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Using a punnett square, indicate the probability that a brother of your friend would have hemophilia, assuming that her mother carries two functional copies of the gene involved in this sex-linked recessive disorder. explain your answer.

Answer :

mathmate
Let
X=functional (affected),
x=unaffected
Haemophilia is an X-linked recessive disease, hence 
For females, it takes XX to have the disease, and Xx will be a carrier.
For males, it take Xy to have the disease, and therefore cannot be a carrier.

Punnett square 1
(father has haemophilia)
      X    X
X   XX   XX
y    Xy    Xy
=>  both son and daugher have haemophilia

Punnett square 2
(father does not have haemophilia)
      X    X
x    Xx  Xx
y    Xy  Xy
=>  daughter is carrier, but son has haemophilia

Comments on outcomes where the mother has haemophilia, it means that both X chromosomes are affected:

If the father has haemophilia (case 1) daughter and son will always have haemophilia.

If the father does not have haemophilia, (case 2) daughter will always be a carrier, since X is recessive in the presence of x.

However, since the father passed on the y-gene to the son, the x-gene must come from the mother resulting in X-y.  Since there is no x-gene to suppress the expression, haemophilia will be expressed in the son.

In conclusion, the son of a mother with haemophilia (XX) will ALWAYS have haemophilia, therefore probability is 100%.

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