Geneticists use the Y-chromosome in population studies such as this because it doesn't recombine as other parts of the genome do. When it comes to eye color, or height, or resistance or susceptibility to particular diseases, each parent contributes half of a child's DNA, which join together to form a new genetic combination.
The Y-chromosome is passed on as a chunk of DNA from father to son, basically unchanged through generations except for random mutations.
These random mutations, which happen naturally and are usually harmless, are called markers. Once the markers have been identified, geneticists can go back in time and trace them to the point at which they first occurred, defining a unique lineage of descent.
In this particular instance, the lineage originated 1,000 years ago. The authors aren't saying that the genetic mutations defining the lineage originated with Khan, who was born around 1162; they are more likely to have been passed on to him by a great great grandfather.
The lineage was found in only one population outside of the former Mongolian empire, in Pakistan.
"The Hazaras [of Pakistan] gave us our first clue to the connection with Genghis Khan," said Wells. "They have a long oral tradition that says they're his direct descendants."
Of course, the connection to Genghis Khan will never be a certainty unless his grave is found and his DNA could be extracted. Until then, geneticists will continue to seek out isolated populations in the hope of unraveling the mysteries of geographic origin and relatedness told by our genes.
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