Incomplete Dominance And Codominance Practice Problems Answer Key

Incomplete dominance and codominance are variations on Mendelian inheritance patterns, where the resulting phenotypes of heterozygous offspring differ from the typical dominant/recessive relationships. Understanding these concepts is crucial for mastering genetics, and practice problems are an excellent way to solidify your knowledge. This comprehensive guide will provide you with practice problems, step-by-step solutions, and the underlying principles of incomplete dominance and codominance.

Understanding Incomplete Dominance and Codominance

Before diving into practice problems, let's clarify the key differences between incomplete dominance and codominance:

• Incomplete Dominance: In this scenario, the heterozygous genotype results in a phenotype that is a blend or intermediate between the two homozygous phenotypes. Neither allele is completely dominant over the other. Think of it like mixing paint: red and white create pink.

• Codominance: Here, the heterozygous genotype results in a phenotype where both alleles are expressed simultaneously and distinctly. Neither allele masks the other. A classic example is the AB blood type in humans, where both A and B antigens are produced.

Incomplete Dominance Practice Problems

Let's work through some examples to illustrate incomplete dominance.

Problem 1:

In snapdragons, flower color is controlled by incomplete dominance. The allele R produces red flowers, and the allele W produces white flowers. A heterozygous (RW) plant produces pink flowers.

1. What phenotypes would you expect in the offspring of a cross between a pink-flowered snapdragon and a white-flowered snapdragon?
2. What genotypic and phenotypic ratios would you expect if you crossed two pink-flowered snapdragons?

Solution:

1. Cross between a pink-flowered snapdragon (RW) and a white-flowered snapdragon (WW):

   • Set up a Punnett square:

         |   R   |   W   |
     ----|-------|-------|
     W   |   RW  |   WW  |
     W   |   RW  |   WW  |
     

   • Genotypes: 50% RW, 50% WW

   • Phenotypes: 50% pink flowers, 50% white flowers

2. Cross between two pink-flowered snapdragons (RW x RW):

   • Set up a Punnett square:

         |   R   |   W   |
     ----|-------|-------|
     R   |   RR  |   RW  |
     W   |   RW  |   WW  |
     

   • Genotypes: 25% RR, 50% RW, 25% WW

   • Phenotypic Ratio: 1 red : 2 pink : 1 white

Problem 2:

Radish color is governed by incomplete dominance. RR produces red radishes, WW produces white radishes, and RW produces purple radishes.

1. If you cross a purple radish with a white radish, what are the possible genotypes and phenotypes of the offspring?
2. A farmer wants to produce only purple radishes. Explain the cross(es) the farmer needs to do and why they are the only way to guarantee only purple radishes.

Solution:

1. Cross between a purple radish (RW) and a white radish (WW):

   • Set up a Punnett square:

         |   R   |   W   |
     ----|-------|-------|
     W   |   RW  |   WW  |
     W   |   RW  |   WW  |
     

   • Genotypes: 50% RW, 50% WW

   • Phenotypes: 50% purple radishes, 50% white radishes

2. Producing Only Purple Radishes:

   The farmer cannot guarantee all purple radishes. Since purple is the heterozygous phenotype, crossing two purple radishes (RW x RW) will still result in some red (RR) and white (WW) radishes (as shown in Problem 1). There is no cross that would yield 100% purple radishes.

Problem 3:

In chickens, the allele for black feathers (B) is incompletely dominant to the allele for white feathers (W). Heterozygous chickens (BW) have blue feathers.

1. What is the probability of obtaining a blue-feathered chicken if you cross a black-feathered chicken with a white-feathered chicken?
2. What cross would produce the greatest percentage of blue-feathered chickens?

