10-3 Consider and autosomal recessive disorder with a population frequency of 1/

Question

10-3 Consider and autosomal recessive disorder with a population frequency of 1/10,000 in individuals of African descent. What would be the approximate risk that a child of first cousins would have alleles identical by descent for this disorder assuming that only one of the child's grandparents was of African descent? 10-4 A woman has a child with Duchenne muscular dystrophy. She has no prior children, no brothers and no prior history of muscular dystrophy. What is the likelihood that she is a carrier for Duchenne muscular dystrophy, as opposed to her son's having a new mutation?

Explanation / Answer

Answer to Q1

As the disease considered is autosomal recessive disorder, so for person to be suffering from that needs to have a pair of recessive alleles.

Chances of the grandparent to get that allele = 1/10,000 = 0.01 %

The chances of grand children to get an allele from a grandparent = 25% (Equal chances from each grandparent)

So, the actual chances to have that allele in the child = 0.25 x 0.0001 = 0.000025 = 0.0025%

Answer to Q2

Duchenne muscular dystrophy is caused by a recessive gene which is linked to X-chromosome. As the child (son) is suffering from the disease and we do not consider any new mutation in the son, the likelihood of mother being carrier is 100 % because son receives X-chromosome from his mother and any kind of allele (even recessive) on that has to be expressed in the son.

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