Genetics Question from EK Bio

[happily123]

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If someone could help with this question, I would be so grateful!

Colorblindness is a sex linked recessive trait. If a colorblind man and woman that is a carrier for the trait have two girls and two boys, what is the probability that at least one of the girls will be colorblind?

A. 0%
B. 50%
C. 75%
D. 100%

The answer is C.

Thanks!

 

[mauroz]

Ok first form a punnet square for a sex linked trait, since they are only asking about the girls, you don't have to worry about filling in the genotypes for the boys. Check out the attached file. So if they only had one girl, the chance would be 50%. Since they had two, the chance of the second one having colorblindness is also 50%. So, 50% + .5(50%) = 75%. For this question you can also look at it like this...there's a 50:50 chance of a girl having colorblindness, so if they have more girls, it will increase the chances of having a girl with colorblindness but does not guarantee it, so you know it will be between 50% and 100%. Hope this helps.

 

[erythrocyte666]

If someone could help with this question, I would be so grateful!

Colorblindness is a sex linked recessive trait. If a colorblind man and woman that is a carrier for the trait have two girls and two boys, what is the probability that at least one of the girls will be colorblind?

A. 0%
B. 50%
C. 75%
D. 100%

The answer is C.

Thanks!

I thought of it like this: from punnett square chance is 1/2 for 1 girl; so:
what's probability that neither of the girls have it? - it's 1/2 x 1/2 = 1/4
Probability of at least one having it is simply: 1 - 1/4 = 3/4