Cystic fibrosis (a disorder of cell membranes) is caused by a recessive…

The idea

Cystic fibrosis is an inherited disorder affecting cell membranes, particularly in the lungs, pancreas and digestive system, where thick sticky mucus is produced. It is caused by a recessive allele, so a person must inherit two copies of the faulty allele, one from each parent, to develop the disorder. Someone with one faulty allele and one normal allele is a carrier: they do not have cystic fibrosis but can pass the faulty allele to their children. In a genetic cross, two carrier parents (Ff × Ff) have a 1 in 4 chance of a child with cystic fibrosis (ff), a 2 in 4 chance of a carrier child (Ff) and a 1 in 4 chance of a child with two normal alleles (FF). The disorder is not caused by infection or lifestyle, and it is not sex-linked.

What earns the marks

• Cystic fibrosis is caused by a recessive allele, so the disorder only appears when two recessive alleles are inherited, one from each parent.
• A person with one recessive allele and one dominant normal allele is a carrier; carriers do not show symptoms but can pass the recessive allele on.
• The disorder affects cell membranes, leading to thick sticky mucus that affects organs such as the lungs and pancreas.
• A genetic cross between two carriers (Ff × Ff) gives a 1 in 4 probability of an affected child (ff), 2 in 4 carrier children (Ff) and 1 in 4 unaffected non-carrier children (FF).
• Because the allele is recessive, an affected person must have the genotype ff; a dominant allele F would mask the recessive allele.
• Family pedigree diagrams can be used to identify carriers and to work out the probability of inherited disorders appearing in later generations.

Common mistakes and how to put them right

• Thinking that a carrier of cystic fibrosis has the disorder in a mild form.

  Put it right: Carriers are unaffected because the dominant normal allele masks the recessive allele.

• Assuming that two carrier parents will definitely have an affected child.

  Put it right: Each child has a 1 in 4 probability, and chance has no memory between pregnancies.

• Confusing cystic fibrosis with a dominant disorder such as polydactyly.

  Put it right: Cystic fibrosis is recessive, so two copies of the faulty allele are needed.

Exam tips

• Use a Punnett square or genetic diagram with clear parent genotypes, gametes and offspring genotypes to show how the recessive allele is inherited.
• State probabilities as fractions, decimals or percentages and link them to the genotypes in the cross, for example 1 in 4 for ff.
• When explaining why a person is a carrier, refer to one recessive allele being present but masked by the dominant normal allele.