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A specific X-linked recessive allele is lethal. Males that inherit the recessive allele and females that are homozygous recessive will abort before birth. What is the probability that a heterozygous mother will give birth to a child with only one copy of the recessive allele?

How would you do this question? I know what bootcamp's answer is (and their explanation) but I never really did punnet squares with XY alleles for some reason. Is this a typical thing to do?


My way of thinking was simply that you have 1/2 probability of getting a girl and then another 1/2 probability of her inheriting the recessive allele from her mother. The boy doesn't matter because he is either aborted or doesn't have the recessive allele. I know there's something wrong with my thinking here but what is it?


Another iffy-ish one is this: A scientist thinks she has discovered a new species of beetle. Which of the following would help determine if the beetle is a new species or not?

Answer was attempting to breed it with other known species -- but I put sequencing DNA. My logic was that breeding doesn't always work (lions+tiger, plants, asexuality), but DNA sequencing does. For example, I learned in uni that for nematode diversity DNA sequencing is the way that the vast majority of nematodes were identified.
 
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A specific X-linked recessive allele is lethal. Males that inherit the recessive allele and females that are homozygous recessive will abort before birth. What is the probability that a heterozygous mother will give birth to a child with only one copy of the recessive allele?

How would you do this question? I know what bootcamp's answer is (and their explanation) but I never really did punnet squares with XY alleles for some reason. Is this a typical thing to do?
Males who have the X-linked allele will abort before birth; therefore, we can be certain that the father would lack the X-linked allele. Moreover, the question asks what is the probability that the mother will give birth to a child with the X-linked allele. Only females can be born with the X-linked allele, therefore the question mandates that the child need be a girl. Therefore, of the four theoretical genotypes that result from the punnet square, we can eliminate the choice of a boy who inherits the lethal allele; he will be aborted before birth.

This leaves us with three possible children that can be born; a male who lacks the recessive allele, and a pair of girls, one of whom will have the lethal allele. Therefore, the answer is 1/3rd, or approximately 33.34%.

My way of thinking was simply that you have 1/2 probability of getting a girl and then another 1/2 probability of her inheriting the recessive allele from her mother. The boy doesn't matter because he is either aborted or doesn't have the recessive allele. I know there's something wrong with my thinking here but what is it?
Your error is that you're operating under the assumption that there is a 50% probability that the child born will be a girl; any boy who inherits the lethal allele will be aborted. Therefore, 2/3rd, or approximately 66.67% chance that a child that is BORN will be a girl.

Please note that the probability of a zygote inheriting a pair of X alleles is different from the probability of a child actually being born with such a genotype.

Another iffy-ish one is this: A scientist thinks she has discovered a new species of beetle. Which of the following would help determine if the beetle is a new species or not?

Answer was attempting to breed it with other known species -- but I put sequencing DNA. My logic was that breeding doesn't always work (lions+tiger, plants, asexuality), but DNA sequencing does. For example, I learned in uni that for nematode diversity DNA sequencing is the way that the vast majority of nematodes were identified.
This is based around the biological species concept, which is the most straight-forward method of determining whether a type of organisms belong to different species. Simply put, if the beetles cannot sexually reproduce, then they must belong to different species. However, being able to reproduce does not mandate that do belong to the same species.

Determining phylogenetic diversity can be greatly facilitated by sequencing DNA, and is particularly valuable when one is working with organisms that reproduce asexually; e.g., binary fission in prokarytoic life. There may indeed be some leeway in this question, but if you're dealing with chordate vertebrates you can take it for granted that the biological species concept is likely the correct answer.
 
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