DAT bio genetics question

[ddsshin]

this is from the destroyer by the way.
q. bill and ann have normal vision, but their fathers are both color blind. what is the probability that their first child will be a normal boy?

i thought it was 50% because 1 out of 2 son will be colorblind, but the answer says that it is 25%..

ok and this one is very similar

q. a woman who is seemingly normal marries normal man. woman's father is hemophiliac. what is probability that couple's son will have hemophilia?

i answered 50% and it was correct.

so, I don't know why the first question answer is 25%...
in the book it says like probability to have boy is .5 and prob to have colorblind boy is .5 and when you multiply them you get .25 which is 25%..

IS the answer different because the first question ask for the 1st son ????
Very confused.. help!!

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[mathcod]

Nevermind, I see your confusion. However, I wrote out an explanation already so you can look at it if you want. However, i demand a cookie.

In Q1, you said "i thought it was 50% because 1 out of 2 son will be colorblind," but the question asked for the probability of the first child, but you assumed you were given information that the child was a boy when you weren't.

In Q2, the question asks for the probability for the first son, that is you were given that the kid was already XY.

This is not a problem of genetics, this is a reading comprehension error.

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Draw a Punnett Square and think about it.


Color blindness is a "sex-linked trait" only affecting the X chromosome.

in the following, B represents X carrying the blindness

BY x XX = BX for mother (she's a carrier)

BY x XX = XY for father

BX x XY

=BX, BY, XX, XY

We want BY, so proba = 1/4 = .25

 

[jdm789]

yea 25% is the right answer. the question asks what the is chance their CHILD is born with the trait, so you have to consider the females in your answer too. If it read what is the probability that their SON has the trait, then it would 50%