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 Biology (1BI0) specification · Updated October 2026