Sue has normal hearing but her sister suffers from deafness determined by a rare recessive allele. what is the probability that sue's phenotypically normal daughter is heterozygous for the gene? (hint: the word "rare" implies that the recessive allele is not frequent in the population; you can assume that the father of sue's daughter is not likely a carrier)

Since Sue's sister suffers from a disorder determined by a rare recessive allele, her sister carries two copies of the recessive gene. Since Sue has normal hearing, she can at most carry one copy, but may have none. Assuming Sue's husband is not a carrier, Sue's daughter could be heterozygous only if Sue passes down the rare allele. If either of Sue's parents has the disorder, then Sue is definitely a carrier, and Sue's daughter has a 50% chance of being a carrier. If neither of Sue's parents have the disorder, then they were both carriers, Sue has a 50% chance of carrying the alelle, and her daughter has a 25% chance of being a carrier.