Question

In a human family,the female has a form of deafness determined by an autosomal recessive allele.What's the probability that her phenotypically normal brother is heterzygous for the gene?

Answer 1

the probability is 1/2. The brother with the normal phenotype could have two
alleles without a form of deafness or he could be a heterozygote. If he has a
heterozygote he has 1 normal allel and 1 recessive allele with a form
of deafness. The probability of the latter is 1/2, that is the answer.