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, inheritable phenotype produce only offspring with that (same) phenotype.
In the case of a gene with multiple different alleles in the population, 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.
The definition of true breeding is : Pertaining to an individual all of whose offspring produced through self fertilization are identical to the parental type. True breeding individuals are homozygous for a given trait.
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