I need someone to explain to me why this pedigree chart is autosomal recessive and not sex-linked.
This is a question from Excel Success One and the answers say that for the condition to be sex-linked, the unrelated females in the second generation must be carriers. I used this reason in my justification, so I'm a bit confused. Sex-linked inheritance can occur even in non-blood-related individuals, right? Or must the inheritance of the 'disorder' be strictly between blood-related individuals?
Hi there!
By looking at the pedigree, we immediately know that if the condition were to be sex-linked, it would have to be recessive, as the mother in the very first generation is a carrier, but is not affected herself.
By filling out the pedigree (attached), you can see that the condition can be both sex-linked recessive and autosomal recessive.
Based on the answer from Success One (which I have put below), I think they are arguing that it would be unlikely that
"two of the unrelated mothers in the second generation must have independently both been carriers (as well as the daughter from the initial parents)", therefore it's more likely to be autosomal recessive.
If I had received this question in the exam and had filled out the pedigree and seen that it's possible for it to be sex-linked recessive, I definitely would've answered that, on the basis that sex-linked inheritance can occur in non-blood-related individuals. Unless the question states that the disorder is
super rare and something crazy like "less than 0.000000006% of the population has it, as long as you can prove that it is sex-linked (by filling out the pedigree and giving clear evidence to how it can be sex-linked) I think you'll be fine.
Answer from success one: The pedigree diagram indicates that only males are affected by the condition; however, as the female has to pass on the affected X chromosome to the son, two of the unrelated mothers in the second generation must have independently both been carriers (as well as the daughter from the initial parents) for this to be sex-linked inheritance. This would suggest the condition is autosomal recessive. Both of the initial parents could have been carriers, resulting in half of their offspring with the condition (ie. homozygous recessive) and their unaffected daughter and son could have been carriers (ie. heterozygous), as well as their partners.