Genetic Question: Different scenarios

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lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Genetic Question: Different scenarios

Post by lotus0618 » Thu Jul 09, 2020 11:02 pm

I came across this question "IF a woman who is a heterozygous carrier for the condition X has a son with a normal man, what's the probability this son will suffer from this condition X?". Here are my questions for you:

1) why is the probability 50%, instead of 25%? the question asks for a male who is affected by this condition. Based on the Punnett square, it's 25%
2) how is this question different from "What's the probability of having a son who is also affected by the condition X?"
3) How is question #2 different from "What's the probability of having a child who is affected by this condition?" is the Prob 25% because based on the pun square, only 1 child out of 4 children is affected.

Thansk!
NS_Tutor_Yuqi
Posts: 57
Joined: Fri May 29, 2020 11:43 pm

Re: Genetic Question: Different scenarios

Post by NS_Tutor_Yuqi » Sun Jul 12, 2020 2:03 pm

Hi! Is there any other information that's provided? From the answer choices you've given, it seems that we're dealing with an X-linked recessive disorder here, since an autosomal disorder would lead to a 0% chance that any offspring would have it, and a dominant disorder would render it impossible for the woman to simply be carrier of the disease.

1. If the assumptions I made above are correct, then the answer would be 50% because we're presupposing that the child will be a son. Since that locks in the Y chromosome from the father, the only two options come from the two X chromosomes from the mother, one of which contains the gene for the disorder. Therefore, there's a 50% chance that the son will inherit the disorder.
2. This second wording of the question makes us also factor in the likelihood of their child being a son, instead of being told that their child would definitely be a son in the original wording of the question. In this case, the chance of the child inheriting the disorder would be 25%, since now we have to add in the two outcomes where they have a daughter instead.
3. This is not all that different from the second question since the daughters cannot get the disease (even if they inherit the gene carrying the disease, they would only be carriers), and therefore the probability is still 25%.
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Re: Genetic Question: Different scenarios

Post by lotus0618 » Sun Jul 12, 2020 6:58 pm

OH wow thank you so much! The wordings really tripped me up. What you said about "PRESUPPOSING" really clicked to me.
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