Linjeavling- eneste riktige måten å avle på for å sikre mangfold?

[Yodel]

Jeg fikk tilsendt denne artikkelen skrevet av en meget kjent basenji oppdretter, som jobber med genforskning/ har skrevet flere srtikler om sykdommer på hund/ nedarvingssystemer osv.

Hennes teori er at den enste måten å drive oppdrett på er igjennom linjeavling. Dette finner hun støtte i gjennom følgende lille prosjekt, der et par oppdrettere ble tildelt en "ny rase" (alt foregikk i cyberspace under oppsyn og laget av en gruppe forskere):

WHAT IS DIVERSITY, REALLY?

by Mary Lou Kenworthy

These are the results of a Diversity Program on a genetics list in which I participated. The program consisted of importing a theoretical breed new to the country where we lived. It started with a limited foundation stock of Cyber-dogs.

The owner of the list, a professor of genetics at a Canadian university, ran a simulated breeding program with 8 breeders founding this theoretical breed. Each breeder was given a dog and a bitch (litter mates) to start. There were two inherited defects in some of the foundation dogs – one early onset and one late onset. Little was known of the background of the dogs because they were imported. A few of the dogs were known to be related. Some dogs carried one problem and some the other but none of these foundation dogs carried both defects. Some didn’t carry either. This was all the information we were given to start. No one knew which dogs carried what or which ones were healthy. From this start everyone bred theoretical litters.

Since this was basically a diversity list, six of the breeders immediately went after diversity by breeding to males as distant from their bitches as they could get. One other breeder and myself believed in line and in breeding. The other breeder’s dogs had known relatives in the program so she bred to them. Not wanting to cause a furor on the list, my first litter was an “accidental” breeding of brother to sister - when it turned out I had a fence climber. <G>

This other breeder’s dogs had the early onset problem. Since she had linebred she quickly discovered this and figured out where it came from so she knew which dogs to avoid in future breeding. In a few generations she had clear stock.

My dogs carried the late onset problem and fortunately it showed up in my first litter so I knew it was there before I had gone too far down the wrong road. With late onset problems you can be in deep, deep do-do before you realize it. Both problems proved to be recessively inherited.

I then bred my dogs, that hadn’t come down with the problem, with dogs from the other breeder’s line. As her dogs were now free of any problems the only thing we had to worry about was carriers of my late onset problem. With further linebreeding, this too could be eliminated. The study was based on 10 generations and in that time the other breeder and I knew exactly what we had and where our one problem may be hiding. Given another few generations we could have eliminated it entirely. Seeing the results other people got from using our dogs helped too. The health problems were randomly computer generated within their possibilities.

The other six people thought they could control problems with outcrossing. Some of them started with the early onset problem, some had the late onset problem and at least one of them had clear dogs with no problems. Each of these six breeders bred to six or seven other people’s dogs. Everyone kept records that were shared. After 3 or 4 generations, these 6 people started seeing the problems. After a few more generations they could no longer outcross because everyone else had done the same thing and bred to all the dogs so there was nothing ‘different’ to go to.

All their pedigrees were the same hoge-podge of the many foundation dogs and now they all carried both problems. Since none of the six had linebred they never discovered which of the foundation dogs were the healthy ones. These were the dogs that should have been used for back-crosses. Now it was too late. The six people with the outcrossed dogs now had affected and/or carriers of two genetic problems and they had no idea who carried what recessively. Instead of there being some clear and some with one problem all of their dogs now carried two problems recessively. Now that the six people had already incorporated everything into their breeding programs there was no diversity left.

The two breeders who line bred had healthy stock with only a few carrying one problem and they knew where to expect that problem to show up. So they were ahead of the game.

Population genetics preaches diversity mistakenly equated to anti-line breeding. And yes, diversity is necessary in the whole population. But short of God, who controls whole populations??

The main problem I notice is that most people try to apply "population genetics" to individual breeding programs and can not separate the two in their minds.

The only way to keep diversity within a breed is by line breeding. Let me say that again so you know I didn’t make a mistake. “THE ONLY WAY TO KEEP DIVERSITY WITHIN A BREED IS BY LINE BREEDING.”

One breeder must line breed on a certain few dogs long enough to establish a separate line (different from all the rest). Another breeder must line breed on a few different dogs and establishes another line. A third breeder should line breed on yet some other dogs and establishes yet another line. A forth breeder – etc., etc., etc. NOW we have diversity! Even if all are descended from the same limited number of ancestors, by having different lines, we maintained diversity.

When you line breed and run into a problem you can always out-cross. If you are already out-crossing and run into a problem (and you will) where do you go then?

My grandmother always used to say, “Sweep your own doorstep first.” If each breeder creates and monitors their own line, the breed, as a whole, will prosper. No breeder can maintain diversity by himself and any attempt to do so will lead to disaster for the breeder and the breed – it takes a network of breeders working together with individual lines to maintain diversity.

Line-breeding and in-breeding get a bad rap because ‘popular studs’ are overused. The big winners owned by the politically correct individuals are bred to the most bitches. Soon they are in everyone’s pedigrees multiple times. Their problems are spread throughout the breed and the use of animals that are not big winner or properly owned is lost along with what diversity they could have offered.

Mener dere at linjeavling det eneste rette for å skape et friskt mangfold i en rase?

[Petra]

Jeg vil tro at de to(?) oppdretterne som linjeavlet lykkes så godt i dette tilfellet fordi forsøket er så forenklet - de har kun to sykdommer å ta hensyn til og that's it. Så snart de har fått identifisert og utelukket "hundene" som bærer disse kan de pare de resterende dyrene de har så mye de vil uten å risikere at noe nytt dukker opp. Slik er det jo gjerne ikke i virkeligheten, hvor hvert dyr gjerne bærer flere recessive gener med negativ effekt som vil økes i antall etter hvert som innavlsgraden øker.

Ellers har de vel rett i at det å pare alle individene med alle ofte er en dårlig løsning, av de grunnene som nevnes i innlegget. Jeg har i hvert fall lært fra avlslæren at den beste måten å få økt genetisk fremgang uten for stor økning i innavl er å ha flere små subpopulasjoner innen en rase med bare begrenset og nøye gjennomtenkt utkrysning mellom subpopulasjonene. Men jeg er litt usikker på hvordan man får til dette i praksis med en så liten startpopulasjon som i dette tilfellet, det er vel fort gjort at man enten utkrysser for lite og får innavlsdepresjon i hver linje, eller utkrysser for mye slik at alle blir i slekt.