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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?
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!