Question

Treat race as a Mendelian trait that shows COMPLETE dominance. If a woman who is homozygous dominant for dark skin and curly hair marries a man who is homozygous recessive for light skin and straight hair, what is the chance that they will have a child with light skin and light hair?

How would this be different if we looked at this gene as INCOMPLETE dominance?

Answer 1

Assuming that skin color and hair texture are controlled by the same gene and follow complete dominance, the woman would have the genotype DDcc (dark skin and curly hair) and the man would have the genotype ddCC (light skin and straight hair). Their possible gametes are Dc and dC, respectively.

The Punnett square for their cross would be:

Dc Dc
dC DdCc DdCc
dC DdCc DdCc

Therefore, all of their offspring would have dark skin and curly hair (genotype DdCc), and none would have light skin and light hair.

If the gene were to follow incomplete dominance, the heterozygous offspring (DdCc) would have an intermediate phenotype (for example, medium skin and wavy hair), and the phenotype of the homozygous dominant (DDcc) and homozygous recessive (ddCC) would be at opposite ends of the spectrum (very dark skin and very curly hair for DDcc and very light skin and very straight hair for ddCC).

In this case, the chance of having a child with light skin and light hair would depend on the expression of the gene in the heterozygous state, which could result in a range of skin and hair phenotypes.