The most common form of colorblindness is a recessive, sex-linked hereditary condition caused by a defect on the X chromosome. Females are XX, while males are XY. Individuals inherit one chromosome from each parent, with equal probability; for example, an individual has a 50% chance of inheriting their father’s X chromosome, and a 50% chance of inheriting their father’s Y chromosome. If a male has an X chromosome with the defect, he is colorblind. However, a female with only one defective X chromosome will not be colorblind. Thus, colorblindness is more common in males than females; 7% of males are colorblind but only 0.5% of females are colorblind.

(a) Assume that the X chromosome with the wild-type allele is X+ and the one with the disease allele is . What is the expected frequency of each possible female genotype: , and  What is the expected frequency of each possible male genotype: X+Y and X-Y ?

(b) Suppose that two parents are not colorblind. What is the probability that they have a colorblind child?

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x+