In this video I explain that inbreeding increases the chance of homozygosity in each locus. This is also true for homozygosity in general. Inbreeding causes an increase in the level of homozygosity, and a decrease in the level of heterozygosity. At 100 % inbreeding no heterozygosity is left.
The measurement of inbreeding in the various breeds can also be regarded as a measurement of differentiation between breeds. Inbreeding in a breed is caused by random loss of variation, whereby the breed loses parts of its original variation. Inbreeding drives the evolution, which is competition between populations on a higher level. Each population, if it is large enough, will acquire new mutations to compensate for the loss of original common variation.
It is known that one of the consequence of strong inbreeding is loss of vitality. A linear negative relationship between production traits and the degree of inbreeding can be predicted. Therefore it is important to ensure that the inbreeding is kept at the lowest level possible among the production animals. This can be done using pedigree information before mating, or by use of more robust systems, such as subdividing the animals into herds, keeping the females in the herd while adding new males from other herds. This system can ensure a low level of inbreeding.
The decrease in the level of the production traits is called inbreeding depression. It is at a magnitude of 0.4 units for an increase in the inbreeding of one unit. For reproduction traits or fitness traits the decrease is much higher, at about one unit. For reproduction traits or fitness traits the decrease is much higher, at about one unit. This cause the fact that it is extremely difficult to produce 100 % inbred domestic animals.