Dr. Barry Starr of Stanford University’s Department of Genetics posted a great entry on a Chinese man who is living normally with just 22 chromosome pairs (humans normally have 23), as a result of a chromosomal translocation that resulted in his chromosome 14 fusing with his chromosome 15. What’s truly amazing is that the man is functioning normally, and does not appear to have any problems often associated with abnormal numbers of chromosomal pairs, like Down Syndrome. He explains why this discovery may be important for our understanding of human evolution:
This is an important finding because it tells us about a key genetic event in human prehistory. All the evidence points to humans, like their relatives the chimpanzees, having 48 chromosomes a million or so years ago. Nowadays most humans have 46.
What happened to this 44 chromosome man shows one way that the first step in this sort of change might have happened in our past. Scientists could certainly predict something like this. But now there is proof that it can actually happen.
So, perhaps this man’s mutation is similar to one that characterized the individual(s) who were the missing link between man and chimp.While evolutionary biologists have long suspected that man’s early migration from the forests and savannah’s of Africa to the North and West gave rise to isolated groups and therefore more pronounced genetic differences, that theory has been weaker at explaining the shift from 48 chromosomes to 46. This finding presents a more compelling explanation, as summed up by commenter “blahedo” on Hacker News:
This is interesting because it provides a mechanism for speciation that doesn’t involve sequestration in a lost valley or something like that: in an already established population, once you have a herd or tribe that has translocated chromosomes floating around, that subgroup has much-lowered chances of (successful) mating outside the subgroup. That, in turn, means that any beneficial mutation that arises by random chance is kept “in-house”, as it were, and refined in the crucible of inbreeding, while the subgroup begins to succesfully outcompete the rest of the local population—and that local population can’t acquire the beneficial mutation because of the breeding problem.
Keep in mind the very large numbers we’re dealing with when we talk about speciation events. All of recorded human history is perhaps six or eight thousand years, but a hundred thousand years is a short time frame on an evolutionary scale; so even an incredibly rare mutation will happen many times.
Whether or not this finding proves to uphold such a theory in the long term remains to be seen. But it is clear that, at the very least, this Chinese man is a genetic miracle made possible by the mating of two closely related individuals, each with their own genetic anomaly:
To get two of the same balanced translocations, both parents need to have the same balanced translocation. This is incredibly rare. Except when the parents are related.
In this case, both parents are first cousins and they share the same translocation. When these parents try to have kids, they run into the same kinds of problems that can happen with one balanced translocation. Except that the problems are doubled. This makes for the many possibilities outlined below:
This very complicated table shows the 36 possible outcomes when two parents with the same balanced translocation attempt to have a child.
In this representation, the father’s possible sperm are shown on the top and the mother’s eggs on the side. Each pregnancy has only an 8 in 36 chance for success. And 1 out of 36 would have two of the same balanced translocation (the circled possibility).
Damjan is a Partner at Rubicon Strategy Group, as well as the Managing Editor of Asia Healthcare Blog. Last summer he worked as a law clerk at the offices of Harris & Moure, a leading boutique international law firm, widely admired for the internationally celebrated China Law Blog. He is currently a completing a J.D./M.A. Law and Chinese dual-degree program at the University of Michigan. He graduated from Duke University, in 2007, with a B.A. in Public Policy, concentration in health policy, and is an alumnus of the Middlebury College Chinese Language School.