A haplogroup refers to a group of haplotypes that share an identical single nucleotide polymorphism (SNP). Haplogroups are designated by letters of the alphabet and are further refined by alternating numbers and letters. They can be used to trace geographical genetic origins for thousands of years.
Paternal and maternal lineages are traced using different haplogroups and are each designated by a different set of letters. Paternal lineages are traced using the Y-chromosome which is only passed from father to son and maternal lineages are traced using mitochondrial DNA which is passed from mother to offspring. 
Haplogroups can also be used to predict risk factors to various diseases  and health conditions .
Y-chromosome haplogroupsEditThe Y-chromosome is used to trace paternal lineages. The Y-chromosme haplogroups can only be traced by males since the Y-chromosome is only passed from father to son. The male haplogroups begin with Y-Chromosome Adam and are designated by the letters A through T. 
Mitochondrial DNA haplogroupsEditThe mitochondrial haplogroups can be traced by anyone since the mother passes her mitochondrial DNA to all of her offspring regardless of the child's sex. These haplogroups begin with Mitochondrial Eve and are designated by the letters A through Z with the exception of O. 
1. Haplogroup. Wikipedia. http://en.wikipedia.org/wiki/Haplogroup
2. Weng SW, Kuo HM, Chuang JH, Lin TK, Huang HL, Lin HY, Liou CW, Wang PW. "Study of insulin resistance in cybrid cells harboring diabetes-susceptible and diabetes-protective mitochondrial haplogroups." Mitochondrion. 2013 Aug 13. doi:pii: S1567-7249(13)00217-1. 10.1016/j.mito.2013.08.001. PMID: 23948373
3. Nardelli C, Labruna G, Liguori R, Mazzaccara C, Ferrigno M, Capobianco V, Pezzuti M, Castaldo G, Farinaro E, Contaldo F, Buono P, Sacchetti L, Pasanisi F. "Haplogroup T is an obesity risk factor: mitochondrial DNA haplotyping in a morbid obese population from southern Italy." Biomed Res Int. 2013;2013:631082. doi: 10.1155/2013/631082. PMID: 23936828