New genetic principles

Most of the new genetic concepts outlined in this review were elucidated during the past decade, and even newer concepts will become apparent in the future. No current discussion of "new genetic concepts" would be complete without the mention of epigenetics. Epigenetics refers to several genetic mechanisms that cause gene silencing or reduce the normal level of expression, thereby influencing gene function. Some of the epigenetic mechanisms include DNA methylation, which occurs in the regulatory element of certain genes, usually located at the 3' of the gene and contain sequences rich in guanine and cytosine (CpG islands). De novo methylation of CpG islands suppresses gene expression. Epigenetics is a relatively novel concept that is being introduced especially in cancer genetics but is likely to be important in other complex genetic disorders. Nonmendelian inheritance is recognized with increasing frequency in a diverse and increasing number of human disorders. With the ever-expanding knowledge stemming from the human genome project, other modes of nonmendelian inheritance may be discovered. Many of the genetic principles taught even recently in medical school may not apply to certain genetic conditions. The clinician is encouraged to remain alert, especially in the setting of preconceptual and prenatal counseling.