 1 Black : 2 Blue : 1 White

Solution 5: Hypothetical Flower Color (Codominance)

a) Cross: Purple flower (P¹P¹) x Flower with purple and yellow petals (P¹P²)

Punnett Square:

|        | P¹  | P¹  |
| :----- | :-- | :-- |
| **P¹** | P¹P¹ | P¹P¹ |
| **P²** | P¹P² | P¹P² |

Genotypes: 50% P¹P¹ (Purple), 50% P¹P² (Purple and yellow)
Phenotypes: 50% Purple, 50% Purple and yellow

b) To result in 50% of the offspring having purple and yellow petals, and 50% having yellow petals, you would cross a flower with purple and yellow petals (P¹P²) with a yellow flower (P²P²).

Punnett Square:

|        | P¹  | P²  |
| :----- | :-- | :-- |
| **P²** | P¹P² | P²P² |
| **P²** | P¹P² | P²P² |

Genotypes: 50% P¹P² (Purple and yellow), 50% P²P² (Yellow)
Phenotypes: 50% Purple and yellow, 50% Yellow

Conclusion: Mastering the Nuances of Inheritance

Codominance and incomplete dominance represent fascinating deviations from simple Mendelian genetics. By understanding these concepts, you gain a deeper appreciation for the complexity of inheritance and the diverse ways in which genes can be expressed. Through clear explanations, illustrative examples, and practice problems with detailed answer keys, this guide has equipped you with the knowledge and skills to confidently navigate these genetic phenomena. As you continue your exploration of genetics, remember that these principles are fundamental to understanding the rich tapestry of life and the mechanisms that drive its diversity.