Biology Question?

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%

Answer 1

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.

Answer 2

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%.

Answer 3

i would ask your doctor about it