Cystic fibrosis is a recessive genetic disorder caused by a mutation of the CTRF gene. The disease is characterized by excessive mucus production along the respiratory and digestive tracts in addition to other symptoms. Cystic fibrosis is a progressive disorder that is most associated with chronic respiratory disease.

Question

Sara and Anthony found out that they are both carriers of the disorder, but neither have the disease. What are the chances their child will be born with cystic fibrosis?
A.
0:4
B.
1:4
C.
1:2
D.
4:4

Answer 1

Cystic fibrosis is caused by a recessive allele, meaning that an individual must have two copies of the mutated allele (one from each parent) to express the disease. Since both Sara and Anthony are carriers, they each have one normal allele and one mutated allele (genotypes: Aa, where 'A' is the normal allele and 'a' is the mutated allele).

When two carriers (Aa x Aa) have a child, the possible combinations of alleles are as follows:

• AA (normal)
• Aa (carrier)
• Aa (carrier)
• aa (affected)

This results in the following probabilities for their child:

• 1 in 4 chance of being AA (not a carrier and does not have CF)
• 2 in 4 chance of being Aa (a carrier, but does not have CF)
• 1 in 4 chance of being aa (having cystic fibrosis)

Thus, the chance that their child will be born with cystic fibrosis is 1 in 4.

The correct answer is B. 1:4.