Inheritance of genes, alleles and cystic fibrosisEdexcel A-Level Biology A: Subtopic test

1. 1
   A healthy couple have a baby girl who is diagnosed with cystic fibrosis (CF), which is caused by a recessive allele of the CFTR gene. Neither parent has any symptoms of CF and there is no known history of CF in either family. The couple are planning to have more children. In this question, C represents the dominant allele and c the recessive allele.

   (a)

   Which term describes the genotype of the baby girl?

   [1 mark]

   • AHeterozygous
   • BHomozygous dominant
   • CHomozygous recessive
   • DCodominant

   (b)

   What is the probability that the couple's next child will be an unaffected carrier of the CF allele?

   [1 mark]

   • A1 in 4
   • B1 in 2
   • C3 in 4
   • DCertain

   (c)

   Explain how two parents who do not have CF can have a child with CF.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A rare inherited condition affects the nervous system in adults. Two affected parents, who both developed symptoms in their forties, have four children. Two of the children have the condition and two do not. The condition is controlled by a single gene with two alleles, and the gene is not on a sex chromosome. Ignore new mutations.

   (a)

   What is the probability that a fifth child of these parents will be unaffected?

   [1 mark]

   • A1 in 4
   • B1 in 2
   • C3 in 4
   • D0

   (b)

   One of the unaffected children later has a child with an unaffected partner. What is the probability that this grandchild is affected?

   [1 mark]

   • A1 in 4
   • B1 in 2
   • C3 in 4
   • D0 (no chance)

   (c)

   Deduce whether the allele that causes the condition is dominant or recessive. Explain your answer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A girl aged six has cystic fibrosis. She has repeated chest infections and, despite a good appetite, she is underweight. Genetic testing shows that she is homozygous for a mutation in which three bases are deleted from the CFTR gene, so one amino acid is missing from the CFTR protein. The CFTR protein is a channel in the cell-surface membrane of epithelial cells that transports chloride ions out of the cells. She takes capsules of digestive enzymes with every meal.

   (a)

   Explain how this mutation leads to the production of thick, sticky mucus.

   [3 marks]

   (b)

   Explain why the girl has repeated chest infections and is underweight, and why the enzyme capsules help her.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A 29-year-old man has cystic fibrosis and his doctor has told him that he is probably infertile. His partner does not have CF, but a genetic test has shown that she is a carrier of the CF allele. The couple hope to conceive with sperm collected surgically and have asked about the risk to any child. In this question, C represents the dominant allele and c the recessive allele.

   (a)

   Explain how the expression of a mutated CFTR allele causes the infertility of men with CF and breathing difficulties in people with CF.

   [6 marks]

   (b)

   Determine the probability that a child of this couple will have CF and the probability that the child will be an unaffected carrier. Explain why the genotype of the couple's first child does not change these probabilities for later children.

   [6 marks]

   Total for question 4: 12 marks

End of questions