Zygosity

Zygosity is a way of classifying genotypes. Simply put, if the two copies of a gene in a diploid organism are the same alleles, e.g. w⁺/w⁺, the individual is a homozygote. If the two alleles are different, e.g. w⁺/w^-, the individual is a heterozygote.

Less commonly we can talk about hemizygotes where an organism only has one copy of a gene. For example most genes on an X-chromosome are present as only one copy in males. Individuals can also be hemizygous because of an insertion or deletion.

A fourth term, nullizygous, seems to have an inconsistent definition. It is sometimes used to refer to cases where both copies of the gene are completely missing, as in a homozygous deletion. However, it sometimes is used to refer to being homozygous for inactive null alleles (amorphs).