Inbreeding, linebreeding, understanding the pricipals

[Whizzy]

The article on selecting a breeder brought up the subject of breeding pricipals; inbreeding, linebreeding and outcrossing. I'd like to go a step further and perhaps help people understand the differences, and explain how and why a breeder will use these tools in a successful breeding program.

We've all heard the term "inbreeding" and in general it is assumed to be bad. Inbreeding, in and of itself, is not bad. It's a tool that can be used, but must be used with discretion and the utmost understanding of what one is doing.

Inbreeding is defined as the production of offspring by parents more closely related than the average of the population. Some breeders erroneously consider close inbreeding (brother/sister or father/daughter) as inbreeding, and the less intense (grandfather/granddaughter) as linebreeding. They are both inbreeding.

In the initial stages of the development of any breed, inbreeding was necessary to create and fix type. The number of animals available for breeding was limited, and most if not all of them were related. Animals of the desired type were bred together to increase the number of offspring with correct type, and weed out those that did not have correct type. Many breeders of purebred companion animals still use inbreeding to concentrate the genetic inheritance of desirable characteristics, and to identify carriers of undesirable characteristics so they can be eliminated from the gene pool.

What does inbreeding do genetically? It tends to make the offspring more homoqygous for the genes they carry. In other words, the genes of each pair that the dog carries tend to be more alike than different. To examine what that means, lets take a purely subjective illlustration.

Animal 1.....AABBCCDD (homozygous for 4 pairs of genes)
Animal 2.....AaBbCcDc (heterozygous for 4 pairs of genes)

Animal 1 can only contribute ABCD to each of it's offspring, while animal 2 can contribute one of 16 different combinations. Obviously the offspring of animal 1 will resemble each other and that parent more than the offspring of animal 2.

In a breeding program using inbreeding, that's what the breeder is hoping to do; produce offspring that show the desired traits of the parents. But there are pitfalls.

Undesirable recessive traits can crop up. Of course, a responsible breeder would ethically remove the parents of such animals from their breeding program, because they have just identified carriers. But inbreeding does not create new recessive genes, it only alows them to be expressed and identified.

Increased inbreeding can result in a decline of fertility in some animals, probably due to pairing of detrimental recessive genes, or partially dominant ones. Inbreeding can also cause a declline in the traits related to physical fitness. The death rate shortly after birth is higher in inbred animals, and inbred animals can lack somewhat in size, although this is not always the case.

To be continued......

Reference: "Abnormalities of Companion Animals"; C.W. Foley, J.F. Lsley, G.D. Osweiler