Let’s take the example from before regarding hair colour. Say you had a couple that wanted a child. Both parents are heterozygous and have one allele for brown hair and one allele for blonde hair.

In order to work out the chances of their child having brown or blonde hair you can use a monohybrid cross. The first stage is known as the parental generation or P1. The offspring of that generation is called the first filial generation or F1 generation. You can then add successive generations which you would call second filial generation or F2 generation and so on.

P1 phenotype brown brown

P1 genotype Bb Bb

gamete’s genotype B b B b

F1 genotype BB Bb Bb bb

F1 phenotype brown brown brown blonde

In sexual reproduction, the offspring get half their gametes from each parent which means that only one allele is present in each gamete.

The result is that there’s a