Solution:

1. Cross between a black-feathered chicken (BB) and a white-feathered chicken (WW):

   • Set up a Punnett square:

         |   B   |   B   |
     ----|-------|-------|
     W   |   BW  |   BW  |
     W   |   BW  |   BW  |
     

   • Genotype: 100% BW

   • Phenotype: 100% blue-feathered chickens

2. Greatest Percentage of Blue-Feathered Chickens:

   Crossing a black-feathered chicken with a white-feathered chicken, as demonstrated above, will produce 100% blue-feathered chickens. Any cross involving two heterozygous chickens (blue x blue) will create a genotypic ratio of 1:2:1 (BB:BW:WW) that reduces the number of potential blue-feathered offspring, so BB x WW is the best option.

Codominance Practice Problems

Now, let's move on to codominance.

Problem 1:

In certain breeds of chickens, feather color is codominant. The allele B codes for black feathers, and the allele W codes for white feathers. Heterozygous chickens (BW) have feathers that are both black and white – often referred to as "erminette."

1. If you cross two erminette chickens, what are the expected genotypic and phenotypic ratios in the offspring?
2. A farmer wants only erminette chickens. What parental cross would give them the best chance of creating more of these chickens?

Solution:

1. Cross between two erminette chickens (BW x BW):

   • Set up a Punnett square:

         |   B   |   W   |
     ----|-------|-------|
     B   |   BB  |   BW  |
     W   |   BW  |   WW  |
     

   • Genotypes: 25% BB, 50% BW, 25% WW

   • Phenotypic Ratio: 1 black : 2 erminette : 1 white

2. Maximizing Erminette Chickens:

   To maximize the chances of obtaining erminette chickens, the farmer should cross a black chicken (BB) with a white chicken (WW). This will produce 100% erminette chickens (BW), similar to the blue chicken problem in incomplete dominance above.

Problem 2:

The MN blood group system in humans is determined by codominant alleles M and N. Individuals with the MM genotype have M antigens on their red blood cells, those with the NN genotype have N antigens, and those with the MN genotype have both M and N antigens.

1. A man with blood type M marries a woman with blood type MN. What are the possible blood types of their children?
2. If both parents are blood type MN, what is the probability that their child will have blood type N?

Solution:

1. Man with blood type M (MM) marries a woman with blood type MN (MN):

   • Set up a Punnett square:

         |   M   |   M   |
     ----|-------|-------|
     M   |   MM  |   MM  |
     N   |   MN  |   MN  |
     

   • Genotypes: 50% MM, 50% MN

   • Phenotypes: 50% blood type M, 50% blood type MN

2. Both parents are blood type MN (MN x MN):

   • Set up a Punnett square (same as the erminette chicken cross above):

         |   M   |   N   |
     ----|-------|-------|
     M   |   MM  |   MN  |
     N   |   MN  |   NN  |
     

   • Genotypes: 25% MM, 50% MN, 25% NN

   • Phenotypes: 25% blood type M, 50% blood type MN, 25% blood type N

   The probability that their child will have blood type N is 25%.

Problem 3:

Consider a plant species where leaf type is controlled by codominance. The allele F produces forked leaves, and the allele R produces round leaves. Heterozygous plants (FR) have both forked and round leaves on the same plant.

1. What genotypes and phenotypes would you expect from a cross between a plant with forked leaves and a plant with both forked and round leaves?
2. What cross would yield the highest proportion of plants with both forked and round leaves?

Solution:

1. Cross between a plant with forked leaves (FF) and a plant with both forked and round leaves (FR):

   • Set up a Punnett square:

         |   F   |   F   |
     ----|-------|-------|
     F   |   FF  |   FF  |
     R   |   FR  |   FR  |
     

   • Genotypes: 50% FF, 50% FR

   • Phenotypes: 50% forked leaves, 50% both forked and round leaves

2. Highest Proportion of Plants with Both Forked and Round Leaves:

   Crossing a plant with forked leaves (FF) with a plant with round leaves (RR) will yield 100% heterozygous offspring (FR), and therefore the highest proportion of plants with both forked and round leaves.

Advanced Practice Problems: Incorporating Mendelian Genetics

These problems combine incomplete dominance or codominance with basic Mendelian inheritance principles.

