Tuesday, November 30, 2004

Secrets of the Y Chromosome

Discover Magazine, Dec. 2004
   I read this Discover Magazine article this morning on the subway. Below is a summary that doesn't do it justice.
   The Y chromosome is unlike other chromosomes. It doesn't have a matched pair. In the process of meiosis to create a sperm, the Xm and Y chromosomes separate in anaphase -- likewise in the creation of an egg, the two Xf chromosomes separate. Our high school biology tells us that the possible outcomes for the child are two XfY and two XfXm. Over generations, the X chromosomes get pretty mixed up in the genetic pool as the offspring gets one from both parents. The Y chromosome in contrast, remains unchanged and gets passed on from father to son. The only changes come from mutations each man provides to the Y chromosome he has received from his father. In fact, the Y chromosome of each man today, is thought to be 99.99% the same as the one carried by 'Adam' who left Africa about 50-60,000 years ago. (Similar work done on mitochondrial DNA -- DNA that's passed down from mother to daughter -- traces 'Eve' back to Africa as well, to around 50,000 years ago.)
   Mutations in DNA that are neutral, doing neither harm nor good, simply accumulate over time in the genome, being passed on from one generation to the next. A mutation that is common in a group of people therefore indicates that they have a shared ancestor. The small mutations each man provides his Y chromosome then, acts as a genetic marker, recording the human species migration pattern across the planet over time. Going back to our roots in Africa, we can now trace our migration using the genetic markers. 50-60,000 years ago, we left Africa for the grasslands of Central Asia. From there, we branched out about 30,000 years ago -- one branch heading to Western Europe, the other for the Altai Mountains of western Siberia and western Mongolia. Roughly 10,000 years later, the group from Siberia/Mongolia branches, with some heading across the Bering Strait to North America. Then, just under 1,000 years ago, the original branches meet, as Europeans arrive in North America.
   The mutations at the single-nucleotide level on the Y chromosome occur rarely and provide the big picture view of migration patterns, spanning thousands of years. The smaller, more frequent mutations however -- those that occur on the microsatellite level -- the repetitious sequence of DNA that can change in the number of repetitions that occur from one generation to the next, such as CACACACA to CACACACACA -- provide a finer resolution to the migration pattern. It was this type of mutation for instance, that was used to profile Thomas Jefferson and prove that he had fathered a son by his slave, Sally Hemings, who's descendant was now living in Pennsylvania today. Looking at DNA samples collected from Central Asia at this fine a resolution, genetists found a pattern that repeated more often than expected. That led to a startling conclusion.
   In 1162, just east of the Altai Mountains, "a prodigious fornicator named Genghis Khan splashed into the gene pool like a cannonball." Genghis Khan has been quoted as saying, "Man's greatest good fortune is to chase and defeat his enemy, seize his total possessions ... use the bodies of his women as a nightshirt and support, gazing upon and kissing their rosy breasts, sucking their lips which are as sweet as the berries of their breasts." He is said to have continued campaigns into his sixties -- massacring and bringing home a new wife from each campaign. His harem was said to have had 500 wives. Genghis did two things in life apparently -- he killed and he copulated. He sowed his genes. His sons and grandsons ruled his empire and like Genghis, would have had all the opportunity to continue sowing his genes. One grandson, Kublai, was emperor of China. What the researchers had found in the repeated pattern, was Genghis Khan's Y chromosome. From a genetic perspective, Genghis is a success story. He remains with many today.
   Genetists are working against time. The migration trend of the past no longer holds true for the future. People move from region to region around the planet on a regular basis, and genetic history is quickly disappearing. The Human Genome Diversity Project is struggling to get off the ground due to misunderstanding and cries of exploitation. It's not that genetic diversity is disappearing -- far from it -- but the history of how the human population migrated across the planet is, another victim of globalization. Hopefully though, we'll come to realize that we're all the same, and all the mixing will create a global population with no colour. After all, we all came from a small gene pool some 60,000 years ago -- and genetically, we're almost identical to each other.
   For too much on this topic, see the links below:
  • The 1998 Expedition that collected DNA samples from Eurasia
  • African Origin of Modern Humans in East Asia: A Tale of 12,000 Y Chromosomes [PDF]
  • A Genetic Landscape Reshaped by Recent Events: Y-Chromosomal Insights into Central Asia
  • The Genetic Legacy of the Mongols
  • A Novel Y-Chromosome Variant Puts an Upper Limit on the Timing of First Entry into the Americas
  • Where West Meets East: The Complex mtDNA Landscape of the Southwest and Central Asian Corridor
  • Polynesian origins: Insights from the Y chromosome [PDF]
  • Reduced Y-Chromosome, but Not Mitochondrial DNA, Diversity in Human Populations from West New Guinea
  • Patterns of inter- and intra-group genetic diversity in the Vlax Roma as revealed by Y chromosome and mitochondrial DNA lineages [PDF]
  • Hierarchical patterns of global human Y-chromosome diversity [PDF]
  • Founding Mothers of Jewish Communities: Geographically Separated Jewish Groups Were Independently Founded by Very Few Female Ancestors
  • Y-Chromosome Lineages Trace Diffusion of People and Languages in Southwestern Asia
  • Genetic Evidence on the Origins of Indian Caste Populations [PDF]
  • Genes, peoples, and languages [PDF]
  • An apportionment of human DNA diversity [PDF]
  • Reconstruction of human evolution: bringing together genetic, archaeological, and linguistic data [PDF]
  • Population genetic implications from sequence variation in four Y chromosome genes [PDF]
  • The Eurasian Heartland: A continental perspective on Y-chromosome diversity
  • Out of Africa: Map showing where different Y chromosome lineages spread around the globe

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