In a diploid organism, the alleles at a specific locus may be either…

The idea

In a diploid organism, homologous chromosomes carry the same genes at the same loci. For autosomal genes, there are two alleles. If they are the same, the organism is homozygous at that locus; if different, it is heterozygous. A homozygous organism produces one type of gamete for that gene. A heterozygote produces two types of gamete in equal numbers due to segregation in meiosis. Two heterozygous parents produce homozygous recessive offspring a quarter of the time. This is a one in four risk per pregnancy for autosomal recessive conditions like cystic fibrosis. Note that males have only one allele for most X-linked genes, so they are not diploid for those loci and cannot be described as homozygous or heterozygous.

What earns the marks

• Defining homozygous as two identical alleles and heterozygous as two different alleles at the same locus on homologous chromosomes.
• Stating that a homozygous organism produces one type of gamete for that gene, while a heterozygote produces two types in equal proportions because of segregation in meiosis.
• Explaining that two heterozygous parents can produce a homozygous recessive offspring, with a one in four probability for each pregnancy.
• Applying the terms to a named gene rather than an organism as a whole, for example stating that a person is heterozygous for the CFTR gene, not that they are heterozygous.
• Recognising that the terms apply to autosomal loci in a diploid organism, and that males have only one allele for most X-linked genes.

Common mistakes and how to put them right

• Describing an organism as homozygous or heterozygous overall, when the terms apply only to a named gene.

  Put it right: State the gene or locus, for example homozygous for the CFTR allele.

• Treating the one in four as a guarantee, so assuming that in a family of four children exactly one will be affected.

  Put it right: Each pregnancy carries a one in four chance independently.

• Using heterozygous to mean carrier in a condition caused by a dominant allele, where the heterozygote is affected.

  Put it right: A carrier is heterozygous for a recessive allele and is unaffected.

• Writing pure bred or hybrid instead of homozygous and heterozygous in an A-level answer.

  Put it right: Use the correct genetic terminology.

• Forgetting that a male has only one allele for most X-linked genes, so he cannot be described as homozygous or heterozygous for those loci.

  Put it right: Refer to hemizygous or state that only one allele is present.

Exam tips

• When a question compares two families, check whether the pattern fits nuclear inheritance from heterozygous parents. Remember that terms like homozygous and heterozygous apply to diploid loci, not to most X-linked genes in males.
• Write homozygous recessive or homozygous dominant in full, because homozygous on its own rarely scores.
• Probability resets with every pregnancy, so write a one in four chance for each child.