F1 hybrid

An F1 hybrid (or filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types.^[1] F1 hybrids are used in genetics, and in selective breeding, where it may appear as F1 crossbreed. The term is sometimes written with a subscript, as F₁ hybrid.^[2][3] Subsequent generations are called F₂, F₃, etc.

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 breeding the parents often are two inbred lines. Mules are F1 hybrids between horse and donkey. Today, certain domestic–wild 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.

Production of F1 hybrids

In plants

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 is done with plants by deactivating or removing male flowers from one population, taking advantage of time differences between male and female flowering or hand-pollinating.^[4]

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 hybrid

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.^[5]

In animals

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.^[6]

Advantages

• Homogeneity and predictability—The genes of individual plant or animal F1 offspring of homozygous pure lines display limited variation, making their phenotype uniform and therefore attractive for mechanical operations and easing fine population management. Once the characteristics of the cross are known, repeating this cross yields 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 increases the likelihood of an optimal version of the enzyme being present and reduces the likelihood of a genetic defect.

Disadvantages

• The main advantage of F1 hybrids in agriculture is also their drawback. When F1 cultivars are used as parents, their offspring (F2 generation) vary greatly from one another. Some F2s are high in homozygous genes, as found in their grandparents, and these will lack hybrid vigour. From the point of view of a commercial seed producer who does not wish customers to produce their own seed, this genetic assortment is a desired characteristic.
• Both inbreeding and crossing the ancestral lines of the hybrid are costly, which translates into a much higher price. In general, the higher yield, etc., offsets this disadvantage.
• F1 hybrids mature at the same time when raised under the same environmental conditions. They all ripen simultaneously and can be more easily harvested by machine. Traditional cultivars and landraces are often more useful to gardeners because they crop over a longer period of time, avoiding gluts and food shortages.

See also

• Backcrossing
• Heterosis ("hybrid vigour")
• Genetics
• Heirloom plant

References