A 30-year-old woman is phenotypically normal but had a brother who died from infantile tay-sachs disease (autosomal recessive lethal condition with 100% penetrance). what is the probability that this woman is a heterozygous carrier for tay-sachs disease?
a. none of the listed
b. 2/3
c. 1/3
d. 1/8
e. 1/4
f. 1/2