Question

28. A red bull is crossed with a white cow and all of the offspring are roan, a color that is caused by the presence of both red and white hairs. This is an example of alleles that are _____.
a. nonhomologous
b. epistatic
c. codominant
d. polygenic
e. completely dominant

Answer 1

Because both are present, that means both traits appear, and only one would work form this

Co-dominance occurs when the contributions of both alleles are visible in the phenotype.

For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (IA, IB) and dominant over the recessive i at the ABO locus. The IA and IB alleles produce different modifications. The enzyme coded for by IA adds an N-acetylgalactosamine to the membrane-bound H antigen. The IB enzyme adds a galactose. The i allele produces no modification. Thus IA and IB alleles are each dominant to i (IAIA and IAi individuals both have type A blood, and IBIB and IBi individuals both have type B blood. But IAIB individuals have both modifications on their blood cells and thus have type AB blood, so the IA and IB alleles are said to be co-dominant.)