Key genetic termsSpec 3.13
In short
Key genetic terms describe how alleles are inherited. A dominant allele is expressed with one copy, but a recessive allele only with two copies. Homozygous means two identical alleles, such as TT or tt; heterozygous means two different alleles, such as Tt. The genotype is the alleles an organism has, and the phenotype is the characteristics expressed.
- Chromosome
- A long molecule of DNA in the nucleus that carries many genes.
- Gene
- A section of DNA that codes for a specific protein.
- Allele
- A different version 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 (no dominant allele).
- 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 (or genes).
- Phenotype
- The characteristics that are expressed and can be observed.
- Gamete
- A sex cell (such as a sperm or egg) with half the number of chromosomes.
- Zygote
- The cell formed when two gametes fuse at fertilisation.
Dominant alleles are written as capital letters (T) and recessive alleles as lower case (t). A heterozygous plant, Tt, shows the dominant characteristic.
Exam tip:
Genotype is the alleles, such as Tt. Phenotype is what you can see, such as tall. Do not swap them.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026