Geneticists are discovering discrete blocks of DNA that differ from one person to the next. And they may be common enough and account for enough of the genome, researchers concluded at a conference held here last week, that it9s time to create a new map of the genome, one that describes its blocky structure. Its creators hope this so-called haplotype map will be a tool for pinning down the genes that contribute to the development of complex diseases such as cancer, diabetes, and mental illness.