Mitochondrial DNA suggests that almost all modern dogs drop into 1 of four monophyletic haplogroups which are named haplogroups A, B, C, and D. The majority of dogs drop in haplogroup A. The mDNA "molecular clock" suggests that 22,800 YBP the initial genetic divergence (break up) transpired in haplogroup A, leading to the lineages A1b and A2. This