Another heredity question (1 Viewer)

[anonymousjmn]

Hey guys, got a question from 2020 James Ruse biology trial. Made the mistake of thinking C was the answer, assuming that the mother was heterozygous dominant. - the answer was D, and the mother is homozygous recessive.
From reading the question, how am I supposed to determine if the X-linked trait is recessive or dominant?

Thanks :3

 

[fallenleaves]

it seems impossible to gather if it's dominant or recessive based only on the information given in the question, so i'm thinking that they might've expected you to already know the inheritance pattern of the disorder?

i've come across probably three or four red-green colour blindness questions in the past few days while revising, so it seems like something that's tested really often. i don't recall this ever being mentioned in class or in the syllabus but it's such a common question that i guess it's just assumed knowledge 🧍

 

[rrrooster]

View attachment 36804

Hey guys, got a question from 2020 James Ruse biology trial. Made the mistake of thinking C was the answer, assuming that the mother was heterozygous dominant. - the answer was D, and the mother is homozygous recessive.
From reading the question, how am I supposed to determine if the X-linked trait is recessive or dominant?

Thanks :3

These questions are super easy once you realise the pattern with X-linked disorders, so here's how I approached this question:

First, I would draw a simple pedigree on the side; in this case, shade in the mother, leave the father and son blank for now.

Let the dominant allele for red-green colour blindness be b and the recessive allele for red-green colourblindness be B. Since this is an X-linked trait, we must depict the alleles of the genotypes for the trait as a superscript on XX (for females) or XY (for males).

We know that the mother is colourblind, and the father is not. Hence, the mother must have the genotype X^b X^b.
Since the father only has ONE X-chromosome from which he can inherit a colour-blind gene, and he is NOT colourblind we know that his genotype must be X^B Y.

Then, draw a simple punnet square to see the possible offspring that can be produced from the mother (X^b X^b) and father (X^B Y). [See attachment below]. From the cross, we can observe that all male offspring (remember: genotype XY), are DOMINANT for the allele for red-green colour blindness, hence there is a 100% chance of their son having red-green colourblindness. If they asked what the chance that their daughter had colourblindness, however, the answer would be 0%, since their daughter is a heterozygous CARRIER and does not actually show the trait (but she would show the trait if she were a homozygous X^b X^b, which she is not in this case.)

Another way I like to differentiate between X-linked dominant and X-linked recessive traits is by recognising that if a female (mother) is affected, then ALL male offspring are affected as they can only inherit ONE copy of the gene from their mother since they only have ONE X chromosome. BUT this rule is only effective for pedigree analysis questions, and not genotype and probability questions like in this case, as we have no knowledge of other siblings, cousins, extended family history, etc.

Hope this helps!

 

[rrrooster]

That’s the problem he had with the question. The question doesn’t specify whether it’s a dominant or recessive trait. It expects you know to it

In this case, it does not matter whether or not the trait is dominant or recessive. We know that since the trait is X-linked, and the mother IS in fact red-green colour blind, she can only have one genotype that actually shows she is colourblind; that is homozygous X^b X^b. Since females have two X chromosomes (XX) from which they can carry an allele for a certain characteristic, this is the only genotype possible that shows she is colourblind. If she had the genotype X^B X^b, she would be a carrier, and not actually show the trait.

Given the "man has normal colour vision" we know that the father does NOT have the trait and hence can ONLY have the genotype X^B Y.

I recommend revising X-linked traits if you're still confused.

 

[airora]

In this case, it does not matter whether or not the trait is dominant or recessive. We know that since the trait is X-linked, and the mother IS in fact red-green colour blind, she can only have one genotype that actually shows she is colourblind; that is homozygous X^b X^b. Since females have two X chromosomes (XX) from which they can carry an allele for a certain characteristic, this is the only genotype possible that shows she is colourblind. If she had the genotype X^B X^b, she would be a carrier, and not actually show the trait.

Given the "man has normal colour vision" we know that the father does NOT have the trait and hence can ONLY have the genotype X^B Y.

I recommend revising X-linked traits if you're still confused.

Sorry, i'm not following. It does matter whether the trait is dominant or recessive. If the disease was dominant X-linked, then the mother could have either the genotype X^B X^B or X^B X^b. These genotypes would also show that she is colourbind. The father would then have the genotype X^b Y. The probability for son to have colour blindness would thus depend on the genotype of the mother.