Inheritance of genes, alleles and cystic fibrosisEdexcel A-Level Biology A: Mind map

Exam questions on Inheritance of genes, alleles and cystic fibrosis

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.
   Explain how two parents who do not have CF can have a child with CF.2 marks
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.
   Deduce whether the allele that causes the condition is dominant or recessive. Explain your answer.2 marks
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.
   Explain how this mutation leads to the production of thick, sticky mucus.3 marks