An F1 hybrid (also known as filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types. F1 hybrids are used in genetics, and in selective breeding, where the term F1 crossbreed may be used. The term is sometimes written with a subscript, as F hybrid. Subsequent generations are called F, F, 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. 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 was 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 10 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. In 1960, 99% of all corn, 95% of sugar beet, 80% of spinach, 80% of sunflowers, 62% of broccoli, and 60%