If one X chromosome is healthy and one is mutated, why does the healthy take control?
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There’s something called dominant and recessive genes. A simplified case is eye color. Blue is recessive and brown is dominant. If you have two brown eye genes, your eyes are brown. If you have two blue eye genes, your eyes are blue. If you have one of each, your eyes are brown. This means two brown-eyed parents can have a blue-eyed child, assuming each has a recessive blue eye gene and they both pass it to the child. But two blue-eyed parents can never have a brown-eyed child as they each have to have two blue eye genes and therefore always pass one each to the child. (Reality has eyes a little more complicated than this especially as there are other colors like green and hazel, but this is the basic idea.)
Colorblindness is a recessive gene, but there’s an interesting foible: it’s on a sex gene, specifically the X, and the Y gene does not have a corresponding dominant gene. So a woman can only have colorblindness if she gets a colorblindness gene from each parent, while a man will be colorblind if he gets a single colorblindness gene from his mother. And it has to specifically be from the mother, as he’s always getting the Y from his father.
If a man is colorblind, he will always pass along that X gene to any daughters he has, but never to his sons. If a woman is colorblind, she will always pass along a colorblind gene to any children she has, which means 100% of her sons will be colorblind, regardless of whether the father is, but her daughters will be colorblind only if the father also was. If a woman is a “carrier” for colorblindness (one colorblind X and one non-colorblind X), it’s 50/50 to pass along the colorblind X to each child, meaning half of her sons on average would be colorblind, and her daughters would be colorblind half the time if and only if the father was also colorblind and carriers the other half, and would be carriers half the time if he wasn’t and fully non-colorblind the other half.
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