How can a child have cystic fibrosis when neither of the parents have the disorder? (5 Marks)

• Cystic fibrosis is an autosomal recessive disorder, which is caused by a mutation in the CFTR (cystic fibrosis transmembrane conductance regulator) gene. • The defective CFTR protein does not allow Cl- ions to leave the cell, which leads to the build up of thick mucous in the lungs and pancreas.• To have the cystic fibrosis phenotype, you need to inherit 2 recessive alleles with this mutation from both parents. • If 2 recessive alleles are mutated, the child will be homozygous recessive. • If neither parent has the disease, they must be heterozygous for the cystic fibrosis allele.