Inbreeding is defined as the mating of related individuals - individuals that share at least one ancestor. Since all cattle are ultimately related, and since we don't think of all matings as producing inbred offspring, inbreeding is more precisely defined as the mating of individuals more closely related than is average for the population. The opposite of inbreeding is outbreeding or the mating of individuals less closely related than is average for the population. Linebreeding is a euphemism for inbreeding, although it usually involves the avoidance of very close matings and may be an effort to retain in the herd the genes of a particularly revered ancestor or group of ancestors. To understand the effects of inbreeding and outbreeding, it is necessary to see what is happening to the genes themselves during the inbreeding and outbreeding processes.
Genes come in pairs, one member of each pair inherited from the sire, the other from the dam.
The genes of a pair may or may not be identical in function. When they are identical, the genes are said to be homozygous , and when they are not, they are termed heterozygous . A gene may function differently depending on the gene it is paired with. A recessive gene, on the other hand, is one whose function can be partially or completely overridden when paired to a dominant gene. The gene for polledness, for example, is dominant and the gene for the horned character is recessive.