Individuals in a population show variation. Genetic variation is caused by differences in alleles and can be inherited by offspring. For example, suppose a hypothetical coat-colour gene has a dominant brown allele $B$ and a recessive grey allele $b$. If a sperm carrying $B$ fertilises an egg carrying $b$, the offspring has genotype $Bb$ and a brown coat. An offspring must have genotype $bb$ to have a grey coat.