family tree showing the pattern of inheritance of colour vision deficiency (some
ID: 9581 • Letter: F
Question
family tree showing the pattern of inheritance of colour vision deficiency (sometimes called colour-blindness). Colour vision is controlled by a single X-linked gene, with two alleles. The allele associated with colour vision deficiency is recessive to the allele associated with normal colour vision. Individuals shown in the family tree either have normal colour vision (shown with circles for eyes) or have colour vision deficiency (shown with crosses for eyes). Females () are represented with long hair, and males () with short hair.
(c) Matthew and Paul are both baby boys, and it is too early to determine whether they have normal colour vision. Given the knowledge that they are male, calculate the statistical probability that:
(i) Matthew will have colour vision deficiency
(ii) Paul will have colour vision deficiency
Explanation / Answer
All right, so we can extract from the question several key pieces of information.
-It is X-linked
-It is recessive
We can work certain genotypes out by basic logic.
Holly, (middle row), is double recessive.
Graham, (middle row), is a mystery from his parents, but if we look at his kids, we see that his son Keith has the trait because his only X chromosome comes from his mother, who has ONLY recessive alleles. There is no way he was getting a dominant allele from either. His father HAS to donate the Y Chromosome, since he is a boy, and that leaves the only source for the X chromosome to be from the mother. So we can safely say that ANY boy this couple has MUST display the trait.
Therefore Matthew will indeed be color-blind with a certainty of 100%.
Now onto Paul.
Paul's father automatically does NOT have the recessive trait, or else he would display the trait, since ALL males have only 1 X Chromosome.
More importantly, Jenny, the mother, does NOT express the trait, but her daughter DOES express it. This means, that since the father doesn't have the trait, and the daughter does, the mother MUST have passed it on. But since she doesn't display the trait herself, we can safely assume she has one dominant allele and one recessive allele, Xx.
Since the Dominant allele father and the half-and-half mother follow basic genetic inheritance, the son MUST inherit the Y chromosome from his father. And there is a 50/50 chance of either X chromosome from the mother to get passed on, since the mother passes on 1 X chromosome.
Therefore there is a 50% chance of Paul becoming colorblind.
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