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#1
12-11-2012
 USMLE Forums Scout Steps History: Not yet Posts: 10 Threads: 5 Thanked 2 Times in 2 Posts Reputation: 12
Calculating the probability of two traits in genetics!

How do I do this question?

Color-blindness is a recessive, X-linked trait. A couple, who are both blood type A and who both have normal vision, have a son who is blood type O and color blind. What is the probability that their next child will be a daughter who is blood type O and has normal vision?
A. 1/2
B. 1/4
C. 1/8
D. 1/16
E. Zero - not possible

How do I do this problem? I got the answer as 1/4 while the correct answer, according to the book is 1/8.

#2
12-11-2012
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 358 Threads: 22 Thanked 204 Times in 116 Posts Reputation: 214

1)probability of having a girl is 50%,boy is 50%

2)they are both AO,therefore they have chances of 25% AA child,50% AO and 25% OO

chances of a girl with OO is 50% * 25%=1/8
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#3
12-11-2012
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 333 Threads: 162 Thanked 284 Times in 147 Posts Reputation: 294

Since both parents are type A and their first child is type O, you know that both parents are heterozygous for blood type A, meaning that one chromosome they have is A and the other is type O. For a child to be blood type O, each parent has to give their type O chromosome to the child. There is a 1/2 chance that one parent will give the recessive O chromosome, so for both parents to give their recessive allele the probability is 1/2 * 1/2 = 1/4

Colour blindness is an X-linked condition, that means that women can be carriers but not manifest the colour blindness because they have one "good" X chromosome which masks the recessive colourblindness gene on the other X. The father cannot have a colourblindness gene or else he'd be colourblind himself, since he's only got 1 X-chromosome. Because dad has healthy eyesight, any girls he has will have healthy eyesight too, since his X will always be healthy and mask the X from mom if she happens to give the recessive colourblind gene. So the probability that the girl will have normal vision is 1/1.

However, it is not a given that the baby born will be a girl, there's a 50/50 chance that it'll be one or the other sex, so we need to consider the probability involved with the child being a girl. this probability is 1/2.

So now we multiply all our probabilities together to calculate what the chances of a normal visioned, type O blood typed girl is:

Normal vision (1) * blood type O (1/4) * girl baby (1/2) = 1/8

 The above post was thanked by: Taayseer (08-23-2014)

 Tags Genetics-, Step-1-Questions

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