The genotype of an organism is their genetic constitution. The genotype is expressed through the phenotype: this is the outward effect of the genes and what you see when you look at an organism.

Chromosomes come in pairs. Each one of these chromosomes that make up a pair has a gene at the same point. However, there are alternative forms of the gene which can be stationed at this point. These are known as alleles: alternate forms of the same gene. There can be multiple alleles for one gene.

If both chromosomes in the same pair have the same allele for the gene they’re known as homozygous. If they have different alleles for the same gene they’re known as heterozygous.

For example, say ‘B’ is the allele for brown hair and ‘b’ the allele for blonde hair. If you had the two alleles ‘BB’ or ‘bb’ you’d be homozygous for that gene. However, if you had ‘Bb’ then you would be heterozygous.

The phenotype is affected by how the alleles are paired. An allele can be either dominant or recessive. A dominant allele reveals itself more strongly than a recessive. In written form, a dominant allele is always written as a capital letter and a recessive gene as a lower case letter. So, in the example above: ‘B’ for brown hair is the dominant allele and ‘b’ for blonde hair is the recessive.

For BB the genotype is homozygous dominant and the phenotype (the colour of your hair) is brown.

Similarly:

bb = homozygous recessive and you have blonde hair

Bb = heterozygous and you have brown hair