Genetic genealogy is the use of genealogical DNA tests, i.e., DNA profiling and DNA testing, in combination with traditional genealogical methods, to infer genetic relationships between individuals. This application of genetics came to be used by family historians in the 21st century, as DNA tests became affordable. The tests have been promoted by amateur groups, such as surname study groups or regional genealogical groups, as well as research projects such as the Genographic Project. As of 2019, about 30 million people had been tested. As the field developed, the aims of practitioners broadened, with many seeking knowledge of their ancestry beyond the recent centuries, for which traditional pedigrees can be constructed. History The investigation of surnames in genetics can be said to go back to George Darwin, a son of Charles Darwin and Charles' first cousin Emma Darwin. In 1875, George Darwin used surnames to estimate the frequency of first-cousin marriages and calculated the expected incidence of marriage between people of the same surname (isonymy). He arrived at a figure of 1.5% for cousin-marriage in the population of London, higher (3%-3.5%) among the upper classes and lower (2.25%) among the general rural population. Surname studies A famous study in 1998 examined the lineage of descendants of Thomas Jefferson’s paternal line and male lineage descendants of the freed slave Sally Hemings. Bryan Sykes, a molecular biologist at Oxford University, tested the new methodology in general surname research. His study of the Sykes surname, published in 2000, obtained results by looking at four STR markers on the male chromosome. It pointed the way to genetics becoming a valuable assistant in the service of genealogy and history. Direct-to-consumer DNA testing In 2000, Family Tree DNA was the first company to provide direct-to-consumer genetic testing for genealogy research. It initially offered eleven-marker Y-chromosome STR tests and HVR1 mitochondrial DNA tests but not multi-generational genealogy tests. In 2001, GeneTree was acquired by Sorenson Molecular Genealogy Foundation (SMGF), which provided free Y-chromosome and mitochondrial DNA (mtDNA) tests. GeneTree later returned to genetic testing in conjunction with its Sorenson parent company until it was acquired by Ancestry.com in 2012. In 2007, 23andMe was the first company to offer saliva-based direct-to-consumer testing, and the first to use autosomal DNA for ancestry testing. An autosome is one of the 22 chromosomes other than the X or Y chromosomes. They are transmitted from all ancestors in recent generations and so