Question:medium
What is the chance of a child having cystic fibrosis if one parent is affected and the other parent is a carrier?
What is the chance of a child having cystic fibrosis if one parent is affected and the other parent is a carrier?
Show Hint
Cross an affected (aa) parent with a carrier (Aa) parent and count the aa offspring.
Updated On: Jun 24, 2026
- 25%
- 50%
- 70%
- 80%
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: Treat this as a simple Mendelian cross. Cystic fibrosis needs two defective CFTR copies, so disease genotype is homozygous recessive (aa), while a single defective copy (Aa) is a silent carrier.
Step 2: Set up the parents. Affected parent = aa, contributing only an a gamete. Carrier parent = Aa, contributing A or a in a 1:1 ratio.
Step 3: Combine the gametes. Half the children get a + A = Aa (carrier, healthy) and half get a + a = aa (cystic fibrosis). That is a 50:50 split.
Step 4: So one in two children, or 50 percent, will be affected when one parent has the disease and the other is a carrier.
\[\boxed{50\%}\]
Step 2: Set up the parents. Affected parent = aa, contributing only an a gamete. Carrier parent = Aa, contributing A or a in a 1:1 ratio.
Step 3: Combine the gametes. Half the children get a + A = Aa (carrier, healthy) and half get a + a = aa (cystic fibrosis). That is a 50:50 split.
Step 4: So one in two children, or 50 percent, will be affected when one parent has the disease and the other is a carrier.
\[\boxed{50\%}\]
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