Genotype - GCSE Biology Definition

What is genotype?

In GCSE biology, a genotype is the combination of alleles that an organism has for a particular gene. The genotype of an organism interacts with environmental factors to determine an organism’s phenotype.

Each individual has two alleles of each gene, one from each of the homologous chromosomes inherited from its parents. This means that an organism’s genotype can be either:

• homozygous dominant: two dominant alleles

• heterozygous: two different alleles

• homozygous recessive: two recessive alleles

Genotype is often represented using letters, where capital letters represent dominant alleles and lowercase letters represent recessive alleles, e.g. for a gene with the alleles B and b the genotypes for the following individuals would be:

• homozygous dominant = BB

• heterozygous = Bb

• homozygous recessive = bb