Help with pedigree question

[virtualmaster999]

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Hey everyone I'm stuck on this pedigree question while studying. So I know it's gonna be recessive but I'm thinking it is x linked dominant since there's more females that have it. What's a good way to distinguish autosomal and x linked dominant?

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[kholley25]

This schematic shows an autosomal dominant trait. We know this particular trait it cannot be X-linked dominant because every daughter of a father that has the dominant X allele will also have the phenotype/trait since the father HAS to donates his only X to a daughter.

X+/Y mates X/X
A daughter would have to be X+/X or X/X+

Generation 3's parents both show the trait but one of their daughters does NOT. This means that this trait cannot be X-linked dominant because, as said above, the phenotype would be shown in the daughter.

 

[FeralisExtremum]

I agree with kholley25's explanation above. Everything you need to answer this question is pretty much in Generation 3: it can't be autosomal recessive because then both offspring of two affected parents would have to be affected as well. It can't be X-linked dominant because all female offspring of an affected father would have to be affected. It can't be X-linked recessive because all offspring of two affected parents would have to be affected. This must be a case of autosomal dominant where both parents are heterozygous.

The giveaway to quickly identifying the answer is both parents affected but not all offspring affected (autosomal dominant).

 

[virtualmaster999]

Thanks everyone!


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