True-Breeding Plants

Fresh peas
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Definition

A true-breeding plant is one that, when self-fertilized, only produces offspring with the same traits. True-breeding organisms are genetically identical and have identical alleles for specified traits. The alleles for these type of organisms are homozygous. True-breeding plants and organisms may express phenotypes that are either homozygous dominant or homozygous recessive. In complete dominance inheritance, dominant phenotypes are expressed and recessive phenotypes are masked in heterozygous individuals.

The process by which genes for particular traits are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.

Examples

The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). The round seed shape is dominant to the wrinkled seed shape. A true-breeding plant with round seeds would have a genotype of (RR) for that trait and a true-breeding plant with wrinkled seeds would have a genotype of (rr). When allowed to self-pollinate, the true-breeding plant with round seeds would produce only progeny with round seeds. The true-breeding plant with wrinkled seeds would only produce progeny with wrinkled seeds.

Cross pollination between a true-breeding plant with round seeds and a true-breeding plant with wrinkled seeds (RR X rr) results in offspring (F1 generation) that are all heterozygous dominant for round seed shape (Rr).

Self-pollination in F1 generation plants (Rr X Rr) results in offspring (F2 generation) with a 3-to-1 ratio of round seeds to wrinkled seeds. Half of these plants would be heterozygous for round seed shape (Rr), 1/4 would be homozygous dominant for round seed shape (RR), and 1/4 would be homozygous recessive for wrinkled seed shape (rr).