Problem 1:

In a certain species of flowering plant, flower color is controlled by incomplete dominance, where R produces red, W produces white, and RW produces pink flowers. Plant height is controlled by simple dominance, where T is tall and t is short.

1. What are the possible genotypes and phenotypes of the offspring from a cross between a tall, pink-flowered plant (TtRW) and a short, white-flowered plant (ttWW)?
2. What proportion of the offspring from the above cross will be tall and pink?

Solution:

1. Cross between TtRW and ttWW:

   • We need to consider each trait separately and then combine the probabilities.

   • Height: Tt x tt yields 50% Tt (tall) and 50% tt (short)

   • Flower Color: RW x WW yields 50% RW (pink) and 50% WW (white)

   • Possible genotypes: TtRW, TtWW, ttRW, ttWW

   • Possible phenotypes: tall pink, tall white, short pink, short white

2. Proportion of offspring that are tall and pink:

   • Probability of being tall (Tt) = 0.5
   • Probability of being pink (RW) = 0.5
   • Probability of being tall and pink = 0.5 * 0.5 = 0.25 or 25%

Problem 2:

In cattle, coat color exhibits codominance. The allele R produces red coats, the allele W produces white coats, and RW produces roan coats (a mixture of red and white hairs). Horn presence is determined by simple dominance, where the allele H is horned and h is hornless.

1. A roan, horned bull (RWHh) is crossed with a red, hornless cow (RRhh). What are the possible genotypes and phenotypes of their offspring?
2. What proportion of the offspring will be roan and hornless?

Solution:

1. Cross between RWHh and RRhh:

   • Coat Color: RW x RR yields 50% RR (red) and 50% RW (roan)

   • Horn Presence: Hh x hh yields 50% Hh (horned) and 50% hh (hornless)

   • Possible genotypes: RRHh, RRhh, RWHh, RWmmhh

   • Possible phenotypes: red horned, red hornless, roan horned, roan hornless

2. Proportion of offspring that are roan and hornless:

   • Probability of being roan (RW) = 0.5
   • Probability of being hornless (hh) = 0.5
   • Probability of being roan and hornless = 0.5 * 0.5 = 0.25 or 25%

Tips for Solving Incomplete Dominance and Codominance Problems

• Clearly Define Alleles: Before setting up any Punnett square, identify each allele and its corresponding phenotype. Using consistent notation (e.g., R for red, W for white) will minimize confusion.
• Recognize the Pattern: Determine whether the problem describes incomplete dominance (blending) or codominance (both traits expressed). This is crucial for assigning phenotypes to genotypes.
• Use Punnett Squares: Punnett squares are invaluable tools for visualizing crosses and determining the probabilities of different genotypes and phenotypes.
• Break Down Complex Problems: When dealing with multiple traits, analyze each trait separately and then combine the probabilities. This simplifies the process and reduces errors.
• Practice Regularly: The more practice problems you solve, the better you'll become at recognizing patterns and applying the concepts of incomplete dominance and codominance.

Common Mistakes to Avoid

• Confusing Incomplete Dominance and Codominance: The key difference is whether the heterozygous phenotype is a blend (incomplete dominance) or a simultaneous expression of both alleles (codominance).
• Incorrectly Assigning Genotypes: Make sure you understand which genotypes correspond to which phenotypes, especially for heterozygous individuals.
• Forgetting Basic Mendelian Principles: Incomplete dominance and codominance are variations on Mendelian genetics, so don't forget the fundamentals of allele segregation and independent assortment (when dealing with multiple traits).
• Not Showing Your Work: Write out your Punnett squares and calculations. This will help you identify errors and understand the reasoning behind your answers.

Conclusion

Mastering incomplete dominance and codominance requires a solid understanding of the underlying principles and plenty of practice. By working through these example problems and following the tips outlined above, you can confidently tackle any genetics problem involving these inheritance patterns. Remember to clearly define your alleles, use Punnett squares effectively, and break down complex problems into manageable steps. Good luck!