I must admit, that my heart made a fond little leap this morning when I discovered genetics on our timetable for the first time! Not only that, but our lecturer made a foray into the one area of genetics that I find most intriguing: genetic geography. Here’s something new that I learned:
Over the course of human history, it appears that women have migrated more, and over greater distances, than men. You may ask, how could we possibly know that? And that’s where a couple convenient features of genetics come into play.
Mitochondrial DNA is passed from mother to offspring. Paternal mitochondria are never normally inherited. Thus, if you are a female reading this, then you can be quite certain that your mitonchondrial DNA came from your mother, which came from your grandmother, and so on up your maternal line of descent.
There is a similar set of DNA for male descent: the Y chromosome. Genotypically normal females lack the Y chromosome, and so males will have inherited their Y chromosome from their father, which came from their grandfather, and so on.
Once a set of DNA can be distinguished for male descent versus female descent, then it becomes possible to compare genetic variation between the two sets. The results of this comparison show that mitochondrial DNA is far more uniform geographically than Y-chomosomal DNA. It can be inferred then, that women were historically more travelled than men because their variation has been distributed more evenly geographically.
A possible anthropological explanation for this finding is that when women were married, it may have been the tradition in most human societies for the wife to move to the husband’s home. I have lingering doubts about this hypothesis, however, because the opposite is what normally happens with our closest ape relatives.
That is very interesting indeed. Don’t worry… I completely know what you mean about the “heart leap”… this kinda happened to me when I figured out one of the construction workers doing the renovation in our basement understood what I was talking about when I referred to the Endosymbiotic theory. He ended up being a Western Biochem graduate who does construction on the side with his cousin the Carleton Systems Design Engineering graduate. Not quite the same situation but my heart leaped nonetheless.
My question now is how do the researchers know this was not a result of natural selection?
Everyone knows that there is not much “usable” genetic material in the Y chromosome and hence mutations can occur without any serious repercussion on the individual and its progeny (ie. the fitness of males with a mutation in their “silent” Y chromosome will not decrease due to the mutation so they the mutation will be passed on to their future generations)
On the other hand, mitochrondrial DNA is known for coding proteins involved in proper mitochondrial functioning including enzymes involved in oxidative phosphorylation, tRNA, rRNA etc (http://ghr.nlm.nih.gov/handbook/basics/mtdna). Knowing this hints at the fact that any mutation to mitochondrial DNA would either increase or decrease the fitness of an individual therefore eradicating the lineage of those individuals with “weaker” DNA (unfit), creating a more uniform DNA pool among the population. As people migrated around the globe, they would have carried this uniform mtDNA, making it geographically uniform.
Wouldn’t you agree this is a possible explanation?
Comment by D — October 9, 2008 @ 8:44 pm
* ADD TO END OF 2nd PARAGRAPH*
Since mutations don’t have an effect on natural selection, mutations would occur with a higher incidence therefore creating a more diverse genetic pool.
Comment by D — October 9, 2008 @ 8:50 pm
Diana
!!! My true genetics soulmate… I stand in salute.
This is a powerful bit of reasoning!! Aside from my Y-chromosome (which is feeling a tad dejected at being deemed mostly unusable
), the rest of me is engrossed in your theory. It’s a good one! Certainly what you have described is logical, and probably does occur. That is, Natural Selection likely pushes for more variation in Y-chromosomal DNA , and less variation in mtDNA.
The only wrench in the works is that data has shown that the opposite is actually true!! There is more variation in mtDNA than in Y-chromosomal DNA (Ref: Dawkins, ‘Ancestor’s Tale’). This boggles my faculties for the very reasons you described.
What’s happening, is that while Natural Selection pushes in one direction, there’s another form of selection pushing in the opposite direction: Sexual Selection. Human societies have developed a mating pattern in which most females are mildly successful breeders, and few are left childless. In contrast, over history males have tended to achieve more disparate reproductive success; some males have lots of children, and many have none at all. For an extreme example, Genghis Khan had so many children that he is likely to be an ancestor to more than half of the men alive today!!
The result of most women being approximately equally reproductively successful, is that polymorphisms in mtDNA are usually passed on to the next generation. They can accumulate, and contribute to the wide variability in mtDNA. In contrast, when only a small subset of men give rise to the next generation, only their peculiar polymorphisms on the Y-chromosome will be inherited, and all others lost. Thus the variation in Y-chromosomal DNA is less.
So Sexual Selection pushes against Natural Selection such that mtDNA is more variable than Y-chromosomal. Actually Sexual Selection has a wonderful habit of confounding Natural Selection
!! The best example is the development of the human brain, but that’s a whole other story,
Despite the differences in variability, we can still look at how variation is spread geographically. Even though mtDNA has more variability, it is still spread more uniformly across the planet. Thus the conclusion that women migrate more often, and greater distances.
Comment by Jason Booy — October 9, 2008 @ 10:33 pm
Thanks for the explanation… what still boggles my mind (and I might be getting a little stuck on semantics here) is what you mean by variability being uniform.
When you refer to variability I assume you’re talking about polymorphisms in different chromosomes. In the case of mtDNA, more variability would mean different arrangements of genes along the mitochondrial genome of different individuals (including deletions, inversions, duplications,etc). If that is the case, how can these polymorphisms be uniform among the population? Better yet, how can they be uniform among the different populations of the world?
Comment by D — October 11, 2008 @ 9:47 pm
Ya, so let’s say that one particular polymorphism in the mtDNA is an inversion of bp 350-489. This polymorphism could be carried by many women, all of whom may have inherited it from a single common ancestor. At the same time, all the women who have this particular polymorphism could be spread far apart geographically: one family in England, two in Spain, one in Tanzania. That’s what’s meant by geographically uniform. You can see how after that one common ancestor gained a polymorphic variant (I like to avoid the words mutation and mutant when talking about people), her descendants moved geographically far apart! Hence the conclusion that these women migrated quite a bit.
Compare that to, say, a duplication of bp 350-489 on the Y-chromosome. Again, many men may have this polymorphism, all having inherited it from a single common ancestor. But when you look at the geographic distribution, you may discover that all of the men having this particular polymorphism are in Tibet. Well, then you must conclude that these related males have stayed in close proximity to each other, ever since that first common ancestor who acquired the polymorphism.
So we look at as many polymorphisms of Y-chromosomal DNA and mtDNA as possible, and their geographic distribution. The results of this demonstrate that, historically, women migrated more often, and farther.
Does that sort of make sense
??? I may just be talking rubbish… in fact that’s actually rather likely,
Comment by Jason Booy — October 12, 2008 @ 11:32 pm
No, no rubbish… I understand completely now.
Thanks for clearing this up for me (I know it would have bugged me for a while)… hahaha this just felt like an “online” study party for 303 or 403.
Comment by D — October 13, 2008 @ 10:29 pm
Yay study party
!!! … Aw, I miss those D!
Comment by Jason Booy — October 14, 2008 @ 12:02 pm