true-breeding

A true-breeding line is one that, when crossed within itself, always yields offspring just like the parents for the trait in question — generation after generation, the trait stays put. A true-breeding tall pea variety produces only tall offspring; a true-breeding white-flowered line produces only white flowers.

This reliability comes from being homozygous for the relevant gene: with two identical alleles, every gamete carries the same version, so no hidden alternative can surface in the next generation. True-breeding stocks were the essential raw material of Mendel's experiments, giving him clean, predictable starting points before he made his hybrid crosses.

A line can be true-breeding for one trait while still varying in others, since the property is judged gene by gene. The term applies most cleanly to single-gene traits; for traits shaped by many genes and the environment, 'true-breeding' is at best an approximation, because the phenotype can drift even when the genotype is fixed.