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I'm having trouble trying to understand TBR's answer to this, but anyways the question is: What is the chance that siblings III-2 and III-4 are carriers of the disease?
I assumed there was a 1/2 chance either sibling would be carriers since 2 out of 4 possible outcomes would be heterozygous. I don't understand how TBR said it would be 2/3. Could anyone explain?? This is page 306 of TBR.
EDIT: By the way, slashes mean carriers. I think for this question though, they are asking in general, what are the chances that the offspring would be a carrier (considering both parents are heterozygous for the disease).

I assumed there was a 1/2 chance either sibling would be carriers since 2 out of 4 possible outcomes would be heterozygous. I don't understand how TBR said it would be 2/3. Could anyone explain?? This is page 306 of TBR.
EDIT: By the way, slashes mean carriers. I think for this question though, they are asking in general, what are the chances that the offspring would be a carrier (considering both parents are heterozygous for the disease).
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