AP Biology Questions on Codominance: Exam Prep

📚 Quick Study Guide

• 🧬 Codominance is a type of inheritance where both alleles for a trait are equally expressed in a heterozygote.
• 🩸 Unlike incomplete dominance, where a blended phenotype is observed, codominance results in both phenotypes being distinctly visible.
• 🐔 A classic example is the feather color in chickens: a black chicken (BB) crossed with a white chicken (WW) produces speckled black and white chickens (BW).
• 🐮 Another example is the ABO blood group system in humans, where both $I^A$ and $I^B$ alleles are codominant, resulting in type AB blood.
• ✍️ When solving codominance problems, remember to use appropriate symbols to represent the alleles, such as superscripts.
• 💡 Understand the difference between codominance, incomplete dominance, and complete dominance to avoid confusion on exam questions.
• 🧮 Use Punnett squares to predict the genotypes and phenotypes of offspring from codominant crosses.

🧪 Practice Quiz

1. Which of the following statements best describes codominance?
   1. (A) One allele is dominant and masks the expression of the other allele.
   2. (B) The heterozygous phenotype is an intermediate blend of the two homozygous phenotypes.
   3. (C) Both alleles are equally expressed in the heterozygous phenotype.
   4. (D) One allele is lethal when homozygous.
2. In shorthorn cattle, the alleles for red coat color (R) and white coat color (W) are codominant. What coat color would a heterozygous (RW) cow have?
   1. (A) Red
   2. (B) White
   3. (C) Roan (a mixture of red and white hairs)
   4. (D) Pink
3. A man with type AB blood marries a woman with type O blood. What are the possible blood types of their offspring?
   1. (A) A and B
   2. (B) AB and O
   3. (C) A, B, and O
   4. (D) AB only
4. In a certain species of flower, the alleles for red petals (R) and white petals (W) are codominant. If a red-petaled flower is crossed with a white-petaled flower, what percentage of the offspring will have red and white petals?
   1. (A) 0%
   2. (B) 25%
   3. (C) 50%
   4. (D) 100%
5. Which of the following is NOT an example of codominance?
   1. (A) ABO blood groups in humans
   2. (B) Roan coat color in horses
   3. (C) Pink flowers resulting from a cross between red and white flowers
   4. (D) Speckled feather color in chickens
6. If two roan cattle are crossed, what percentage of their offspring is expected to be roan?
   1. (A) 25%
   2. (B) 50%
   3. (C) 75%
   4. (D) 100%
7. A plant with the genotype $C^RC^W$ produces flowers with both red and white patches. This is an example of:
   1. (A) Complete dominance
   2. (B) Incomplete dominance
   3. (C) Codominance
   4. (D) Sex-linked inheritance