Imagine that you are a genetic counselor, and a couple planning to start a famil
ID: 83141 • Letter: I
Question
Imagine that you are a genetic counselor, and a couple planning to start a family comes to you for information. Carl was married once before, and he and his first wife had a child with Tay-Sachs disease, an inherited recessive disorder. The brother of his current wife, Erica, died of Tay-Sachs. Carl and Erica wish to have two children. What is the probability that they will have two children without Tay-Sachs? (Neither Carl, Erica, nor their parents have Tay-Sachs.) Enter your answer as a fraction - e.g., 1/2 - and do not put anything else in the box.
Explanation / Answer
Tay-Sachs disease is autosomal recessive. Let the symbol for Tay-Sachs disease be 'tt'. So, a person with 'tt' genotype will have the disease, and a person with 'Tt' or 'TT' genotype will not have the disease.
Carl had a child with Tay-Sachs disease with his first wife. This means that Carl is a carrier of the disease. So, his genotype will be 'Tt'.
The brother of his current wife was having the genotype 'tt'. It is given that Erica's parents were not having the disease. This means that both Erica's parents were carriers with genotype 'Tt.
Now the probability of having the disease with Carl and Erica depends on Erica's genotype, which she inherits from her heterozygous parents. She can be either 'TT' or 'Tt'. If Erica's genotype is 'Tt'', then there is 1/4 chance of having a child with Tay-Sachs.
But if Erica's genotype is 'TT', then there is no chance of having the disease.