They must be patentable in order to promote the development of genetics. Research into genetics is costly, and overall the only way to motivate businesses into developing these products is by allowing the opportunity for profit to be made. Patents allow for development of these genetic related products, which will help better our society as a whole.
Companies such as Myriad, whose research has led to knowledge about the genes BRCA1 and BRCA2, have attempted to claim that they can own a patent on a gene sequence, simply because they discovered something about it. This claim is without merit, as has recently been decided by the United States Supreme Court. The reason being, genes were not developed by the companies claiming ownership of them. Genes occur and exist in nature, and thus are not patentable.
Patents, originally meant to promote scientific research, has largely shown (in recent years) to hinder it in many cases. While patent law is desperately in need of an overhaul, a current threat in the scientific community deals with the patenting of gene sequences. Gene sequences should not be patented because society stands to lose far more from the lack of open research on genes than from companies not gaining a monopoly on them. Science isn't currently advanced enough to fully understand all gene-gene interactions, so any patent issued couldn't possibly state its own scope.