Colorblindness question

[fingerscrossedd]

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From EK In-class exam, Biology, Ch9:

205. Color-blindness is a sex-linked recessive trait. If a color blind man and a woman that is a carrier for the trait have two girls and two boys, what is the proability that at least one of the girls will be colorblind?

Had a tough time with this one, logically (the answer is 75%). Any help?

Thanks guys! 😍😍😍

 

[milski]

If the father is color blind, the girls are inheriting the recessive gene from him. The mother is a carrier, so she is heterozygous. The chance for each girl to be color blind then is 50%. The only way for none of them to be CB is if they're both ok - the probability for that is 0.5x0.5=0.25. So the chance for at least one of them to be CB is 1-0.25=0.75. What do they have as an answer?

 

[Postictal Raiden]

The best way to approach this problem is the same way milski did.

However, you could also do it the long way:

Let's label the girls, girl A and girl B. Now what is the chance that girl A alone is CB? 0.5 (girl A CB) x 0.5 (girl B normal) = 0.25. Similarly, the chance that girl B alone is CB is 0.25. Now, what is the change that both girls are CB? 0.5 x 0.5 = 0.25. Add all probabilities to one another and you get 0.75

 

[adrakdavra]

I hate probabilities , when I saw this Q I was like WT*