# Genetics Probability question: ?

Tay-Sachs disease (TSD) is an inborn error of metabolism that results in death by the age of two. It is caused by a mutation in the HEXA gene located on chromosome 15. You are a genetic counselor and one day you interview a phenotypically normal couple. The husband had a female first cousin on his father’s side, who died from TSD. The wife had a maternal uncle, who died from TSD. There are no other known cases in either of the families and none of the matings were/are between related individuals. Assume that this trait is very rare. [Note: a first cousin is a child of one’s parent’s sibling.]

(a) What is the probability that the wife is a carrier for TSD?

(b) What is the probability that the husband is a carrier for TSD?

(c) What is the probability that both of them are carriers for TSD?

(d) What is the probability that neither of them are carriers for TSD?

(e) What is the probability that one of them is a carrier and the other one is not?

### 1 Answer

- ZirpLv 72 months ago
"HEXA gene located on chromosome 15" means you can disregard male and female. We're talking about a recessive disease

"The husband had a female first cousin on his father’s side, who died from TSD" means both her parents were carriers, so one of the husband's grandparents must have been a carrier. 50% of it being passed on to his parent, 25 % of it reaching him. So answer B is 25%.

"The wife had a maternal uncle, who died from TSD."

Assuming we're not talking about halfsiblings or babies resiulting from extramarital affairs (!)... that means both of the wife's mother's parents were carriers, and since her mother doesn't have the disease, the wife's mother has 50% chance of being a carrier, and the wife has 25% (answer A)

C: 1/4 times 1/4 is 1/16

D: 3/4 times 3/4 equals 9/16

E: (3/4 times 1/4) plus (1/4 times 3/4)= 3/8

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