Friday, April 13, 2018

Bottle Necks, Sire Selection, and Disease


The University of California at Davis
has a nice post entitled How a Genetic Mutation From 1 Bull Caused the Loss of Half a Million Calves Worldwide


A bull born in 1962, by the name of Pawnee Farm Arlinda Chief, produced 16,000 daughters, 500,000 granddaughters, and more than 2 million great-granddaughters. His sons were also popular sires.

As a result, Chief’s chromosomes account for almost 14 percent of the genome in the current Holstein population in the United States. It seems Chief was the source of a haplotype (a group of genes that are inherited from one parent) on chromosome 5 of Holstein cows that was associated with lower fertility rates and embryo loss.

Because Chief was such a popular sire, his genes alone were responsible for over 100,000 cumulative Holstein calf losses over 30 years in the United States alone, and about half a million worldwide.

What does this have to do with dogs?
A bit. As I noted on a post on this blog more that a decade ago:

When pressed about the poor genetic quality of today's "pure bred" dogs, most Kennel Club breeders parrot the Kennel Club apologia: "We only register dogs, we don't breed them."

In fact, the line is pure bunk.

The Kennel Club does far more than register dogs -- it sets the rules that guarantee more and more dogs will suffer serious (and often painful) genetic problems.

The problem, in a nutshell, is the closed registry system.

With all Kennel Club breeds, the "founding stock" has always been small in number, and often fairly inbred going in, since breed creation is a product of inbreeding and line breeding to "set" the look of a dog.

Because a closed registry never adds new blood, it becomes progressively more inbred over time.

Genetic diversity is never increased in the Kennel Club -- it is only reduced. In practice, it is often reduced quite rapidly due to the fact that show-winning males are in great demand to "cover" as many bitches as possible -- the so-called "popular sire effect."....

How did the Kennel Club come to embrace a closed registry, and why does it maintain this system?

The adoption of a "closed registry" by the Kennel Club is an artifact of its history, while the continuation of this practice is driven by the economics of dog breeding and the political construct of the Kennel Club.

The Kennel Club was created in Victorian England in 1873, at a time when new theories about genetics were being promulgated by learned men who did not yet have a very good idea of what was going on in the natural world.

As noted in American Working Terriers, the "speciation" of domestic breeds of livestock began with the work of Robert Bakewell in the 18th Century, and the control of sires. Bakewell's work helped speed the rise of the Enclosure Movement, which in turn led to large estates, fox hunting, and the rise of terrier work.

Bakewell had no real knowledge of scientific genetics, and his breeding program was largely limited to the control of sires (made easier by enclosures) and the admonition that "like begats like" and that success was to be found by "breeding the best to the best".

The first stud book to document the breeding of animals was the General Stud Book of 1791 which tracked a small pool of racing horses. A stud book for Shorthorn Cattle was produced in 1822.

As more and more farmers followed the tenets of Robert Bakewell, sire selection became increasingly prevalent and inbreeding and line breeding more common. By selecting the best beef and milk producers, and pairing them, rapid improvements in cattle breeds were made. 

What was different with farmers, than with dogs, however, was that down on the farm there was a clear axis of production.

Farmers inbreeding animals for improvement began to notice that fertility rates began to drop after a few generations. In some lines disease popped up, or defects such as weak hocks appeared. Inbred animals were not better if they remained increasingly inbred, and so hybrids or out-crosses began to become the rule.

Because farm herds are large and often kept by families for generations, farmers were able to "tease out" data indicating drops in production, increases in mortality, declining fecundity, and a steady rise in disease and illness.

Inbreeding, which had initially boosted production, now appeared to be reducing it.

Because farmers had a clear "steak and eggs" axis for evaluation of stock, they were ready and willing to outcross to achieve the best results for their needs and their land. Consumers, after all, do not much care what breed of chicken their eggs come from, or what "champion" bull sired their steak.

Through experimentation, farmers discovered that outcrosses and hybrids of two "pure" types produce as well or better, while remaining more disease resistant, more fecund, and longer-lived than deeply homogeneous stock.

What may appear to be a pure Angus (the most common breed of beef cattle in the world) is likely to have a wide variety of cattle genes coursing through its system. In fact, entire breeds of cattle are now kept solely for their outcross potential. On today's farms the cattle in the field may be Brangus (Brahman-Angus crosses), Braford (Brahmam-Hereford crosses), Beefmasters (a cross of Hereford, Shorthorn and Brahman), or any other combination or mix.

And so, once again, man learns and forgets, learns and forgets, but slowly, slowly we make progress.

2 comments:

Daniƫlle Termijn said...

Hello Terrierman, I'm just reading these days in the book Managing Breeds for a secure future by Sponenberg, Martin and Beranger. I recognize some what you have shared in your blog and believe that this second edition is a good read. I'm a admire and owner of the Dobermann breed and try to understand also through this book more about their status quo and what the next step should be. It is a complex matter !

hj said...

That's a great article.
I'm sad that people don't want mixed breed dogs.
Many people want purebred dogs, and because of that,
purebred dogs suffer from genetic diseases.