Things could get a bit complicated with heredity:
A radiologist I once thought I was going to be working with was interviewing me. I was sorry that he left as I joined. I didn’t really know I was to be his replacement but I suspect his departure was his own idea. What I do remember was his teasing reference to a colleague who belonged to a wine tasting club. They would get some bottles of wine, divvy them out and sip their aliquots. Then they would say things like, “Yes, that’s an interesting wine,” or “Yes, that’s wine all right.” The knee slapper was the enhanced seriousness with which he would imitate them. Similarly there is a joke told on the Scots. “The Irish invented the bagpipes and gave them to the Scots; the Scots haven’t realized that it was a joke.”
So many years ago it was with high seriousness that we addressed genetics. DNA was inherited. DNA existed in long highly specific chains that were transcribed into shorter highly specific chains of messenger RNA. These in turn are transcribed into highly specific proteins that do the work of running the structure and function of tissues. The poster child of this mechanism was a disease called sickle cell anemia. A single mutation in the gene for hemoglobin produced a single protein change in hemoglobin that resulted in an abnormally behaving molecule and a consequent disease. The mental set was: One DNA base pair change, one amino acid change in a protein, one disease.
Against that expectation it is no surprise that the notion that a degree of consanguinity is needed for normal fertility was not realized. There just seemed no way it could happen. Genes were either all right or they were not.
Things have become more complicated. Only a small percentage of the human genome follows the logic above, which of course is right so far as it goes. DNA that did not code for proteins was dismissed for a long time as simply being noise in the system. But other things have been found to be going on. (Elizabeth Pennisi ENCODE Project Writes Eulogy For Junk DNA SCIENCE vol. 337. no. 6099 September 7, 2012 page 1159 and The Origin of Species ECONOMIST vol. 405 no. 8813 December 1, 2012 page 83)
For me there is a terminology problem. The question is asked, “How much DNA has a purpose?” Personally I would say the answer is none. Unless you are a creationist, which means you probably aren’t interested in this at all, (not so sure about Francis Collins) none of it has a purpose. It happened by chance and what has worked has persisted.
The next question is, “How much DNA has a function?” Some does, that’s for sure. And some is no doubt actual junk doing nothing at all. But that hardly covers the field.
My notion about biology is shaped a bit by my observation of politics. You can look at the wrangling of American politicians or if you have a stomach of iron you can read a bit about politics in Congo (huge, fertile, rich in mineral wealth, with bounteous rainfall), or Syria (crossroads Europe, Asia and Africa, rich in cultural diversity and ancient of power). There is no question in my mind that many people in those unhappy countries do have their hearts in the right place and do what they can to make things right. They have a “purpose.” But there are destructive elements in effect as well. Those elements seem to me to have no “function” as such. They are dysfunctional. Intuitively it seems to me that within the toils of sub cellular biology there are also areas of DNA that are functional and those that are dysfunctional. So the real question is, “How much DNA has an effect?”
The answer to that is, “Most of it.” DNA gets transcribed into RNA that then usually does things other than build proteins. It controls other stretches of DNA and I know not what other things it is doing. The article I cite is helpful, but I am way over my head. They helpfully point out that 80% of DNA has “function” rather than the 1% once suspected. They also point out that there are 442 researchers with the ENCODE project that has been sorting this out. I would have guessed thousands.
Then there is the issue of “conserved DNA.” Some DNA stretches that can be found are about the same in humans and in animals, which is taken to mean that it is probably important; this I agree with. Some DNA stretches vary between humans, which is taken to mean it has no effect; I fail to see how that must follow.
And there are “transposable elements” pieces of DNA (specifically a type called “lincRNA”) that can move around on the genome and which seem to be yet another control system and which seem to have much to do with embryonic development. And they are sort of the opposite of being conserved. Should we call them “liberated?” Instead of being alike between species, they vary between species more than other parts of the genome. So the suggestion is that it is these lincRNA stretches rather than mutations in the DNA of genes that actually induce speciation.
Hmm. I catch a scent of the quarry. My friends sometimes say of me, “He’s interested in hybrid infertility,” which is not really the case. But what I pursue here is related in that given the predictable emergence of hybrid infertility, that is to say given the emergence of speciation, then some mechanism must have evolved to keep that from wiping out populations. The ECONOMIST article points out that we have learned little about speciation since Charles Darwin published his book On The Origin of Species, and I assure you he didn’t understand beans about it. Although speciation is not the quarry, the quarry cannot be far away from it.
So there is much nobody yet understands. And that is before you even bring epigenetic effects into the picture.
The good news is that there is certainly enough complexity in the process so that the consanguinity-fertility relationship could be tucked in there somewhere. The bad news is that the whole thing is so horribly complex that there is no chance that somebody looking at some stretch of DNA is going so say, “Hello. What’s this? This DNA strand determines how epigenetic effects are laid out, and now that I look at it, those epigenetic effects will produce a consanguinity-fertility relationship.
So I watch in awe, but I fear any helpful breakthrough like that will be a long time in coming if ever.
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