Some disorders are inherited. These disorders are caused by the…

The idea

Inherited disorders are conditions passed from parents to offspring because they are coded for by particular alleles carried in gametes. A disorder allele is a version of a gene that alters the protein or trait it controls, so the condition appears when that allele is present in the offspring's cells. Some disorder alleles are dominant, so one copy is enough; others are recessive, so two copies are needed. For example, a recessive disorder such as cystic fibrosis requires both parents to pass on the recessive allele, while a dominant disorder appears if either parent passes on the dominant allele. Genetic diagrams using letters show the possible allele combinations and the probability of offspring being affected, unaffected or carriers.

What earns the marks

• Inherited disorders are caused by alleles passed from parents to offspring in gametes, not by lifestyle or infection.
• A gene can have different alleles; a disorder allele is a version that produces a faulty or altered protein leading to the condition.
• Dominant disorder alleles cause the condition when at least one copy is inherited.
• Recessive disorder alleles cause the condition only when two copies are inherited, one from each parent.
• Carriers have one recessive disorder allele but do not show the condition; they can pass the allele to offspring.
• Genetic diagrams and Punnett squares use letters to predict the probability of offspring inheriting disorder alleles.

Common mistakes and how to put them right

• Thinking inherited disorders are caught from the environment.

  Put it right by stating that the cause is the inheritance of certain alleles from parents.

• Assuming a dominant disorder must be more common than a recessive one.

  Put it right by explaining that dominance affects whether one copy is enough, not how frequent the allele is in a population.

• Confusing a carrier with an affected person for a recessive disorder.

  Put it right by stating that a carrier has one recessive allele and one dominant allele and does not show the condition.

Exam tips

• Define the key terms allele, dominant, recessive and carrier before applying them to a named disorder.
• When using a genetic diagram, state the parent genotypes, gametes and possible offspring genotypes clearly.
• Link each outcome to the alleles inherited, for example two recessive alleles are needed for a recessive disorder to be expressed.