Question:easy
Cystic fibrosis is inherited as an autosomal recessive condition. A normal couple has one daughter affected with the disease. They are now planning to have another child. What is the chance of their next child being affected by the disease?
Cystic fibrosis is inherited as an autosomal recessive condition. A normal couple has one daughter affected with the disease. They are now planning to have another child. What is the chance of their next child being affected by the disease?
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Both healthy parents of an affected child must be Aa carriers.
Updated On: Jun 24, 2026
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- 1/2
- 1/4
- 3/4
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The Correct Option is C
Solution and Explanation
Start by fixing the parents' genotypes. Cystic fibrosis needs two mutant alleles to show. Both parents look healthy, yet they have an affected child who must be homozygous for the mutation. That child got one mutant allele from each parent, which proves both parents are silent carriers.
Write the carrier as Aa, where A is the working allele and a is the faulty one. When two carriers have a child, simple Mendelian counting gives the genotypes AA, Aa, Aa, aa in equal quarters. Only the aa child shows the disease.
That single aa outcome out of four equally likely combinations gives a risk of one in four for any pregnancy. Genetics has no memory, so the previously affected daughter does not raise or lower the odds for the next baby; the chance stays the same each time.
Therefore the chance that the next child is affected is one quarter.
\[\boxed{\tfrac{1}{4}}\]
Write the carrier as Aa, where A is the working allele and a is the faulty one. When two carriers have a child, simple Mendelian counting gives the genotypes AA, Aa, Aa, aa in equal quarters. Only the aa child shows the disease.
That single aa outcome out of four equally likely combinations gives a risk of one in four for any pregnancy. Genetics has no memory, so the previously affected daughter does not raise or lower the odds for the next baby; the chance stays the same each time.
Therefore the chance that the next child is affected is one quarter.
\[\boxed{\tfrac{1}{4}}\]
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