Alleles may be dominant, recessive or codominant.

The idea

A dominant allele is expressed whenever it is present, so the homozygous dominant and the heterozygote share a phenotype. A recessive allele is expressed only in the homozygote, which is why a recessive characteristic can skip generations and reappear when two heterozygous carriers have children. Codominant alleles are both expressed in the heterozygote, producing a third phenotype: I^A and I^B together give blood group AB, and in cattle C^R and C^W together give a roan coat of red and white hairs. Codominance is not blending, because both proteins are made and both characteristics appear rather than an average. Dominance is a relationship between alleles, not a measure of how common an allele is, so a dominant allele can be rare and a recessive allele common.

What earns the marks

• Defining dominance as the allele expressed in the heterozygote, and recessiveness as the allele expressed only when homozygous, with the heterozygote showing the dominant phenotype.
• Explaining codominance as both alleles being expressed in the heterozygote, giving a third phenotype, illustrated by I^A I^B blood group AB or C^R C^W roan cattle.
• Using correct notation: a capital and lower-case letter for a dominant/recessive pair, and a shared capital with superscripts for codominant alleles, such as C^R and C^W.
• Distinguishing codominance from multiple alleles: ABO shows both, but codominance concerns expression in the heterozygote while multiple alleles means more than two alleles exist in the population.
• Explaining that dominance does not determine allele frequency, so a dominant allele may be rare and a recessive allele common in a population.

Common mistakes and how to put them right

• Arguing from how common a characteristic is; a characteristic can be common because the allele is frequent, not because it is dominant. Correction: dominance describes expression in the heterozygote, not population frequency.

• Describing codominance as blending, so predicting an intermediate colour instead of patches of both colours.

  Put it right: Both alleles are expressed, so both characteristics appear, as in roan cattle with red and white hairs.

• Writing codominant alleles as a capital and a lower-case letter, which implies one is recessive.

  Put it right: Use a shared capital letter with superscripts, such as C^R and C^W.

• Confusing codominance with multiple alleles.

  Put it right: Codominance is about both alleles being expressed in the heterozygote; multiple alleles means more than two alleles exist for the gene in the population.

• Claiming that two parents with the same phenotype producing a different offspring proves dominance.

  Put it right: This pattern can also arise with codominance, for example two roan parents producing red and white offspring, so codominance must be excluded first.

Exam tips

• To infer dominance from a family tree, first check that codominance is excluded; then two parents with the same phenotype producing a different offspring indicates the parents are heterozygous and the offspring homozygous recessive.
• For codominant alleles use the board's notation, one common capital letter carrying superscripts, such as C^R and C^W, and write them clearly.
• Do not write anything you are unsure of next to your correct evidence, because a wrong statement can cancel the mark.