Beyond Mendel

Life, and genetics in particular, would be much easier if all genetic inheritance was "Mendelian"... that is to say, if all genes were inherited in the simple way described by Mendel. In actual fact a lot of genes ARE inherited in this way, which is why Mendel is still taught.

Several issues cloud up Mendelian Genetics. The first has to do with genes which have limited expression. Two of these are the si gene which causes a silvering of the coat of mice, and the hs gene, which causes a spot on the head of a mouse. In both of these cases, the genes are labelled as being "heavily affected by modifiers." What this means is that other genes get in the way of the "expression" of the genes. A mouse can have si/si yet have no silvering of its fur, and it can have hs/hs without a spot on its head. This means that genetic conditions have to be just right for the gene to be able to properly express itself.

Another confusing issue is the topic of linked genes. In order to understand about linked genes, let's imagine that a person's genetic makeup, on chromosomes, is simply words in a book. Each chromosome is one incredibly large volume, larger than can truly be conceived of, with each letter in the book being a different pair of genes. If there are 26 chromosomes in a creature, it would have 26 volumes. So let's take volume 7 from the mouse chromosome library. If we zero in on that part of the volume which deals with albino mice, we find that very close, possibly even on the same "page" is the gene for pink eyed mice. They are so close that they are considered to be linked.

Linked genes are those genes which are "close together" on the chromosome. It turns out that genes are inherited in "packets" or clumps. When two genes are located close together they are more likely to be inherited together. For example, let's take Mouse A. This mouse has two parents. One is a champagne (pink eyed brown) mouse with no albino genes at all, and the other parent mouse is an albino with no pink-eyed genes. That means that Mouse A has the following genes: C/c and P/p. When this mouse has offspring, it will pass on these genes. According to Mendel, the inheritance of the albino genes would be completely independent of the inheritance of the pink--eyed gene. But in reality, since they are so close together on Chromosome 7, the inheritance is completely non-mendelian. While sometimes there is a crossover in general, when Mouse A procreates, that mouse will normally pass the linked genes on in the manner they were given to her. She will pass the C gene on with the p and the P gene with the c. Only rarely, perhaps 10% of the time, will one of her offspring inherit both her p, and her c genes.