As shown in the Punnett square below, one variety of deafness is caused by an autosomal recessive allele. Two parents who do not show any signs of the disease but are carriers could therefore have a child who is deaf, because that child could inherit one recessive deafness-causing gene from each parent. Imagine that a deaf male mates with a hearing female. We know the deaf male must have the genotype dd, but the female could be either Dd or DD. Such a mating is essentially a testcross, like the one shown in Figure 9.10. If the parents’ first child has hearing, can you say with certainty what the mother’s genotype must be? What if the couple has four children (none twins), all with hearing—can you say with certainty the mother’s genotype? What would it take for a definitive genotype to be assigned?
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