Alleles and genetic termsSpec 3.19, 3.20
In short
Alleles are different versions, or alternative forms, of the same gene. A dominant allele is expressed when only one copy is present, and a recessive allele only when two copies are present. Homozygous means two identical alleles, such as TT, and heterozygous means two different alleles, Tt. Genotype is the alleles, and phenotype is the observed characteristic.
Genes exist in alternative forms called alleles. A gene for flower colour may have an allele for purple flowers and an allele for white flowers.
In body cells, chromosomes are in pairs, so an organism has two alleles of each gene: one from each parent. Different combinations of alleles give rise to differences in inherited characteristics.
- Allele
- A different version (alternative form) of the same gene.
- Dominant
- An allele that is expressed in the phenotype even when only one copy is present.
- Recessive
- An allele that is only expressed in the phenotype when two copies are present, so no dominant allele is there.
- Homozygous
- Having two identical alleles of a gene, for example TT or tt.
- Heterozygous
- Having two different alleles of a gene, for example Tt.
- Genotype
- The alleles an organism has for a gene, for example Tt.
- Phenotype
- The characteristic that is expressed and can be observed, for example tall.
Dominant alleles are written as capital letters (T) and recessive alleles as lower case letters (t). A heterozygous plant, Tt, shows the dominant characteristic.
Genotype is the alleles, such as Tt. Phenotype is what you can see, such as tall. Do not swap them.
An allele is not a different gene. It is a different version of the same gene, found at the same position on a chromosome.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026