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Chapter 3: Problem 18

A certain type of congenital deafness in humans is caused by a rare autosomal (not X-linked) dominant gene. (a) In a mating involving a deaf man and a deaf woman (both heterozygous), would you expect all the children to be deaf? Explain your answer. (b) In a mating involving a deaf man and a deaf woman (both heterozygous), could all the children have normal hearing? Explain your answer. (c) Another form of deafness is caused by a rare autosomal recessive gene. In a mating involving a deaf man and a deaf woman, could some of the children have normal hearing? Explain your answer.

Short Answer

Expert verified
Answer: The likelihood of deaf children being born depends on the inheritance pattern and the parents' genotypes. In an autosomal dominant pattern, where both parents are heterozygous for the gene, there is a 75% chance their offspring will be deaf. In an autosomal recessive pattern, if both parents are homozygous for the recessive gene, all of their offspring will be deaf, but if both parents are heterozygous, there is a 25% chance their children will be deaf and a 75% chance they will have normal hearing.

Step by step solution

01

Part (a)

In this scenario, both the deaf man and the deaf woman are heterozygous for the autosomal dominant gene. When two heterozygotes mate, the possible genotypes of the offspring are as follows: 1/4 homozygous dominant (deaf), 1/2 heterozygous (deaf), and 1/4 homozygous recessive (with normal hearing). Therefore, not all of their children would be deaf. Only 75% (1/4 + 1/2) of the offspring would be expected to be deaf.
02

Part (b)

In this situation, both parents are heterozygous for the autosomal dominant gene causing deafness, as stated in Part (a). The offspring have a 25% (1/4) chance of being homozygous recessive (with normal hearing). However, that does not imply all the children would have normal hearing; it only indicates the likelihood for an individual offspring. It is statistically very unlikely that all children would have normal hearing, as each offspring's genotype is an independent event with a 75% chance of being deaf.
03

Part (c)

In this part, the cause of deafness is a rare autosomal recessive gene. Consider the case where both parents are homozygous for the recessive gene, which means they have the genotype "rr." In this situation, all of their offspring will be deaf due to inheriting two recessive alleles. However, if both parents are heterozygous (a genotype of "Rr"), their offspring have a 25% chance of being homozygous dominant (with normal hearing), a 50% chance of being heterozygous (with normal hearing), and a 25% chance of being homozygous recessive (deaf). In such a case, their children could have normal hearing.

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