The Y chromosome has been studied intensely and variations have been divided into Human Y-chromosome DNA haplogroups, based upon the results of the number of alleles in certain positions called markers. These "markers" have been chosen for their demonstrated ability to predict the haplogroup of the tested individual.
The study of the Y chromosome and its variations, specifically the number of alleles at certain positions (markers), has led to the categorization of these variations into Human Y-chromosome DNA haplogroups.