Mating based on phenotype is called assortative mating. Mating like to like is positive assortative mating. Mating like to unlike (i.e., bringing a new trait in or correcting a fault) is negative assortative mating.
Matings based on genotype depend on the relationship between the two individuals being bred. If two parents within a breed are less related than the average relationship within the breed (measured as an inbreeding coefficient), then it is an outbreeding.
If the mating of two parents produces an inbreeding coefficient higher than the breed average, it is a linebreeding or inbreeding. Linebreeding concentrates the genes of specific common ancestors behind both parents.
Inbreeding is the mating of close relatives, such as parent to offspring, or full brother x sister.
Linebreeding and inbreeding increase homozygosity, or the pairing of like genes (aa or AA) in the offspring. This can increase the expression of both positive and negative recessive traits.
Outbreeding increases heterozygosity, or the pairing of unlike genes (Aa). Outbreeding propagates both positive and negative recessive genes in the carrier state. Linebreeding does not create deleterious genes, and outbreeding does not eliminate them.
These breeding practices only affect the expression or masking of deleterious genes through homozygosity or heterozygosity. Selection of breeding stock is what affects the increase or decrease in the frequency of desirable and deleterious genes.
Inbreeding or tight linebreeding should only be attempted with knowledge of the range of genes that the parents may carry based on prior matings or genetic testing.
Crossbreeding is the production of offspring by mating individuals from two or more breeds. This produces maximum heterozygosity. Designer breeds are created through crossbreeding. Their offspring do not reproduce themselves if bred.