TopScore Test 3 Bio Question 34

[oxxomxxo]

Hey can any of you guys please help me with this....im not really getting it. thanks.

A woman carries a sex-linked lethal gene that causes spontaneous abortions. She has six children. how many of her children would you expect to be boys.

A.) None
B.) One
C.) Two
D.) Three
E.) Four

thanks in advance.

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[Contach]

woman is XX
if she carries the gene, but survived: the gene must be rececessive. she must have one of the affected genes.
Thus Xd for death
XXd is her genotype

XXd x XY (man who we assume to be normal, and must be normal because if he had an Xd then he would have died in the embryo and would not be alive today)
they will produce these types of kids:
XX
XY
XdX
XdY
Clearly XdY will die in the embryo.
Thus each kid will be either XX XY or XdX (another female carrying this same gene)

If she has 6 kids, you can expect them, or assume probabilities to be equal, to be 2 XX, 2XY, 2 XdX
Since XY are the boys, and the XX's are the girls. 2/6 are boys, and 4/6 are girls
answer C? hope that's right.

 

[alanan84]

That sounds good. Why doesn't XdX die?

 

[oxxomxxo]

It makes much more sense now, and thanks for your time.

 

[Mamona]

That sounds good. Why doesn't XdX die?

Because this genotype is recessive, you need both XdXd to be lethal or in the case of the males only one.
The mother genotype is XdX and obviously she is alive, this is the same genotype of at least 1 of the females if you do the punnet square.

Hope this help