DNA and the genome: understanding and treatment of inherited disorders

The idea

Inherited disorders arise from alleles passed from parents to offspring. A recessive disorder, such as cystic fibrosis, only appears when both alleles are recessive (for example, both parents are carriers, Cc × Cc, giving a 1 in 4 chance of an affected child, cc). A dominant disorder, such as polydactyly, appears if at least one dominant allele is present (for example, Pp × pp gives a 1 in 2 chance). Understanding inheritance lets families use genetic diagrams and Punnett squares to predict probabilities, and allows screening, genetic counselling and informed decisions. Treatment may involve managing symptoms, for example physiotherapy and enzyme replacement for cystic fibrosis, or surgery for polydactyly; gene therapy is an experimental approach that aims to replace or repair faulty alleles.

What earns the marks

• State that inherited disorders are caused by alleles passed from parents to offspring, and that the disorder may be recessive or dominant.
• Use a genetic diagram or Punnett square with correct parental genotypes to work out the probability of offspring inheriting a disorder, for example Cc × Cc giving a 1 in 4 chance of cystic fibrosis.
• Distinguish recessive from dominant inheritance: a recessive disorder needs two recessive alleles, whereas a dominant disorder needs only one dominant allele.
• Describe how understanding inheritance supports genetic counselling, carrier testing and prenatal screening so families can make informed choices.
• Describe treatments that manage symptoms, such as physiotherapy and enzyme replacement for cystic fibrosis, or surgery for polydactyly, and note that gene therapy is still being developed.

Common mistakes and how to put them right

• Writing that a carrier of a recessive disorder is affected.

  Put it right: A carrier has one recessive allele and one dominant allele and is usually unaffected but can pass the recessive allele on.

• Confusing the probability of each child with the proportion of a family.

  Put it right: Each pregnancy is independent, so a 1 in 4 chance applies to each child, not to a fixed number of children.

• Assuming a dominant disorder must be more common or more severe.

  Put it right: Dominance describes which allele is expressed, not how common or serious the disorder is.

Exam tips

• Always define the letters you use, for example C for the dominant allele and c for the recessive allele, and keep the same letters throughout the genetic diagram.
• Show the parental genotypes, the gametes and the offspring genotypes in a clear grid so the examiner can follow your reasoning.
• When asked about treatment, link the treatment to the specific symptom or faulty protein rather than giving a general answer such as 'take medicine'.