The genomes of plants, animals and human consist of two components: euchromatin (representing the genes) and heterochromatin (consisting of non-coding DNAs). Unlike euchromatin, the biological role of heterochromatin is not known. To date, two types of chromosomal heterochromatin have been discovered: C- and Q-heterochromatin. C-heterochromatin is encountered in chromosomes of all higher eukaryotes, while Q-heterochromatin is present in only three higher primates: man, the chimpanzee and the gorilla. Wide variability of Q-heterochromatin has been shown to be mainly inherent in human populations. This study of Q-heterochromatin variability in natives of Eurasia and Africa highlights that interpopulation differences are related to physical environmental factors rather than to racial or ethnic features, and that the human ability to adapt to extreme conditions depends on the amount of Q-heterochromatin. The book also shows that Q-heterochromatin plays a role in the pathogenesis of certain forms of purely human pathologies such as obesity, alcoholism, addiction and atherosclerosis.