# EK bio number 193 Lecture 9

#### Steelersfan2009

Ok i don't understand this question

Color-blindness is a sex linked recessive trait. A woman who is a carrier for the trait has two boys with a color blind man. What is the probability that both boys are color-blind.

From my own punnet square. I thought that because she is a carrier that she would have a wild type X chromosome and a Xc (recessive carrier chromosome) The man has Xc Y ..

From my punnet square I get that there will be 1/4 of a chance to get Xc Y in the progeny so 25% chance a boy will have this disease.. EK says 50% because the wild type chromosome can also be passed on an Xc to another progeny???

Ends up with 25% as answer since .5 x .5 = .25

Been a while since my genetics class lmao these use to be easy.

Thanks

#### wanderer

10+ Year Member
5+ Year Member
Ok i don't understand this question

Color-blindness is a sex linked recessive trait. A woman who is a carrier for the trait has two boys with a color blind man. What is the probability that both boys are color-blind.

From my own punnet square. I thought that because she is a carrier that she would have a wild type X chromosome and a Xc (recessive carrier chromosome) The man has Xc Y ..

From my punnet square I get that there will be 1/4 of a chance to get Xc Y in the progeny so 25% chance a boy will have this disease.. EK says 50% because the wild type chromosome can also be passed on an Xc to another progeny???

Ends up with 25% as answer since .5 x .5 = .25

Been a while since my genetics class lmao these use to be easy.

Thanks
I'm not sure I'm understanding you, but, you are already given the fact that there are two boys. Since the gene is on the X chromosome you ignore the father since he doesn't pass his X to his sons. So there's a 50% chance that one boy will be colorblind. There is then a .5*.5 = 25% chance that both will be colorblind.

Also check this thread out (similar but different question):

#### ratatat

I guess the question can seem ambiguous, but I think what they're asking is what % of boys that could be born would be color-blind, not what % of kids that could be born would be color-blind boys.

Therefore, pretend that the couple can only have boy children/only do a percentage for boys. You end up with 50% and then 25% for two boys.

OP
S

#### Steelersfan2009

I'm not sure I'm understanding you, but, you are already given the fact that there are two boys. Since the gene is on the X chromosome you ignore the father since he doesn't pass his X to his sons. So there's a 50% chance that one boy will be colorblind. There is then a .5*.5 = 25% chance that both will be colorblind.

Also check this thread out (similar but different question):