The combination of these genes determine the appearance of the animal. Genes occur in pairs, one gene comes from the mother, the other comes from the father. These genes are reflected as letters. The letter indicates what locus, this is the place on a gene, is mentioned. A capital points out that the gene is dominant, a small letter means the gene is recessive.
Dominant genes have a stronger effect than recessive genes.
If there is one dominant gene, and one recessive gene, than the dominant gene will overrule the recessive gene, and determine the appearance. A recessive gene will occur in the appearance only when there are two recessive genes present. For instance the A locus. Having the A gene means that the animal has the agouti colour, while the a gene causes the black colour. Only an animal with aa will be black.