A man, Waldo, whose sister died in early childhood from a recessive lethal disea

Question

A man, Waldo, whose sister died in early childhood from a recessive lethal disease marries a woman, Ethel, with the same family history. Because this man survived beyond childhood, he does not have the disease, but he may be a carrier ( = heterozygous, as may also be the case with his wife). What is the probability that their first child will suffer from the disease? [Hint: first calculate the probability that Waldo is heterozygous; then determine the probability that both parents are carriers. Remember that he has survived to adulthood when calculating this probability].

Explanation / Answer

A is the dominant allele over b the recessive diseaae type in male Waldo

and so also Ethel.- A is the dominant allele over b the recessive diseaae type. thus the first born is more likely to be affected by the disease.

Matings between relatives are important for observing recessive alleles, because when that recessive allele is rare, it is more likely to become homozygous through inheritance from a common ancestor with the history of the dideased condition than from parents who are completely unrelated. The reason is that the carrier of a rare allele may have many descendants who are carriers. If two of these carriers should mate (for example, in a first-cousin mating or two unrelated people who are carriers) the recessive allele can become
homozygous with a probability of 1/4.

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