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Cystic fibrosis (CF) is a recessive genetic disorder - Edexcel - GCSE Biology - Question 2 - 2016 - Paper 1
Question 2
Cystic fibrosis (CF) is a recessive genetic disorder. Matthew and Selina both have the genotype Ff. They are carriers of this disorder. The diagram shows how CF wa... show full transcript
Worked Solution & Example Answer:Cystic fibrosis (CF) is a recessive genetic disorder - Edexcel - GCSE Biology - Question 2 - 2016 - Paper 1
Step 1
(i) State the percentage of the male children with CF.
Answer
To determine the percentage of male children with cystic fibrosis (CF), we first consider that each parent carries the genotype Ff. When performing a Punnett square, the possible genotypes of their offspring are:
- FF (homozygous dominant)
- Ff (heterozygous, carrier)
- Ff (heterozygous, carrier)
- ff (homozygous recessive, affected)
This yields a 1 in 4 chance of having a child with the ff genotype, which has CF. Since there are three male children (Chris, Megan, and Stewart), only Stewart (the male with CF) is affected. Therefore:
Percentage of male children with CF = ( \frac{1}{2} \times 100 = 50% )
Step 2
(ii) State the genotypes of Chris and Megan for cystic fibrosis.
Answer
Chris is affected by cystic fibrosis, so his genotype must be ff (homozygous recessive). On the other hand, Megan is not affected and must therefore possess at least one dominant allele, making her genotype FF (homozygous dominant). In conclusion:
- Chris: ff (homozygous recessive)
- Megan: FF (homozygous dominant)