Q: "I thought inbreeding always caused problems. Couldn't it have caused the problems in ferrets we see like adrenals?" A: Ever notice in the "Back to the Future" trilogy that when Marty goes forward in time, his son and daughter look just like him, and when he goes back to the old west and sees his great grandfather, the two look exactly alike, and his mother looks just like his great grandmother? I've always suspected THAT was the result of the yucky kind of inbreeding! Zip your McFly, will ya? I don't think adrenal disease in ferrets is due to inbreeding; as I've said before,I think it is a species trait. Nonetheless, there is no doubt inbreeding can cause problems, but the same kind of problems can occur regardless of the genealogy of the two breeding lines. Breeding domesticated animals is a lot like baseball; you've got your bats, your balls, and you have statistics that rule the show. Like a spitball increasing the chance a batter won't get a hit, inbreeding increases the chances (the risks) that recessive traits will become problems, just like they increase the chance a preferred trait will show up. For this discussion, there are three basic kinds of inheritance in genetics: dominant, incomplete, and recessive (it is somewhat more complex than this, but lets keep the discussion simple). If a problem gene is dominant, you only need one copy of the gene for it to be expressed, but you know it is there because you can see it, so if you are ethical or intelligent you don't breed the line and the problem ends there. If the trait is incomplete, it means different forms of the gene are not dominant over the other ones, and the two genes act together in governing the trait. In this case, while you might see a difference, it may not be bad for the ferret, which can fool you into thinking both of the genes are benign. The quandary is that one gene may be faulty, but the other gene fills in, hiding the trouble. The crisis occurs when you have either two copies of a bad gene that is incompletely dominant over other genes, or when you have two bad genes that are recessive. In those cases, the genes can express bad things--not good, not good at all. If you recall your high school biology, you might recall a guy named Gregor Mendel, an Austrian monk who could hold his peas for years. Mendel was the first person to work out the statistical ratios for basic genetic traits, first publishing his results in 1866. What Mendel discovered was that genetic traits are displayed in predictable mathematical ratios, which means they can be statistically predicted. For example, suppose you had 4 unrelated ferrets: two pure sables (dominant trait, SS) and two albinos (recessive trait, aa). If you bred the sables to the albinos, all the offspring would be sables, but each one would carry the hidden recessive albino trait (Sa). If you bred one of the offspring from the first group to one of the offspring from the second group, you could statistically predict the sables would out-number the albinos in a 3:1 ratio (3 sables to 1 albino). You could additionally predict that out of every four kits, one would be pure sable (SS), two would be half sable and half albino (Sa), and one would be pure albino (aa). Those are statistical probabilities, which means for the ratios to really be seen, you need a large population, because random chance can throw off the ratios in individual litters; you might have half albinos, or all sables. Just like in baseball, you know someone with a .250 batting average will get a hit once out of four times at bat--the problem is, that is only true in the long run. In the short run, you have streaks of hits or outs that superficially appear to be proving the ratios wrong. You can prove this yourself, just by tossing a coin. Each time you flip the coin, the chance is 50% it will land on the heads, yet you might flip 4 or 5 heads in a row. Later, you might have streaks of tails. In the long run, such chance streaks balance each other out, preserving the ratio. Likewise, individual litters might not seem to have the proper ratios, but if you breed for a number of years, or run a huge ferret farm, the ratios are surprisingly accurate. What inbreeding does is to change the probabilities of getting the desired trait, making it more likely to get albinos, if that is what you are breeding for. As such, inbreeding is a very valuable tool to insure offspring will have the best chance to have the trait you want. The reason this works is because inbreeding DECREASES genetic variation. This is how it works: Jill and Hob are unrelated, and--for the sake of this example--they have completely different sets of genes. In other words, Hob might be albino, Jill is sable, Hob is longhaired, Jill is shorthaired--that sort of thing. In this hypothetical situation, there is 100% variability and 0% relatedness. After mating, all the offspring have half of their genes from Jill and half from Hob, so they have 50% variability and 50% relatedness (by relatedness, I mean they share 50% of their genes with their siblings). If a couple of kits show a trait you are looking for and you decide to inbreed them to conserve it, the resulting offspring will have a range of variability and relatedness depending on the genetic shuffle. It can be as low as 50%-50% (unlikely), or as high as 100%-0% (also unlikely); they will likely average to 25% variability and 75% relatedness, meaning you roughly have a 75% chance of any one ferret showing the favored trait. If you inbreed long enough, the genetic variability can drop close to zero, with nearly 100% relatedness. If you just compared females to females, and males to males, they can theoretically be identical twins of each other. That is how scientists bred certain strains of animals to be used for medical research; if you want to study the effect of a single gene mutation, it is really handy to have a population where the rest of the genes are nearly identical; one of the many reasons why learning how to clone is so attractive to genetic researchers. At the end of this type of inbreeding, the probability of any one individual having the favored trait is nearly 100%. The problem is that while you are breeding for your favored trait, there are thousands of other genes on that one chromosome, not to mention all the other chromosomes that make up the ferret's genetic code. Inbreeding decreases the amount of variability there as well. If the genes are healthy and there are no mutations, inbreeding can be done with relative safety. However, it is a different story if defective or mutated genes are present. In that case, as the percent of relatedness increases, the chances of bad ones being expressed increases as well. By inbreeding, it is possible to accidentally generate a strain of ferrets that will develop a specific cancer by a certain age, are deaf, or have clubfeet, when all you wanted was a bigger or darker ferret. [Posted in FML issue 4786]