True breeding organism

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A true breeding organism, sometimes also called a pure-bred, is an organism having certain biological traits which are passed on to all subsequent generations when bred with another true breeding organism for the same traits. In other words, to "breed true" means that two organisms with a particular, heritable phenotype produce only offspring with that (same) phenotype.

By way of contrast - a non-true-breeding organism can (and will, some of the time) produce offspring with different phenotypes (physical characteristics). For instance: if you plant a seed from a red apple, the resultant tree may produce only yellow apples. In this (grossly oversimplified) example: the "parent" genes of the red apple may have been Cc and cc -- the dominant Red allele (C) from one parent overriding the recessive yellow allele (c) from the other -- yet the genes in the offspring (the red apple) comprise the two recessive yellow alleles (which would happen, statistically, half the time) — and will thus create a yellow-apple tree. (A more common example is for two brown-eyed parents to have a blue-eyed child, which has surely led to more than a few heated arguments in the course of human history.)

In the case of a gene with multiple different alleles, the genotype of a true breeding organism is homozygous. For example, a pure-bred variety of cat, such as Siamese, only produce kittens with Siamese characteristics because their ancestors were inbred until they were homozygous for all of the genes that produce the physical characteristics and temperament associated with the Siamese breed.

True breeding is also used to refer to plants that produce only offspring of the same variety when they self-pollinate. For example, when a true-breeding plant with pink flowers is self-pollinated, all its seeds will only produce plants that also have pink flowers. Gregor Mendel cross-pollinated true-breeding peas in his experiments on patterns of inheritance of traits.