VMBO Year 3 · Inheritance and evolution · Section 3.2

Genes and alleles

Homozygous and heterozygous

For most genes, you inherit one allele from each parent. If the two alleles are the same, you are homozygous for that gene. If they differ, you are heterozygous. In a simplified example, AA and aa are homozygous combinations, while Aa is heterozygous.

Dominant and recessive alleles

With complete dominance, the phenotype associated with a dominant allele appears in a heterozygote. The phenotype associated with a recessive allele appears only when no dominant allele is present. Dominance describes the relationship between alleles; it does not mean an allele is physically stronger, better or more common. Real human characteristics such as hair texture usually involve more than one gene, so simple examples are models.

Using letters for genotypes

We usually use a capital letter for a dominant allele and the same letter in lower case for a recessive allele.

• AA: homozygous dominant; the dominant phenotype.
• Aa: heterozygous; the dominant phenotype under complete dominance.
• aa: homozygous recessive; the recessive phenotype.

An intermediate phenotype

In incomplete dominance, neither allele completely masks the effect of the other. The heterozygote has an intermediate phenotype. In snapdragons, crossing a red-flowered plant with a white-flowered plant can produce pink flowers. We can represent the alleles as R and W: RR is red, RW is pink and WW is white. The alleles remain separate and can be passed on unchanged; they do not permanently blend.