Tay sachs disease is a metabolic disorder that is inherited as an autosomal recessive. Individuals who are homozygous for the Tay-sachs allele die in infancy or early childhood. Carriers are healthy although they show some biochemical abnormalities. A couple who are both carriers of the Tay-sachs allele are planning to start a family. What is the probability that their first child will have Tay-sachs disease?

There is a 25% probability that their child will have Tay-Sachs disease.

Explanation:

We know that:

People that are homozygous for the Tay-Sachs allele die, that means that the two copies of that allele are affected.

The members of the couple are both carriers, which means that they are both heterozygous for the Tay-Sachs allele, so they did not die because only one copy of the allele's pair is affected.

If we make a Punnet square where A stands for the affected allele and a for the normal one, we can see that both members of the couple are Aa and that there is only a 25% of probability that their child will have Tay-Sachs disease, as well as a 50% chance of being a carrier.

║ A ║ a

A ║ AA ║Aa

a ║ aA║aa