ELI5 what is true breeding?
In the context of Darwin's Variation under Domestication, &quot;true breeding&quot; 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.