Tess’s sister is known to carry an allele for a single-gene recessive genetic disorder that is fatal in very early life. Tess’s mother was tested for the allele and found not to carry it. What are the odds that Tess is a carrier for the allele?

Question

Tess’s sister is known to carry an allele for a single-gene recessive genetic disorder that is fatal in very early life. Tess’s mother was tested for the allele and found not to carry it. What are the odds that Tess is a carrier for the allele?

1/2
1/8
1/1
1/4

Answer 1

1/4

Since Tess's mother was tested and found not to carry the allele, she must have two normal alleles for that gene. Therefore, there is a 1/2 chance that she passed a normal allele to Tess. The father is unknown in this scenario, but we assume that he does not carry the allele since it is not mentioned. Therefore, there is a 1/2 chance that Tess inherited the allele for the genetic disorder from him. Multiplying these probabilities together (1/2 x 1/2 = 1/4), we get the odds that Tess is a carrier for the allele.