Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Red-green colour vision loss is X-linked and recessive. In a certain region a fraction of the men have it. Men and women are equally numerous, partners are chosen without regard to colour vision, and the gene is in equilibrium. Write for the frequency of the recessive allele among X chromosomes.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Males show the allele frequency directly. Females follow , and .
Allele frequency. The affected males are , so .
(a) , so females.
(b) .
(c) The son takes one X chromosome of his mother at random, and a random X chromosome carries the allele with probability . So the probability is . (Adding gives the same value.)
(d) , so affected females for every affected males.
(e) Among healthy women the carrier share is . A carrier mother passes on the allele to a son with probability , so the probability is . The father does not matter for a son, because a son gets his X chromosome from his mother.
Check. Drawing the mother's two X chromosomes one by one with exact fractions gives for a random woman and for a healthy woman, which is .
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