Question

Imagine you are working as a genetic counselor. A married couple makes an appointment to speak with you. Both have a genetic disorder, but they already have one child who does not have the disorder. They would like to have more children and were told by a friend that since their first child was normal their future children would be normal as well.

Question: Explain to them the probability of future offspring being normal or having the disease (assume that this is an autosomal disease). Be sure to tell them whether the disease is brought about by a dominant or recessive allele and what fraction of their children (if any) could be carriers for the disease.

Answer 1

The disorder is autosomal dominant.

Normal = aa

Affected = AA, Aa

If the parents are Aa, then they would be affected and there are 25% chances that their child would be unaffected. The cross is as follows -

In future also, there are 3/4 or 75% chances that the child will be affected and only 1/4 or 25% chances that the child will be normal.

Aa × Aa

Gametes	A	a
A	AA, affected	Aa, affected
a	Aa, affected	aa, normal