In human genetics, a human Y-chromosome DNA haplogroup is a haplogroup defined by mutations in the non-recombining portions of DNA from the male-specific Y chromosome (called Y-DNA). Many people within a haplogroup share similar numbers of short tandem repeats (STRs) and types of mutations called single-nucleotide polymorphisms (SNPs). The human Y-chromosome accumulates roughly two mutations per generation. Y-DNA haplogroups represent major branches of the Y-chromosome phylogenetic tree that share hundreds or even thousands of mutations unique to each haplogroup.