Topscore question about genetics

[HangOnSloopy]

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I think this is from Bio test 2, but I'm not sure.

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

A 0

B 1

C 2

D 3

E 4

The answer is D but I cant seem to wrap my mind around why. Any help would be appreciated. My test is on Thursday, wish me luck!

 

[Rezdawg]

Im definitely no expert here, so someone shut me up if Im off.

First off, the gene causes spontaneous abortion. Therefore, if her child is born, then the child does not have this gene, otherwise it wouldnt have made it that far in the process. However, this is irrelevant to the question at hand because...

The father's sex chromosomes determine the sex of the child. If he passes his X, then the child will be female. If he passes his Y, then the child will be male.

Therefore, regardless of the lethal gene the mother has, there will always be a 50% chance the child is female and 50% chance the child is male. If she has 6 kids, odds are that 3 are male and 3 are female.

Basically, they are trying to throw you off with the fact that the mother carries a sex-linked lethal gene.

 

[HangOnSloopy]

DOH! this is total proof that the answer is usually right there, and that thinking too much does no good. Thanks Rezdawg, the help is much appreciated.

 

[Awara]

Hey Guys I think you have got it wrong. Answer is not 3. This is how it works I think. The women is carrier so only the boys will be affected here. So out of 6 there will be 3 boys. Now half of these boys will get "lethal gene" and will result in abortion so the answer would be 1.5 boys will result in abortions. you can't have 1.5 boys so probably 1 boys will born and two will result in abortions. Answer choices suck here. This is why I don't like some of the TS questions. this is a trick question. Just think a little hard.

 

[Rezdawg]

Awara, the woman has 6 children already. They are alive, making it passed the spontaneous abortion that takes place. If they were going to be aborted, it would have happened at the very beginning of zygote development, thus the "spontaneous abortion". Since they are alive, we assume the males do not carry the lethal allele.

Yes, it is true that out of every 2 boys, one of them will be prematurely aborted. It is also true that 1 out of every 2 girls will be a carrier. However, this is irrelevant to the question.

The odds of the offspring being male or female is still 50%. She already has 6 kids, therefore 3 of them are boys and 3 are girls.

Now, if it asked the question, "How many females carry the disease?", then the answer would be 1.5.