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.
Production of F1 hybrids 
In plants 
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.
F2 hybrid 
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.
In animals 
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.
- Homogeneity and predictability - If the parents are homozygous pure lines, there is limited genetic variation between individual F1 plants or animals. This makes their phenotype extremely uniform and thus attractive for mechanical operations and makes it easier to fine-tune the management of the population. Once the characteristics of the cross are known, repeating this cross will yield exactly the same result.
- Higher performance - As most alleles code for different versions of a protein or enzyme, having two different versions of this allele amounts to having two different versions of the enzyme. This will increase the likelihood of having an optimal version of the enzyme present and reduce the likelihood of a genetic defect. This effect is referred to in genetics as heterosis.
- F1 hybrids can give higher yields than traditional varieties.
- The main advantage of F1 hybrids in agriculture is also their drawback. When F1 cultivars are used for the breeding of a new generation, their offspring (F2 generation) will vary greatly from one another. Some of the F2 generation will be high in homozygous genes, as found in the weaker parental generation, and these will have a depression in yield and lack the hybrid vigour. From the point of view of a commercial seed producer which does not wish its customers to produce their own seed, this genetic assortment is a desired characteristic.
- Both inbreeding and crossing the lines requires a lot of work, which translates into a much higher seed cost. In general, the higher yield offsets this disadvantage.
- F1 hybrids mature at the same time when raised under the same environmental conditions. This is of interest for modern farmers, because all ripen at the same time and can be harvested by machine. Traditional varieties are often more useful to gardeners because they crop over a longer period of time, avoiding gluts and food shortages.
See also 
- Marschall S. Runge and Cam Patterson (editors) (2006). Principles of Molecular Medicine. Humana Press. p. 58. ISBN 978-1-58829-202-5.
- Peter Abramoff and Robert G. Thomson (1994). Laboratory Outlines in Biology--VI. Macmillan. p. 497. ISBN 978-0-7167-2633-3.
- William Ernest Castle and Gregor Mendel (1922). Genetics and eugenics: a text-book for students of biology and a reference book for animal and plant breeders. Harvard University Press. p. 101.
- Hand Pollination
- Lawrence D. Hills 1987. F2 and open pollinated varieties. Growing From Seed (The Seed Raising Journal from Thompson & Morgan) 1(2) 
- "Guide to selecting and breeding high quality cichlids". bigskycichlids.com.
- Hand pollination
- USDA, Seeds: The Yearbook of Agriculture, 1961.