Question

why is it possible for two healthy parents to give birth to a child with a genetic defect such as cystic fibrosis?
○ cf is only evident in individuals with two mutant alleles; a healthy parent could carry both a defective allele and a normal allele.
○ cf is caused by the inheritance of too many
ormal\ genes; the parents would be unaffected.
○ cf is normally caused by a mutation that occurs only in the sperm and eggs of the parents, so their bodies are not affected.
○ cf is not passed to children from their parents; it comes from their grandparents.
○ cf is normally caused by a mutation that occurs in the fetus during pregnancy, and it does not affect the mother.
incorrect answer

Explanation:

Cystic fibrosis (CF) is an autosomal recessive disorder. For a child to have CF, they must inherit two mutant alleles (one from each parent). Parents who are carriers (heterozygous - having one normal and one mutant allele) are healthy. When two carriers have children, there is a 25% chance of the child inheriting two mutant alleles (as per Punnett square analysis of autosomal recessive inheritance: \(Aa\times Aa\) gives \(AA:Aa:aa = 1:2:1\), where \(aa\) represents the affected child with CF). The other options are incorrect:

• CF is not caused by too many "normal" genes.
• It is not only due to mutations in sperm/eggs (parents can be carriers).
• It is passed from parents (not just grandparents).
• It is not mainly due to fetal - stage mutations (though new mutations can occur, but the common case is inheritance from carrier parents).

Answer:

CF is only evident in individuals with two mutant alleles; a healthy parent could carry both a defective allele and a normal allele.