In biology, hybrid has two meanings.
The first meaning is the result of interbreeding between two animals or plants of different taxa.
Hybrids between different species within the same genus are sometimes known as interspecific hybrids or crosses.
Hybrids between different sub-species within a species are known as intra-specific hybrids.
Hybrids between different genera are sometimes known as intergeneric hybrids.
Extremely rare interfamilial hybrids have been known to occur (such as the guineafowl hybrids).
The second meaning of "hybrid" is crosses between populations, breeds or cultivars of a single species.
This second meaning is often used in plant and animal breeding.
An example of an intraspecific hybrid is a hybrid between a Bengal tiger and an Amur (Siberian) tiger.
Interspecific hybrids are bred by mating two species, normally from within the same genus.
The offspring display traits and characteristics of both parents.
The offspring of an interspecific cross very often are sterile, this hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct.
Sterility is often attributed to the different number of chromosomes the two species have, for example donkeys have 62 chromosomes, while horses have 64 chromosomes, and mules and hinnies have 63 chromosomes.
Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes because the extra chromosome cannot make a homologous pair at meiosis, meiosis is disrupted, and viable sperm and eggs are not formed.
However, fertility in female mules has been reported with a donkey as the father.
Most often other mechanisms are used by plants and animals to keep gametic isolation and species distinction.
Species often have different mating or courtship patterns or behavours, the breeding seasons maybe distinct and even if mating does occur antigenic reactions to the sperm of other species prevent fertilization or embryo development.
The Lonicera fly is the first known animal species that resulted from natural hybridization.
Until the discovery of the Lonicera fly, this process was known to occur in nature only among plants.