truebreeding

True-breeding, also spelled true-breeding or pure-breeding, is a term in genetics and plant breeding describing organisms that, when self-fertilized or crossed with an identical genotype, produce offspring with the same phenotype across generations. This stability typically occurs because the individuals are homozygous at the genes governing the trait, carrying two identical alleles (for example, AA or aa).

True-breeding lines are usually produced by repeated self-fertilization or inbreeding, selecting for the desired trait until uniformity is achieved. In plants, self-pollination is a common method; in animals, controlled matings or line breeding are used. Because the trait is fixed, crossing two true-breeding individuals that differ for a trait results in a uniform F1 generation that reflects the dominant or recessive alleles involved.

In Mendelian genetics, true-breeding lines of contrasting traits were used to reveal inheritance patterns. Crossing a true-breeding dominant line with a true-breeding recessive line yields an F1 that is phenotypically uniform, typically displaying the dominant trait, and heterozygous at the locus. When F1 individuals are selfed, the F2 generation shows a characteristic segregation of traits.

The term is commonly described as pure-breeding or inbred lines in modern breeding programs, which aim for predictable, stable phenotypes. While true-breeding lines offer stability, extensive inbreeding can reduce vigor in some cases, a phenomenon known as inbreeding depression.