True breeding organism
A true breeding organism, sometimes also called a pure-bred, is an organism that always passes down a certain phenotypic trait to its offspring. An organism is referred to as true breeding for each trait to which this applies, and the term "true breeding" is also used to describe individual genetic traits. In Mendelian genetics, this means that an organism must be homozygous for every trait for which it is considered true breeding. Apomixis and parthenogenesis, types of asexual reproduction, also result in true breeding, although the organisms are usually not homozygous.
A pure-bred variety of cat, such as Siamese, only produces 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.
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.
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