The Better Breeding Blog Breed Better Animals

The concept of natural selection is well known. Animals mate, animals are born, and those best suited to their environment survive to pass their genes on to the next generation. Populations, over time, improve their ability to cope with their environment.

A good breeding programme also improves an animal population over time. But this improvement is artificial, in that it is driven by people. And not with nature in mind, but with an end use for that animal in mind. Yes, nature still gets involved (eg deaths during birth, congenital problems, diseases, accidents), and breeders who go against the environment their animals live in do so at their peril. But ultimately with artificial selection it is the breeder, not nature, who selects which animals get to mate, and to whom. The breeder, not nature, decides who is and isn’t suitable for breeding.

How a breeder does this is two-fold. The first is the selection process, deciding which animals get to pass their genes on to the next generation. The second is the actual mating process of matching selected males with selected females. We’ll cover these in the next two posts.

Artificial selection begins when a breeder selects what to him are the “best” animals to be parents and deselects those that aren’t. The expectation is that what makes animals “best” will be passed to their progeny, while what isn’t “best” is removed from future generations as much as possible. In some industries, artificial insemination or embryo transfer introduces “best” into a herd through genetic material rather than physical animals.

Animals selected for breeding may be deselected in turn as even better animals are brought in later. Replacement selection­ is when animals are chosen to be parents for the first time in a breeder’s herd (they may already have produced progeny elsewhere), while animals removed as parents are said to have been culled from that herd. (Culled animals may still be sought-after by other breeders to be first-time parents in their own herds.)

Phenotypic selection is when animals are chosen purely by how they look or perform as individuals. For example, mate biggest to biggest, or fastest to fastest, and repeat. After a few generations you may well have animals bigger, or faster, than the ones you began with. Or not, as anyone who’s mated the same two white alpacas of unknown ancestry year after year and had every colour but white may know!

Many breeders also take into consideration phenotypic records of relatives.

For example, a dog breeder may like the visible phenotype of a puppy, but draws further on pedigree data to assess what other qualities may have been acquired through its parents from their ancestors. This approach is subjective, as nothing can be measured definitively.

A sheep breeder on the other hand may go the other way and consider progeny data rather than ancestral data. Traits like birth weight, weaning weight, fleece weight and micron are routinely measured in commercial industries, and vast amounts of data accumulate this way. This is a more objective approach. A sire who is consistently producing quality, measurable, offspring is likely to keep doing so. But there is still a degree of subjectivity, in that gains may not be as strong if the new dams themselves are not as “best” as previously covered dams.

Whether subjective or objective, the end goal is the same: assessing the ability of a particular animal to pass on the “best” qualities, based on its and its relatives’ performance (phenotype). In other words, assessing that animal’s “breeding value”. Using relatives’ performance helps increase the accuracy of such predictions: if ancestors or progeny show very similar characteristics to each other then an individual animal closely related to those is also likely to be similar. And if closely-related animals have similar performances, it follows that the phenotype of interest is highly heritable. We’ll be covering heritability and the measurement of breeding value in much more detail in future posts.

Once animals have been selected the next step is mating, or deciding which of those selected animals to put to whom. We’ll cover this in the next post.

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.

And that concludes the “Background Information” section of this blog!

I avoided discussing concepts like “genes” and “genotype” in much detail, as I wanted to focus more on the foundations behind breeding. It’s worth stressing that the mechanisms of heredity were unknown for most of the history of breeding, and it wasn’t until the work of Gregor Mendel in the mid 19th century that genetics as a field of study could even arise (more on this to in the next section). And yet this lack of understanding in no way prevented the development of the thousands of breeds of dogs, cats, horses, cattle, sheep, goats, pigs, ad poultry in the centuries prior. Many of those breeds are still with us today, a tribute to the countless astute and observant breeders who shaped them.

Genetics is now a well-established field with more than a hundred years of study behind it. It is an invaluable tool in modern breeding programmes, but to use it  well requires understanding it. The next section of this blog will provide a good introduction to genetics and how heredity works.