Problem: Cystic Fibrosis (read about it here http://www.ncbi.nlm.nih.gov/pubmedh

Question

Problem: Cystic Fibrosis (read about it here http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001167/)

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.

Joe and Mary are considering having a second child, but want to know their chances of having another child with cystic fibrosis. What is this chance?

Fill out a Punnett square on your own paper to solve this problem. To help you get started, an example of how this is done is shown below.

0 (0%)

1/4 (25%)

3/4 (75%)

1 (100%)

Explanation / Answer

The child may have

FF, Ff, Ff and ff

Only the presence of two recessive genes will able to cause the cystic fibrosis.

So the chance of the child having cystic fibrosis would be 25%

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