TBR Pedigree Question

[90210]

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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).

 

[90210]

ahhh. Wrong forum. Could mods move this to Q&A please. Thanks.

 

[SN2ed]

Thread moved to Study Q&A.

 

[aln012]

is there anything more to the question? Like, are Homozygous dominants fatal? Because if that is so, and assuming that the siblings are alive, then the chances that they would be carriers would be 2/3rd.

 

[MT Headed]

since 2 out of 4 possible outcomes would be heterozygous.

Except there aren't four possible outcomes. If the two daughters had the "aa" genotype they would have the disease phenotype, which would be really obvious and they would be dark circles.

Since they are white circles, we know they aren't "aa". There are three possible outcomes, "AA", "Aa", and "aA". All three produce the phenotype of no disease, and two of the three possible outcomes would be heterozygous.