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F1 hybrid is a term used in genetics and selective breeding. F1 stands for Filial 1, the first filial generation seeds/plants or animal offspring resulting from a cross mating of distinctly different parental types. The term is sometimes written with a subscript, as F1 hybrid. The offspring of distinctly different parental types produce a new, uniform variety with specific characteristics from either or both parents. In fish breeding, those parents frequently are two closely related fish species, while in plant and animal genetics those parents usually are two inbred lines. Mules are F1 hybrids between horse and donkey. Today, certain domestic hybrid breeds, such as the Savannah cat, are classified by their filial generation number.
Gregor Mendel's groundbreaking work in the 19th century focused on patterns of inheritance and the genetic basis for variation. In his cross-pollination experiments involving two true-breeding, or homozygous, parents, Mendel found that the resulting F1 generation were heterozygous and consistent. The offspring showed a combination of those phenotypes from each of the parents that were genetically dominant. Mendel’s discoveries involving the F1 and F2 generation laid the foundation for modern genetics.
Crossing two genetically different plants produces a hybrid seed (plant). This can happen naturally, and includes hybrids between two different species (for example, peppermint is a sterile F1 hybrid of watermint and spearmint). In agronomy, the term “F1 hybrid” is usually reserved for agricultural cultivars derived from two different parent cultivars. These F1 hybrids are usually created by means of controlled pollination, sometimes by hand-pollination. For annual plants such as tomato "hybrids" and "hybrid inbred" maize, F1 hybrids must be produced each season.
For mass-production of F1 hybrids with uniform phenotype, the parent plants have to have predictable genetic effects on the offspring. Inbreeding and selection for uniformity for a number of generations ensures that the parent lines are almost homozygous. The divergence between the parent lines promotes improved growth and yield characteristics in the F1 offspring through the phenomenon of heterosis ("hybrid vigour").
Two populations of breeding stock with desired characteristics are subject to inbreeding until the homozygosity of the population exceeds a certain level, usually 90% or more. Typically this requires more than ten generations. After this happens, both populations must be crossed while avoiding self-fertilization. Normally this happens in plants by deactivating or removing male flowers from one population, taking advantage of time differences between male and female flowering or hand-pollinating.
In 1960, 99 percent of all corn planted in the United States, 95 percent of sugar beet, 80 percent of spinach, 80 percent of sunflowers, 62 percent of broccoli, and 60 percent of onions were hybrid. Beans and peas are not commercially hybridized because they are automatic pollinators, and hand-pollination is prohibitively expensive.
While an F2 hybrid, the result of self or cross pollination of an F1, does not have the consistency of the F1 hybrid, it may retain some desirable traits and can be produced more cheaply as no intervention in the pollination is required. Some seed companies offer F2 seed, particularly in bedding plants where consistency is not as critical.
F1 crosses in animals can be between two inbred lines or between two closely related species or subspecies, where such crosses are possible. In some fish, such as cichlids, the term F1 cross is used for crosses between two different individuals known to have been collected from the wild, and therefore assumed to be from different genetic lines.