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What can you conclude about the risk (with each pregnancy) that a couple could have a child affected with an autosomal dominant disorder if...

One parent is affected (heterozygous) and the other parent is unaffected.

I did a punnet square and concluded it was 50 percent because half were Aa and half were aa.

But a classmate told me to line up Aa and aa and conclude that there is only a 25 percent chance that a child is affected because there is only one capital or dominant A and three are "a". Which do I do?

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closed as off-topic by WYSIWYG, AliceD, Nandor Poka, fileunderwater, Chris Mar 24 '15 at 17:12

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Your classmate is forgetting that the child will have to inherit one allele (A or a) from each parent. Suppose the dad is affected---so his genotype is Aa. He has a 50% chance of passing the A on to his kid. That means the kid has a 50% chance of being affected.

If your classmate was right, then it would be possible for both the kid's alleles to come from just one parent. You have to pick one allele from each parent.

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Agree with the answer by @A.Kennard. Here's a punnet square to clarify your answer further. enter image description here

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