Alleles are different forms of the same gene, for example blue and brown are both alleles of the eye colour gene.

Similar genes occupy the same position on both chromosomes in the pair.

Monohybrid genetic crosses
These are genetic diagrams consisting of a single characteristic controlled by a single gene with two alleles.

Alleles can be dominant or recessive.

Dominant alleles will be expressed even if a recessive allele is present.

Recessive alleles will be overridden by the presence of a dominant allele.

Two recessive alleles (no dominant) need to be present for the recessive characteristic to be expressed in the phenotype.

The phenotype is the outward expression of a gene (i.e. the physical appearance).

The genotype is a set of paired symbols used to represent the alleles present. The genotype can be homozygous (the same, BB or bb) or heterozygous (different, Bb).

During meiosis, gametes with only one gene for each characteristic are produced, meaning parents can only pass on one allele for each characteristic to their offspring.