The Lhasa and the Chocolate Cake

Fortinbras

Genes are like recipes - before the age of Xerox. If we have a recipe for "chocolate cake", it may get modified when it is passed on to someone else. Many of the modifications will make no noticeable difference, or only a very subtle one. Some may improve the cake and others will not. If a critical ingredient is forgotten, or it is cooked too long or at the wrong temperature, we have a recipe for disaster. (If we don't understand what has gone wrong, we will likely throw out the recipe and look for a new one.) We may even make deliberate modifications in an attempt to get a more memorable cake. Among the population of cooks, there will be a variety, or diversity, of recipes, and therefore of cakes.

This, I would say, is a *good* thing. Do we always want the same chocolate cake? Surely we will tire of it, and even if we don't, we lose the pleasure of anticipation. If, for some unforeseen reason,everyone suddenly loses their taste for THE chocolate cake, it will surely go extinct. To have the potential for evolution and adaptation, we must risk the possibility of the bad. That is the cost.

Genes are like recipes. When the gene is copied, the copy is not always exact. In a large, naturally breeding population, we will end up with a number of versions (alleles), some so slightly different that we will never notice, some perceptibly different (but still functional), and some that just don't work at all. Remove the diversity and we lose the potential for evolution, and for surviving unexpected change. To have the potential for evolution and adaptation, we must risk the possibility of the bad. Geneticists call that cost "genetic load".

In most populations every individual carries a portion of the load - 4 or 5 bad recipes out of several thousand. The load is so well distributed that if two individuals compare recipes they will generally not have two copies of the same bad recipe. However, suppose we start a new population with only 6 or 8 founders? (A number of breeds have started with that few.) We will get rid of hundreds of bad recipes, but the remaining dozen or two will be encountered much more frequently.

To further compound the problem, about 15 years ago a group of breeders decided they liked "Harry's" recipes (or at least the ones they could see readily) and abandoned the other recipes with little thought to the eventual consequences. Now, almost everyone has Harry's bad recipes as well as his good ones.

How precious is an individual that comes along with many of the missing recipes and few of the "Harry collection"? Do we hesitate because there are a few bad recipes in this alternate collection? Are we now so accustomed to dealing with the Harry collection that we have lost the vision of the "memorable" chocolate cake?

Think about it.

Fortinbras

Submitted by Catherine Marley, MD
Kai-La-Sha