allele: One of the different forms of a gene that can exist at a single locus.

filial generations: Successive generations of progeny in a controlled series of crosses, starting with two specific parents (the P generation) and selfing or intercrossing the progeny of each new (F1, F2,...) generation.

zygote: The cell formed by the fusion of an egg (gamete from female parent) and a sperm (gamete from male parent); the unique diploid cell that will divide mitotically to create a differentiated diploid organism.

homozygote: An individual having a homozygous gene pair (having identical alleles in both copies).

heterozygote: An individual having a heterozygous gene pair (having different alleles in the two chromosome sets of the diploid individual).

testcross: A cross of an individual of unknown genotype or a heterozygote (or a multiple heterozygote) to a tester individual.

tester: An individual homozygous for one or more recessive alleles, used in a testcross.

reciprocal cross: A pair of crosses of the type shown below.

genotype A female X genotype B male
genotype B female X genotype A male