The selection process forms the group of animals that are to be parents. The mating process then matches males to females from within that group. These two steps are repeated over and over, and ideally each generation will improve on the previous one through careful selections and controlled breeding. 

Breeders match animals for several reasons:

Corrective Mating
Corrective matings aim to produce animals without the faults of one or both parents, ultimately creating a more uniform and/or more sound group of animals. One example is to mate unusually small females to an unusually large male to produce more optimally-sized animals. The concerted efforts of reputable dog breeders to remove hip dysplasia is an example of long-term corrective matings breeding out common genetic disorders over time.

Rapid Genetic Gain
Depending on the phenotype sought, and its heritability, a high-performing sire over poor-performing females can produce progeny significantly superior to their dams. Breeders with access to good genetics can quite quickly breed up a quality herd from lesser foundation animals this way. For example, most alpacas in the early years of the Australian alpaca industry were of low fibre density and coverage compared to now. Animals of greater density and coverage were imported from the mid 1990s on, and the changes in the national population were significant and rapid. The older styled animal disappeared from view in just a few years as the newer styled animal took precedence.

Complementarity
This is where two parents complement each other such that the progeny is optimal for those phenotypes. Examples follow.

Complementarity: Cross-Breeding
Rapid change within established breeds is not easy to do, as the range of variance is usually small — outliers in the population that could increase genetic gain don’t occur that often.

A much quicker way to introduce rapid change is to perform between-breed selection, also known as cross-breeding. This is because desirable elements from one breed can be introduced quickly to another breed that lacks these same elements, simply by mating the two together. The progeny have the best of both worlds in many cases. A common example in the Australian beef industry is the “Black Baldy”, a cross between Hereford and Angus cattle that produces a black animal with a white face. The Black Baldy cross takes on the “best” phenotypes of both breeds and is highly regarded for rapid weight gain, quality meat and general all-roundedness.^[1]

Some poultry exhibitors crossbreed their breed with another, then backcross the progeny to the original breed. With careful selections and matings they can “fix” the phenotype they sought from the introduced breed into their birds whilst still staying true to the characteristics of their breed.

Complementarity: Hybrid Vigour (Heterosis)
This is the improved fertility, productivity and survivability of progeny that often arises from cross-breeding and is very desirable in commercial industries — see the “Black Baldy” example above.

Hybrid offspring when mated together often show reduced vigour however, which is why parental pure lines are retained and cross-breeding continually done.

Inbreeding/Linebreeding
Inbreeding is the mating of close relatives. This can lead to inbreeding depression (reduced vigour) and genetically defective animals, but when done carefully is a very useful way of “fixing” particular phenotypes. Linebreeding is a mild form of inbreeding and routinely used in purebred breeding programmes to good effect. We’ll be covering this topic in more detail later on.

Reference:

1. Miller, C. 2015. Black Baldies Come Up Trumps. Retrieved 13th January, 2019.