Genetic inheritance: dominant

The idea

A dominant allele is an allele whose characteristic is expressed in the phenotype even when only one copy is present. In a heterozygous individual, the dominant allele determines the visible characteristic, while the recessive allele is not expressed. Dominant alleles are represented by a capital letter, such as A, while the recessive allele is represented by the same letter in lower case, a. For example, if A codes for brown eyes and a codes for blue eyes, a person with genotype Aa has brown eyes because A is dominant. Dominance does not mean an allele is better, stronger or more common; it describes how the allele affects the phenotype. A dominant phenotype can result from a homozygous dominant genotype (AA) or a heterozygous genotype (Aa).

What earns the marks

• A dominant allele is expressed in the phenotype even when only one copy is present.
• In a heterozygote, the dominant allele determines the phenotype and the recessive allele is masked.
• Dominant alleles are conventionally represented by a capital letter.
• A dominant phenotype can arise from a homozygous dominant genotype or a heterozygous genotype.
• Dominance describes the effect of an allele on the phenotype, not its frequency in a population.
• Genetic diagrams can be used to predict the proportion of offspring showing a dominant characteristic.

Common mistakes and how to put them right

• Thinking a dominant allele is always the most common allele in a population.

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

• Believing a dominant characteristic cannot skip a generation.

  Put it right by noting that a recessive allele can be carried without being expressed.

• Writing that a dominant allele is stronger than a recessive allele.

  Put it right by saying the dominant allele is expressed when present.

Exam tips

• Use a capital letter for the dominant allele and the matching lower-case letter for the recessive allele.
• When predicting offspring, draw a Punnett square and label parental genotypes, gametes and offspring clearly.
• Check whether the question asks for genotype, phenotype or probability, and answer in the form requested.