Inherited Disorders: Polydactyly and Cystic Fibrosis — AQA GCSE Biology

Some disorders are caused by the inheritance of certain alleles. Two examples are required.

Polydactyly

• Polydactyly means having extra fingers or toes.
• It is caused by a dominant allele.
• Because the allele is dominant, a child can inherit the disorder if just one parent carries it.

Cystic fibrosis

• Cystic fibrosis is a disorder of cell membranes, causing thick, sticky mucus in the lungs and digestive system.
• It is caused by a recessive allele.
• A child must inherit the recessive allele from both parents to have the disorder. A person with one copy is a carrier but does not have the condition.

Genetic crosses with disorders

You should be able to use a Punnett square to work out the probability of a child inheriting a disorder. For cystic fibrosis, two carriers (Ff × Ff) have a 1 in 4 (25%) chance of an affected child.

Exam tips

• Polydactyly = dominant; one affected parent can pass it on.
• Cystic fibrosis = recessive; needs two copies; carriers are unaffected.
• Use a Punnett square to find the probability of an affected child.
• A 25% (1 in 4) chance comes from a carrier × carrier cross.