Assume the pedigree below is for a family carrying an X-linked recessive disease

Question

Assume the pedigree below is for a family carrying an X-linked recessive disease.

1)Calculate the probability that a boy offspring of III-1 and III-2 (?) will have the disease. Assume that individuals marrying into the family beginning in generation II do not carry the disease allele.

a)1/16

b)1/8

c)3/8

d)1/4

2) Using the same pedigree, recalculate the probability that an offspring of III-1 and III-2 (?) will have the disease if it is inherited as an autosomal recessive trait. Assume that individuals marrying into the family beginning in generation II do not carry the disease allele.

d)1/144

3)Assuming all of the same parameters as above, how will the answer change if the parents are interested in knowing whether the child will be a boy with the disease?

Explanation / Answer

(1)

Answer: a)1/16

In an X linked recessive inheritance, two copies of a disease allele on the X chromosome are required for an individual with two X chromosomes (a female) to be affected with an X-linked recessive trait.

Xh= x linked recessive allele

X=normal allele

XXh x XY = XhX, XhY, XX, XY

II-5 is ½ chance of being a carrier

III-2 is ¼ chance of being a carrier

For a heterozygous mom, ¼ chance of having a kid with x linked recessive illness

Therefore the total probability= ¼ x ¼ = 1/16

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