The human haplotype map (or HapMap) is out. Basically, very small polymorphisms in the human population were identified via sequencing methodologies (I don't know the details). Quick science lesson. Disease is caused by genetics and environment, and interactions between the two. Science can't help much if you decide to subside on red meat and get heart disease, but genetics can be helped, from treatment to cures. For example, a certain percentage of people get familial hypercholerterolemia; for whatever reason, otherwise normal people have really high levels of cholesterol. Some drugs can treat this more or less effectively. How do we identify people with this disease? If we know the gene that's one thing. But what if you don't? What if it is only known that your entire family has very high levels of cholersterol. Well, you get a bunch of DNA samples from this family, and get some DNA samples from otherwise normal people. Then you look for a link between genetic markers and the phenotype; high cholerstrol. What is a genetic marker? It's anything DNA related that will give a variable result depending on the person. The least precise are chromosome spreads, looking at whole chromsomes. The most precise is looking at polymorphisms in individual DNA base pairs; for example I have a A while you have a C. Then you analyize all these markers, and look for linkage. If I have high cholesterol, and 100 other people also have high chol, and the 'A' marker, but you and 100 other low chol. people have the 'C' marker, then it's a match. The more markers there are, the finer map it is. The finer the map, the easier it is to find genetic disease. My masters dealt with exactly this issue, using two strains of mice that were genetically different in certain areas which caused a change in toxic effect of chemotherapy drugs. I eventually got a very very high linkage (something like 1 to a million) on chromosome 17, near a gene-rich cluster. Problem is, that cluster has 250 genes in it, so we have no idea which gene it is. Finer mapping would help with that issue.