For any given location on a chromosome pair, we find two versions of a gene – one belonging to the ‘left’ strand of the pair, and one belonging to the right. With the exception of the sex chromosomes, only one of these genes will ever be expressed to build the creature, depending on their relative dominance.

This is the classical Mendelian inheritance we learn about in school. For example, imagine a gene that produces a particular enzyme. Perhaps there are several versions of this enzyme, with slightly different properties. Any one creature will inherit two copies of the gene that produces this enzyme, but which version actually gets produced will depend on which of the two alleles is dominant.

Suppose there are only two varieties of the enzyme, which we’ll call E and e. E is dominant over e. So if the creature inherits E+E then obviously it will produce enzyme E, because that’s the only option. But it will also produce E if the gene is E+e or e+E, because E dominates over e. Only if the gene is e+e will it produce the e form of the enzyme.

By the way, E+E is called homozygous dominant (homo meaning same). e+e is called homozygous recessive, and both E+e and e+E are called heterozygous.