Bio Qpack 1: autosomal vs sex-linked

[br2pi5]

How do we figure out if a gene is on an autosome or sex-linked chromosome?

@NextStepTutor_1 since you're always so helpful!

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[PhilzCoffeeAddict]

The passage will or question will inform you. They'll either state it or mention something along the lines of, "All males in the group have said trait." This clearly indicates that it is sex-linked specifically to males. If, lets say for this learning purpose, P gene results in female sex determination and all female have red hair. This could mean that the red hair locus is on the female P gene. Therefore it is sex-linked.

I believe answer is D here because it doesn't indicate whether the gene is autosomal or sex-linked. Unless its in the passage, I only see a question here.

 

[NextStepTutor_1]

Hi @br2pi5 -

Yes, as @PhilzCoffeeAddict points out, you need information from the passage or question to figure this out, and they don't give you any such information in the passage either, which is why the correct answer is D. The information they give you could be relatively explicit or relatively implicit -- for instance, if you can pick up on a pattern where males are much more likely to be affected than females (although this doesn't necessarily mean that all males are affected), it's likely that you're dealing with an X-linked trait.

In general, when determining modes of inheritance, there are two quick questions you need to ask: (1) does a trait skip generations? (if yes, it must be recessive; if not, it is likely to be dominant [although recessive traits can manifest in multiple consecutive generations too, depending on the parental genotypes and random chance]) and (2) are males disproportionately more affected than females? (if so, the trait is X-linked, and likely to be X-linked recessive -- X-linked dominant is a theoretical possibility, but it has some slightly distinct inheritance trends).

Hope this helps!!

 

[br2pi5]

Hi @br2pi5 -

Yes, as @PhilzCoffeeAddict points out, you need information from the passage or question to figure this out, and they don't give you any such information in the passage either, which is why the correct answer is D. The information they give you could be relatively explicit or relatively implicit -- for instance, if you can pick up on a pattern where males are much more likely to be affected than females (although this doesn't necessarily mean that all males are affected), it's likely that you're dealing with an X-linked trait.

In general, when determining modes of inheritance, there are two quick questions you need to ask: (1) does a trait skip generations? (if yes, it must be recessive; if not, it is likely to be dominant [although recessive traits can manifest in multiple consecutive generations too, depending on the parental genotypes and random chance]) and (2) are males disproportionately more affected than females? (if so, the trait is X-linked, and likely to be X-linked recessive -- X-linked dominant is a theoretical possibility, but it has some slightly distinct inheritance trends).

Hope this helps!!

I guess I thought since the passage said that the dewlaps appear on males and those are the ones that reflect UV, then my mind went to it must be on a Y chromosome (not X chromosome bc no females had dewlaps and chances are if it were a recessive X-linked trait, it would appear often on males but could also appear on females)... maybe I thought too much about it?

 

[br2pi5]

Also, for this question I think the wording got me... I crossed the answer that says "differentiation of neighboring cells" because if neighboring cells were induced when the optic cup failed to develop then the lens would still form right?

 

[aldol16]

I guess I thought since the passage said that the dewlaps appear on males and those are the ones that reflect UV, then my mind went to it must be on a Y chromosome (not X chromosome bc no females had dewlaps and chances are if it were a recessive X-linked trait, it would appear often on males but could also appear on females)... maybe I thought too much about it?

Y chromosome genes are very rare since the Y chromosome is very small. Sex-linked traits are usually inherited in an X-linked manner. When considering sex-linked traits, also consider random X-inactivation that occurs in females - i.e. phenotypes can be mosaic, where even one copy of a recessive gene can be expressed phenotypically in certain cells/tissues.