In chinchillas, genes determine the colour of the fur. The genes are set of instruction for producing the proteins that cause different fur colours. There are several different forms of the fur colour gene. The different forms are called alleles. We can refer to these alleles using letters as symbols. For example, we can call the allele that gives grey fur G, and the allele that gives charcoal fur g. In each cell in a chinchilla’s body, there are two copies of the gene giving instructions about which kind of fur colour protein to make. This means that there are three possible combinations of alleles. A chinchilla might have two G alleles. It might have one of each, Gg. Or it might have two g alleles, gg. If the two alleles for this gene in its cells are the same - that is, GG or gg - the chinchilla is said to be homozygous. If the two alleles are different - that is, Gg - then it is heterozygous.