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An F1 hybrid (or Filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types. F1 hybrids are used in genetics and selective breeding. The term is sometimes written with a subscript, as F1 hybrid. The offspring of distinctly different parental types produce a new, uniform phenotype with a combination of characteristics from the parents. In fish breeding, those parents frequently are two closely related fish species, while in plant and animal genetics the 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 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 the phenotypes from each parent that were genetically dominant. Mendel’s discoveries involving the F1 and F2 generations laid the foundation for modern genetics.
Crossing two genetically different plants produces a hybrid seed. This can happen naturally, and includes hybrids between 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 parent cultivars. These F1 hybrids are usually created by means of controlled pollination, sometimes by hand-pollination. For annual plants such as tomato and maize, F1 hybrids must be produced each season.
For mass-production of F1 hybrids with uniform phenotype, the parent plants must have predictable genetic effects on the offspring. Inbreeding and selection for uniformity for multiple generations ensures that the parent lines are almost homozygous. The divergence between the (two) parent lines promotes improved growth and yield characteristics in offspring through the phenomenon of heterosis ("hybrid vigour" or "combining ability").
Two populations of breeding stock with desired characteristics are subjected to inbreeding until the homozygosity of the population exceeds a certain level, usually 90% or more. Typically this requires more than ten generations. Thereafter the two strains 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 sunflower, 62 percent of broccoli and 60 percent of onions were F1 hybrids. Beans and peas are not commercially hybridized because they are automatic pollinators, and hand-pollination is prohibitively expensive.
F2 hybrids, the result of self or cross pollination of F1s, lack the consistency of F1s, though they may retain some desirable traits and can be produced more cheaply, because hand pollination or other interventions are not required. Some seed companies offer F2 seed at less cost, particularly in bedding plants where consistency is less critical.
F1 crosses in animals can be between two inbred lines or between two closely related species or subspecies. In such fish as cichlids, the term F1 cross is used for crosses between two different wild-caught individuals that are assumed to be from different genetic lines.