A dominant allele is always expressed, even if only one copy is present. A…

The idea

Alleles are different versions of a gene. A dominant allele is expressed in the phenotype even when only one copy is inherited, so a heterozygous organism with one dominant and one recessive allele shows the dominant characteristic. A recessive allele is expressed only when two copies are present, meaning the organism is homozygous recessive and no dominant allele is present. For example, in pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). A plant with genotype Tt is tall because T is expressed; a plant with genotype tt is short because no dominant T allele is present. Genetic diagrams use these rules to predict the ratios of phenotypes in offspring, and the 3:1 ratio in a monohybrid cross between two heterozygotes follows from dominance.

What earns the marks

• State that a dominant allele is expressed in the phenotype when at least one copy is present, including in a heterozygote.
• State that a recessive allele is expressed only when two copies are present, so the organism is homozygous recessive.
• Use correct genetic terminology, including dominant, recessive, homozygous, heterozygous, genotype and phenotype.
• Construct or interpret a genetic diagram, such as a Punnett square, using upper-case letters for a dominant allele and lower-case letters for a recessive allele.
• Predict phenotype ratios from a cross, for example a 3:1 ratio when two heterozygous parents are crossed.
• Explain that a recessive phenotype appears only when no dominant allele is present to mask it.

Common mistakes and how to put them right

• Thinking a dominant allele is the most common allele.

  Put it right by explaining dominance is about expression in the phenotype, not frequency in a population.

• Writing that a recessive allele is 'weaker' or 'hidden' in a heterozygote.

  Put it right by saying it is not expressed because the dominant allele is expressed instead.

• Using the same letter case for both alleles in a genetic diagram.

  Put it right by using a capital letter for the dominant allele and the matching lower-case letter for the recessive allele.

Exam tips

• Define the letters you use before drawing a genetic diagram, for example T = tall and t = short.
• Circle or label parental genotypes and gametes clearly so the examiner can follow your reasoning.
• When predicting ratios, count the outcomes in the square and simplify the ratio, stating which phenotype each number represents.