A colourblind man has a colourblind sister bu | vedclass

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(A) Colour blindness is an $X$-linked recessive trait. Let $X^c$ be the allele for colour blindness and $X$ be the normal allele.$1$. $A$ colourblind

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Father colourblind and mother normal

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(A) Colour blindness is an $X$-linked recessive trait. Let $X^c$ be the allele for colour blindness and $X$ be the normal allele.$1$. $A$ colourblind man has the genotype $X^c Y$.$2$. The man has a colourblind sister,meaning her genotype must be $X^c X^c$. Since she receives one $X^c$ from each parent,the father must be $X^c Y$ (colourblind) and the mother must contribute at least one $X^c$ allele.$3$. The man also has a normal brother,whose genotype is $XY$. He receives the $Y$ chromosome from his father and the normal $X$ chromosome from his mother.$4$. Therefore,the mother must be a carrier $(X^c X)$ to have both a colourblind daughter and a normal son.$5$. Thus,the father is colourblind $(X^c Y)$ and the mother is a carrier (phenotypically normal,$X^c X$). The correct option is $(A)$.

$A$ colourblind man has a colourblind sister but a normal brother. What is the phenotype of his parents?

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