In humans, a recessive allele of a gene on the X chromosome causes humans to be "colorblind." A colorblind XY man and an XX woman whose genotype is homozygous for normal color vision have children together. What percentage of their offspring are expected be colorblind?

The answer is 0%.

Explanation:

If X⁻ represent the recessive allele that causes colorblind, and X⁺ represents the dominant allele, thus:

X⁻Y is the genotype of the colorblind man (he is carrier and he has the disease)

X⁺X⁺ is the genotype of the woman (she has normal vision and is not carrier)

When they have children together, the offspring will have the following genotypes:

X⁻Y x X⁺X⁺ = X⁻X⁺; X⁻X⁺; X⁺Y; X⁺Y

Summarizing, they will be 50% X⁻X⁺ (women with normal vision, carrier) and 50% X⁺Y (men with normal vision, non carrier).

No child will have the disease because its expresses when the genotype is homozygous for colorblind allele (X⁻X⁻).