DAT Destroyer BIO Question Please

[marymatthews]

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I have a quick question from the DAT Destroyer Book please... Bio section.
Question 101, A normal woman had a colobring mother. She marries a colobring man. WHat is the probability that her first daughter will be colorblind? the probability the daughter is colorblind is 50%
But Question 120, Bill and Ann have normal vision, but their fathers are both colorblind. What is the probability that their first child will be a normal boy? the probability the son is colorblind is 25%, because you multiply 1/2 x 1/2 = 1/4
How come you dont do that for Question 101? What's the difference?
They seem pretty much the same question to me. One is for a daughter, the other for a son. Why do you need to multiply by 1/2 for question 120 then?

 

[Huskk]

Question 101: Colorblindness is an X-linked recessive gene. The mother is Xx and the father must be xY. When you do a punnet square you only need to look at the female row since we are talking about the daughter. The father will always give his x-linked recessive allele but the mother has two choices: the dominant allele or the recessive allele. The only two outcomes then are Xx and xx, with xx being the colorblind daughter.

__X x
x Xx xx
Y ------


Question 120: Since they both have normal color vision, Bill must be XY and Ann has to be Xx (her father has to pass on his recessive allele so that Ann has 2X chromosomes). When you do a punnet square:

__X x
X XX Xx
Y XY xY

One of the 4 possibilities will result in a boy with the dominant X allele, meaning that he will have normal colorvision.

I think the problem was the way you're reading it. The first problem is just concerning the daughters and the second is concerning all possibilities.

 

[UCB05]

My guess about the reason behind your confusion is that the first question wants to know the probability of colorblindness, given already that the child will be a girl ("what is the probability that her first daughter..."). The second question wants to know the probability of getting a child that is both normal and a boy (what is the probability that their first child...").