Biology Genetics Problem -- Colourblindness is a recessive X linked trait

[TytoAlba95]

Homework Statement

Colourblindness is a recessive X linked trait. A couple, who are both blood type A and with normal vision have a son who is blood type O and colourblind. What is the probability that their next child would be a daughter and with blood type O and normal vision?

Relevant Equations

a. 1/2
b. 1/4
c. 1/8
d. 1/16

Ans is a.

I'm getting 3/32.

P= Probability of a daughter x Probability of O blood type x Probability of Normal vision
= 1/2 x 1/4 x 3/4
= 3/32

 

[Ygggdrasil]

A looks like the wrong answer to me.

3/4 is the correct overall probability of colorblind ness, but you need to account for the fact you've already included the probability that the child is female. Given that the child is female, what is the probability that she will be colorblind?

 

[TytoAlba95]

P= Probability of a daughter x Probability of O blood type x Probability of Normal vision girl
= 1/2 x 1/4 x 1/2
= 1/16

Thank you :)

 

[Ygggdrasil]

Still not correct. It may be helpful to write out a Punnet square for gender and colorblindness.

 

[TytoAlba95]

Therefore P= 1/8.
I was taking the father as XcY, which actually should be XY.

 

[Ygggdrasil]

Correct!