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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 offspring of distinctly different parental types produce a new, uniform variety with specific and/or desirable 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.

Crossing specific parent plants produces a hybrid seed (plant) by means of controlled pollination. To produce consistent F1 hybrids, the original cross must be repeated each season. As in the original cross, in plants this is usually done through controlled hand-pollination, and explains why F1 seeds can often be expensive. F1 hybrids can also occur naturally, a prime example being peppermint, which is not a species evolved by cladogenesis or gradual change from a single ancestor, but a sterile stereotyped hybrid of watermint and spearmint. Unable to produce seeds, it propogates through the vining spread of its own root system.

In agronomy, the term “F1 hybrid” is usually reserved for agricultural cultivars derived from two different parent cultivars, each of which are inbred for a number of generations to the extent that they are almost homozygous. The divergence between the parent lines promotes improved growth and yield characteristics through the phenomenon of heterosis ("hybrid vigour"), whilst the homozygosity of the parent lines ensures a phenotypically uniform F1 generation. Each year, for example, specific tomato "hybrids" are specifically recreated by crossing the two parent heirloom cultivars over again.

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 all phenotypically resembled the dominant parent plant. Mendel’s discoveries involving the F1 and F2 generation lay the foundation for modern genetics. Today, certain domestic hybrid breeds, such as the Savannah cat, are classified by their filial generation number.

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[1].

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. Such figures are probably higher today. Beans and peas are not commercially hybridized because they are automatic pollinators, and hand-pollination is prohibitively expensive.

Unlike most plants, commonly bred fish species as well as all mammals and birds are not hermaphrodite, and therefore it is much easier to prevent self-fertilization during an F1 cross. F1 crosses in fish can be between two inbred lines or between two closely related fish species, such as cichlid subspecies.[2]. The cross is usually performed by natural or artificial insemination.

• 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 non-junk DNA 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 the genetic hysteresis effect.

• The main advantage of homozygous F1 hybrids is also their drawback. When homozygous F1 cultivars are used for the breeding of a new generation, their offspring (F2 generation) will be high in homozygous genes, as found in the weaker parental generation, and thus will have a depression in yield and lack the hybrid vigor. From the point of view of a commercial seed producer which does not wish its customers to produce their own seed, this is a desired characteristic.
• Both inbreeding and crossing both lines requires a lot of work, which translates in a much higher seed cost. In general, the higher yield offsets this disadvantage.
• F1 hybrids can give higher yields than traditional varieties.
• Homozygous 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.
• Homozygous F1 hybrids lack genetic diversity, especially if they are derived from inbred parental lines. F1 hybrids from very distinctive parents are highly heterozygous and often have large levels of genetic diversity.

• Heterosis ("hybrid vigour")

• Hand pollination
• USDA, Seeds: The Yearbook of Agriculture, 1961.