Counting the number of DNA units called short tandem repeats on chromosomes can allow scientists to group individuals according to probable ancestry. One such repeat, AAAG, occurs between two and seven times in people with African heritage but between five and eight times in those whose ancestors came from Europe or the Middle East. (Every person inherits one set of repeats from their mother and one from their father.) Accordingly, someone who carries two and three repeats is likely to have African heritage, whereas someone with six and eight repeats probably has ancestors from Europe or the Middle East. People with between five and seven repeats occur in both populations, however, making these individuals more difficult to classify using only this particular repeat.