Genes located in the mitochondrial DNA are passed on along the maternal line from the mother to her offspring. Multiple analyses of the mitochondrial DNA obtained from modern horses as well as from horse bones and teeth from archaeological and palaeological finds consistently shows an increased genetic diversity in the mitochondrial DNA compared to the remaining DNA, showing that a large number of mares has been included into the breeding stock of the originally domesticated horse. Variation in the mitochondrial DNA is used to determine so-called haplogroups. A haplogroup is a group of closely related haplotypes that share the same common ancestor. In horses, seven main haplogroups are recognized (A-G), each with several subgroups. Several haplogroups are unequally distributed around the world, indicating the addition of local wild mares to the domesticated stock. One of these haplotypes (Lusitano group C) is exclusively found in the Iberian Peninsula, leading to a hypothesis that the Iberian peninsula or North Africa was an independent origin for domestication of the horse. However, until there is additional analysis of nuclear DNA and a better understanding of the genetic structure of the earliest domestic herds, this theory cannot be confirmed or refuted. It remains possible that a second, independent, domestication site might exist but, as of 2012, research has neither confirmed nor disproven that hypothesis.