# Biology Question?

Tay-Sachs disease is caused by a recessive allele. Onset of symptoms begins at about six months of age and death occurs in early childhood. Alicia's parents are both normal, but she had a sister who died of Tay-Sachs disease. What is the probability that Alicia is a carrier of the Tay-Sachs allele?

a. 25%

b.50%

c.0%

d.75%

e.100%

### 3 Answers

- JazSincLv 72 months ago
Bad question. Your teacher sucks. The question does not give Alicia's age! Your teacher really sucks.

Parents are both heterozygous, which you know because they had an afflicted child.

TtxTt => TT Tt Tt tt

If Alicia is less than six months old, then you have those four possibilities, and two of the four are carriers, so Alicia would have a 50% chance of being a carrier.

However, if Alicia is six months or older, and you're asking this question, then you already know she's not afflicted (tt) so you need only consider these three TT Tt Tt; two of them are carriers, so Alicia would have a 2/3 = 67% chance of being a carrier.

This is a variant of the "Monty Hall Problem." Your teacher sucks.

- Formerly Peter SLv 72 months ago
R = normal allele

r = recessive allele

Parents are both carriers (Rr)

Rr x Rr give rr (sister with the disease), RR (totally normal) and Rr + Rr (carriers)

Cahnace of being a carrier = 2 in 4 = 50%.