Codominance Definition: Examples in Genetics

📚 Codominance Definition: A Quick Study Guide

• 🧬 Definition: Codominance occurs when two alleles for a gene are equally expressed in a heterozygote. Both alleles contribute to the phenotype.
• 🩸 Blood Types: The ABO blood group system provides a classic example. Individuals with the $I^AI^B$ genotype express both A and B antigens on their red blood cells, resulting in blood type AB.
• 🌸 Roan Color in Horses/Cattle: In roan horses and cattle, both red and white hairs are present. The alleles for red coat color (R) and white coat color (W) are codominant, so a RW individual displays both colors simultaneously.
• 🐔 Feather Color in Chickens: In some chicken breeds, the allele for black feathers (B) and the allele for white feathers (W) are codominant. A BW chicken will have both black and white feathers.
• 🔬 Key Characteristic: Unlike incomplete dominance (where the heterozygote shows an intermediate phenotype), in codominance, both parental phenotypes are fully expressed.

🧪 Practice Quiz

1. Which of the following is the best definition of codominance?

   1. Both alleles are equally expressed in the phenotype.
   2. One allele is dominant over the other.
   3. The phenotype is a blend of both alleles.
   4. One allele masks the expression of the other allele completely.

2. In a certain species of flower, the alleles for red petals (R) and white petals (W) are codominant. What would be the phenotype of a flower with the RW genotype?

   1. Red petals
   2. White petals
   3. Pink petals
   4. Red and white speckled petals

3. Which of the following is an example of codominance in humans?

   1. Eye color
   2. Hair color
   3. ABO blood type
   4. Height

4. A roan cow has both red and white hairs. What type of inheritance pattern is this?

   1. Complete dominance
   2. Incomplete dominance
   3. Codominance
   4. Sex-linked inheritance

5. In chickens, the allele for black feathers (B) and the allele for white feathers (W) are codominant. If a black feathered chicken (BB) is crossed with a white feathered chicken (WW), what percentage of the offspring will have black and white feathers?

   1. 0%
   2. 25%
   3. 50%
   4. 100%

6. How does codominance differ from incomplete dominance?

   1. In codominance, one allele is completely dominant.
   2. In codominance, both alleles are fully expressed.
   3. In incomplete dominance, both alleles are fully expressed.
   4. In incomplete dominance, the heterozygote expresses both parental phenotypes.

7. A man with blood type A and a woman with blood type B have a child with blood type AB. What are the most likely genotypes of the parents?

   1. Man: $I^AI^A$, Woman: $I^BI^B$
   2. Man: $I^Ai$, Woman: $I^Bi$
   3. Man: $I^AI^B$, Woman: $I^AI^B$
   4. Man: ii, Woman: ii