(Genetics) Pedigree question! :(

[frog301]

The answer to the question is A. I originally answered B, to be homozygous dominant for female A. So if the originally male didn't carry the disease (X^H Y) and the female did (X^h X^h), the females would be X^H X^h. [I'm also confused as to why there aren't 2 girls and 2 boys, instead of the given 3 girls to 1 boy.]

But if that was the case for female A (X^H X^h), mixing with carrier male (X^h Y) would yield 1 boy + 1 girl WITHOUT the disease and 1 boy + 1 girl WITH the disease. How can she be a heterozygous carrier, if none of her offspring had the disease when she mated with a male carrier?

The reason I picked homozygous dominant for female A (option B) was because the rest of her offspring didn't carry the sex linked disease. Because it's recessive, it must be complete dominance of the X^H that masks the disease. ?????????

Answer

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[etp123]

Why does it have to be half boys half girls? Every time you have a child there's a 50% chance it will be a particular sex, not a guarantee it will be one and then your next will be the other. I'm sure you've seen plenty of families in your everyday life where they have all girls or all boys, or a disproportionate amount of one sex to the other. Having 3 girls and 1 boy isn't totally unbelievable statistically. Having 1000 kids and not a single one of them being boys is a different story where the statistics is actually worth noting.

Anyway, again just because female A carries the allele doesn't mean she is guaranteed to pass it on to her offspring. Punnet squares are a statement on likelihood not a guarantee. There's just the possibility her kids will inherit it. And so it's totally possible that she carried it but she happened to only pass on the good allele to her kids. You know there is no way she could be homozygous dominant because she MUST receive one chromosome from her mom, and her mom only had the (X^h) to offer. And dad was (X^H), so he could only give her (X^H). Therefore she has to be (X^H)(X^h)

 

[frog301]

Why does it have to be half boys half girls? Every time you have a child there's a 50% chance it will be a particular sex, not a guarantee it will be one and then your next will be the other. I'm sure you've seen plenty of families in your everyday life where they have all girls or all boys, or a disproportionate amount of one sex to the other. Having 3 girls and 1 boy isn't totally unbelievable statistically. Having 1000 kids and not a single one of them being boys is a different story where the statistics is actually worth noting.

Anyway, again just because female A carries the allele doesn't mean she is guaranteed to pass it on to her offspring. Punnet squares are a statement on likelihood not a guarantee. There's just the possibility her kids will inherit it. And so it's totally possible that she carried it but she happened to only pass on the good allele to her kids. You know there is no way she could be homozygous dominant because she MUST receive one chromosome from her mom, and her mom only had the (X^h) to offer. And dad was (X^H), so he could only give her (X^H). Therefore she has to be (X^H)(X^h)

Gotcha, thank you!