Phenylektonuria is a human hereditary disease resulting from the inability of th

Question

Phenylektonuria is a human hereditary disease resulting from the inability of the body to profcess the amino acid phenylalanine, which is contained in many of the proteins we eat. PKU is manifested in early infancy and, if it remains untreated, generally leads to mental retardation. PKU is cause by a recessive allele with simple Mendelian inheritance. A couple who intends to have children consults a genetic counselor because the man has a sister with PKU and the woman has a brother with PKU. There are no other known cases in their families. They ask the genetic counselor to determine the probability that their first child will have PKU. What is the probability?

Explanation / Answer

PP:normal homozygous

Pp:normal heterozygous

pp:diseased person

as PKU is a disease caused by recessive allele so the persons having PKU (man's sister and woman's brother) both are homozygous i.e pp

as the man and woman both are normal they must have a genotype PP or Pp

so the probability of them having their first child being a PKU patient is 0.

three possible combinations are shown below:

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