Answers are already included, but I would like to understand how they got those

Question

Answers are already included, but I would like to understand how they got those answers.

A man hemizygous for both hemophilia A and color blindness mates with a normal woman. If both disease traits are recessive, completely linked and on the X chromosome, what is the probability 1. that a mating between their children will produce a grandson with both hemophilia A and colorblindness? (5 points) Parents are Fl intercroSS: (Prob of Y) (prob of h c) - (1/2) (1/2)-1/4 2. that a mating between their children will produce a granddaughter with neither hemophilia nor colorblindness? ( 5 points)

Explanation / Answer

Let us denote a normal X chromosome by X-HC and recessive X chromosome by X-hc.

Since the man is hemizygous, meaning his X part of chromosome in XY contain the recessive X linked genes. Thus his genotype would be X-hc Y. Now gametes from man will have either Y chromosome or X-hc. Gametes from a normal woman will have X-HC chromosome, since she is not even a carrier, she has to be dominant homozygous.

Gamete

from

mother

As can be seen from the punnett square, all male offspring will have X-HC Y sex chromosome and all female offspring will have X-HC X-hc sex chromosomes.

1. When these offspring of F1 generation mate, for their son(F2 generation, referred to as grandson in question) to have both haemophilia A and colour-blindness, his sex chromosome need to be X-hc Y. Now probability of inheriting Y chromosome from father in F1 generation is 1/2. Similarly, probability of inheriting X-hc from mother in F1 generation is 1/2. Multiplying them we get the probability of the grandson to have the specified condition, that is equal to 1/4.

2. For their daughter to have neither haemophilia A nor colour-blindness, her genotype has to be either X-HC X-HC or X-HC X-hc. Now probability of inheriting X-HC from father in F1 generation is 1/2. Probability of inheriting X-HC or X-hc from mother is 1. Thus multiplying, we get the final answer as 1/2.

Please upvote if the answer cleared your doubts. If would be of great help to me.

gamete from father

Gamete

from

mother

Y X-hc X-HC X-HC Y X-HC X-hc X-HC X-HC Y X-HC X-hc

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