|MadSci Network: Genetics|
I did a quick web search by trying "max cat breeding" in Google and came up with: http://www.fanciers.com/breed-faqs/manx-faq.html I know nothing first hand about the subject, so this is the best I can find on the specific subject. In terms of general selective breeding, a mutation happens which causes a certain characteristic to be pronounced. This may be a white blob, a short tail, etc. The breeder simply chooses to those with desireable (or useful) characteristics and they form the next generation. The "no tail" characteristic is not acheived by breeding, it is promoted by it. OK, so "short tail" may seem a very dramatic change. Just because it looks like a large change, doesn't mean there has to be a large change at the genetic level. Indeed, small changes can have dramatic effects. Imagine, the way the tail might grow. In one way, it's a little like a branch on a tree, but with a more definite location. Irrespective of the length, or other characteristic, if this "hot spot" of cells needed to begin the tail are not present, the tail will simply not grow. In some ways, "no tail" is more likely than "small tail". I'd be happy to expand this in any direction.
Try the links in the MadSci Library for more information on Genetics.