All modern Thoroughbreds can trace their ancestry back to three stallions which were imported into England in the 17th and 18th century - Byerley Turk (1680s), Darley Arabian (1704) and Godolphin Arabian (1729). Incidentally, Byerley Turk is reputed to have been used by his owner Captain Robert Byerley at the Battle of the Boyne.
The origin of the maternal lineage is less clearcut as the influence of the female line was thought to be less influential than the male line in the early years of horse breeding.
Now, a group of UK and Irish scientists have used mitochondrial DNA from 1929 horses to determine the origin of these foundation mares.
Their results show that Thoroughbred foundation mares were not exclusively Arab or Oriental but, in fact, were of 'cosmopolitan European origin' with some contribution from 'Barbs' (i.e. North African) and significant contributions from British and Irish Native horses.
published in Biology Letters) British and Irish Native horses made twice the contribution to the heritage of modern Thoroughbreds that Oriental (term used in historic records referring to Mid-East and West-Asia) horses did.
Mitochondrial DNA analysis is a perfect way of studying female lineage because mitochondria are normally inherited exclusively along the female line. So, all of your mitochondrial DNA was inherited from your mother, becuase mitochondria in sperm cells are usually destroyed by the egg cell after fertilisation has occurred.
You can read all of the research here (pdf).