Question

Codominance is a situation in which the phenotypes produced by both alleles are completely expressed. A cattle breeder owns a pure-breeding red bull (rr) and a pure-breeding white cow (ww). What is the probability that offspring from these two cows will exhibit the roan (rw) phenotype?

Answer Choices:
a. 25%
b. 50%
c. 75%
d. 100%

Answer 1

In codominance, both alleles are equally expressed in the phenotype; crossing pure-breeding red (rr) and white (ww) cattle will always result in offspring with a roan (rw) phenotype, making the probability 100%.

Step-by-step explanation:

When discussing codominance, it is important to differentiate it from incomplete dominance. Codominance occurs when both alleles for a gene are expressed equally in the phenotype of a heterozygote. For example, in human MN blood groups, the M and N alleles are both expressed in heterozygotes. The ABO blood group system is another classic example of codominance. If a pure-breeding red bull (rr) is crossed with a pure-breeding white cow (ww), all offspring will inherit one red allele and one white allele, resulting in the roan (rw) phenotype. Since all offspring will be rw, the probability is 100%.