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If the woman is colorblind, then both of her X chromosomes is carrying a colorblind gene and she is homozygous for that trait. The father is not colorblind and his one X chromosome carries the normal gene for color vision. Let X represent the normal gene and let X- represent the colorblind gene. The woman is X-X- and the man is XY. His Y chromosome doesn't play a role in the inheritance of colorblindness. When each produces sex cells, the woman's eggs will only contain the X- with the colorblind gene and the man's sperms will contain only the normal X or Y chromosome. The resulting offspring will be both daughters will have X-X and are heterozygous carriers of colorblindness. However, their color vision is normal since they each have one normal gene which is dominant to the colorblind gene. The sons will inherit X-Y. Since each son has the colorblind gene, they will both be colorblind. They only get one copy of the X chromosome from their mother and she passed along a colorblind gene to each son. Males are hemizygous for colorblindness, because they only receive one copy of this gene on their X chromosome.
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