For most genes on the human X chromosome, what percent of XY males with a mutant allele on the X chromosome will express the mutant phenotype?

A., 0%

B., 25%

C., 50%

D., 100%

E., There is insufficient data to predict

D. 100%

Explanation:

In humans, males have only one copy of the X chromosome in their cells. Therefore, human males express all the X-linked genes present on their X chromosomes irrespective of the recessive or dominant nature of the allele. This occurs since males do not have one more copy of the X chromosome to carry another allele for the gene. So, all the X-linked alleles present on the X-chromosomes of the human males are expressed.

For example, human males with genotype X^cY and X^hY express colorblindness and hemophilia respectively. Though, human females express these recessive X-linked disorders in homozygous genotype only.