Colorblindness disproportion

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Basically, all the websites I’ve seen claim that red-green colorblindness is a recessive gene attached to X chromosome, and is more prevalent in men because there is less chance of this gene being muted by a dominant gene of normal vision. But this should lead to 2x difference in red-green colorblindness rate ([as in diagram](https://en.wikipedia.org/wiki/Color_blindness#/media/File:Punnett_square_colour_blindness.svg)), while real difference is 20x (8% of males and 0.4% of females). Why is this?

TIA

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3 Answers

Anonymous 0 Comments

It would be 2x if the colorblind and non-colorblind genes were equally common (that is, if the columns and rows of your Punnet squares had equal probability). But they aren’t.

We can infer from the 8% rate among men that it’s present on about 8% of X-chromosomes (since an XY man is colorblind if it’s present on his X, and because only about 0.2% of [genetic] men are [non-XY](https://en.wikipedia.org/wiki/Klinefelter_syndrome)). That means that, if X-chromosome selection were totally independent, women should have it about 8% times 8% = 0.64% of the time.

The fact that we get a bit less than that suggests that one of (a) it’s a bit underdiagnosed in women, (b) it’s a bit overdiagnosed in men, (c) population genetics aren’t perfectly mixed, or some other factor. My guess would be a mix of (a) and some of the more complicated non-Mendelian genetic quirks around X-chromosome inactivation in women, but I don’t know.

Anonymous 0 Comments

It would be 2x if the colorblind and non-colorblind genes were equally common (that is, if the columns and rows of your Punnet squares had equal probability). But they aren’t.

We can infer from the 8% rate among men that it’s present on about 8% of X-chromosomes (since an XY man is colorblind if it’s present on his X, and because only about 0.2% of [genetic] men are [non-XY](https://en.wikipedia.org/wiki/Klinefelter_syndrome)). That means that, if X-chromosome selection were totally independent, women should have it about 8% times 8% = 0.64% of the time.

The fact that we get a bit less than that suggests that one of (a) it’s a bit underdiagnosed in women, (b) it’s a bit overdiagnosed in men, (c) population genetics aren’t perfectly mixed, or some other factor. My guess would be a mix of (a) and some of the more complicated non-Mendelian genetic quirks around X-chromosome inactivation in women, but I don’t know.

Anonymous 0 Comments

It would be 2x if the colorblind and non-colorblind genes were equally common (that is, if the columns and rows of your Punnet squares had equal probability). But they aren’t.

We can infer from the 8% rate among men that it’s present on about 8% of X-chromosomes (since an XY man is colorblind if it’s present on his X, and because only about 0.2% of [genetic] men are [non-XY](https://en.wikipedia.org/wiki/Klinefelter_syndrome)). That means that, if X-chromosome selection were totally independent, women should have it about 8% times 8% = 0.64% of the time.

The fact that we get a bit less than that suggests that one of (a) it’s a bit underdiagnosed in women, (b) it’s a bit overdiagnosed in men, (c) population genetics aren’t perfectly mixed, or some other factor. My guess would be a mix of (a) and some of the more complicated non-Mendelian genetic quirks around X-chromosome inactivation in women, but I don’t know.