什么是 The vocabulary of crosses？

A gene is a stretch of chromosome; its alleles are its sequence variants; the locus is its position. A diploid individual carries two alleles at each locus: its genotype is homozygous ($AA$, $aa$) or heterozygous ($Aa$). The phenotype is the observable character. An allele is dominant when the heterozygote shows its phenotype, recessive when it shows only in the homozygote; the distinction is a property of the pair of alleles and of the character observed, not of the allele alone — most recessive alleles are loss-of-function alleles, hidden because one working copy suffices. A pure line breeds true (homozygous); a test cross mates an individual of dominant phenotype with a recessive homozygote, so that its offspring reveal the alleles of its gametes directly.