ELI5 what is true breeding? In "Variation under Domestication", Darwin makes several references to the concept of true breeding: > They believe that every race which breeds true, let the distinctive characters be ever so slight, has had its wild prototype. And: > I crossed some white fantails, which breed very true, with some black barbs. Before diving into the book, I never heard of the term, so I looked it up and found this definition in the Wikipedia article on Zygosity: > A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits that are to be held constant. But this is rather technical and it's not the kind of explanation I would give to a child. What is true breeding, in simple terms? When is breeding not considered true?

In the context of Darwin's _Variation under Domestication_ , "true breeding" is a phenotypic characteristic rather than a genetic one. True-breeding organisms produce offspring that are identical to themselves, concerning some trait -- _i.e._ white fantails, when bred with white fantails, produce characteristically white offspring. For diploid organisms, true-breeding typically implies that the parents are homozygous at the locus conferring the trait of interest.