Color Blindness Statistics

[Nim]

I read that the most common form of color blindess (red/green), occurs in 7% of males, but only in .4% of females.

Men just have to get the gene from their mother to have color blindess. But women have to get it from their father also. So shouldn't that make it only half as likely for females to be color blind? Shouldn't 3.5% of females have color blindess?

[Hurkyl]

Nope.

Consider a similar situation:

What are your odds of rolling a "1" on a single die? (1/6)
What are your odds of rolling two "1"s on two dice? (1/36)

In order to find the probability that a women gets two color-blindness genes, you have to multiply the problabilities of getting each individual color-blindness gene.

You'll note that 0.07 * 0.07 = 0.0049, which is about .4%.

(However, for completeness's sake, I don't think that both genes have to have the same probability of occuring in this case)

[iansmith]

The colour blindness gene is on the sex chrosome X. Therefore male only carry one X (it is from the mother) and absent from the Y. Therefore as soon as the gene is not functional, the person will have colour blindness. The gene is also dominant that female carry two XX and require both gene to be not functional for colour blindness.

Hukyl gave a good example of stats.