General Stud Book
Although much attention is paid to the sire line, Thoroughbred horses are also traced through the distaff line, alternately called the mare line or tail-female line. This maternal line is known as a "family". This practice dates to the beginning of the General Stud Book (GSB). This was done because the mares produce far fewer foals than stallions and many leading breeders maintained and built families all tracing to a single mare. However, modern genetic studies have revealed that there are some cases where the haplotype in the mtDNA of modern Thoroughbreds, which should not mutate or alter, differs from the records in the General Stud Book, indicating that some female families contain deep rooted pedigree errors.