Joe and Mary have a son named Billy. He is only 2-years-old, and has been experi

Question

Joe and Mary have a son named Billy. He is only 2-years-old, and has been experiencing repeated respiratory infections, sometimes requiring hospitalization due to pneumonia. His respiratory specialist suspects he may have cystic fibrosis, and orders a genetic test to determine if he has the underlying mutant alleles on chromosome 7 for a dysfunctional protein in his chloride ion channels. The sticky mucus that results from having two copies of this mutant allele leads to congestion, recurrent bouts of pneumonia, salty-tasting skin, poor fat digestion, inflammation and dysfunction of the pancreas, and infertility. He is tested by a respiratory specialist, and it is confirmed. Billy has cystic fibrosis.

Neither Joe nor Mary show any symptoms of cystic fibrosis, yet their son, Billy, has the disorder.

We will use F to designate the dominant, normal allele, and f to represent the recessive allele for cystic fibrosis.

What is Billy's genotype?

Explanation / Answer

Joe and Mary don’t show any symptoms of cystic fibrosis hence it is very likely that they are carriers of cystic fibrosis genes hence their genotypes are Ff. Cystic fibrosis is an autosomal recessive disorder. Hence, both the alleles of cystic fibrosis should be recessive in Billy because he shows the symptoms of disease.

Mating of parents with Ff genotypes result in children with either FF or ff genotypes. Children with ff genotypes will be suffering from cystic fibrosis as in case of Billy.

Therefore, genotype of Billy is ff.

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