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How do genes get associated with specific traits?

Genetics as it applies to evolution, molecular biology, and medical aspects.

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How do genes get associated with specific traits?

Postby simphilip » Sat Jul 08, 2006 1:50 pm

Hello everyone,

I am curious as to how genes get associated specific traits in nature. For example, how did it evolve so that, say, some gene (let's call it G) is associated with having blue eyes or brown eyes? In other words, how exactly does nature "know" that having this gene G is good for the population? What made G more important than all those other traits that never made it into human DNA, and, specifically, what mutation (or other process) occurred that changed human DNA from not caring about G to caring about G?

Obviously, I am not only interested in the answer for blue eyes, that is just an example. If anyone could shed any light on this question I would really appreciate it. Thanks very much.

--Phil
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Postby mith » Sat Jul 08, 2006 3:55 pm

Natural selection?
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Postby simphilip » Sat Jul 08, 2006 4:13 pm

Well, yes, I actually knew that. :)

My question was more along the lines of, what actually happens to change the linkings of the genes to the traits? I'm not really concerned with the chemical nature of this, more the algorithmic nature (i.e. what would a computer do to simulate this).

In case anyone is wondering, I am wondering this not just out of idle curiosity, but because I am trying to construct a genetic algortihm and am unsure of how to evolve the traits that are coded for themselves so that they will evolve more intelligently towards a solution to the problem I am working on.

But anyway all that is not really relevant right now; I just want to know how nature does it.

Thanks again.

--Phil
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Postby herb386 » Sun Jul 09, 2006 11:27 am

I'm not really sure what you are asking for the most part. When you say "how exactly does nature "know" that having this gene G is good for the population?" the answer is that nature doesn't know anything about it. Gene G will change by random mutation so that it performs some function (like making the eyes blue) entirely by chance. If this function is good for the organism then the gene will spread through the population by natural selection. If it is bad then it will be lost. There is no prior knowledge of whether the gene is good or bad. It is selected in hindsight not foresight.

You talk about the links between genes and traits. As I see it the linking stages are fairly static and not really important when considering a single gene. It is the gene itself which controls the trait and the trait will only change if the gene changes. The only factors other than the gene itself in controling a trait are the environment and the genetic context of the gene.
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Postby simphilip » Sun Jul 09, 2006 12:25 pm

Ok...let me see if I can clarify what I am asking...with an example....

Take a regular human being today. Suppose that, in some large amount of time, that humans descendants have antennae on their head. How did that happen genetically? I am assuming that before that point there was no "having antennae" gene that suddenly got switched on...is that a correct assumption? How did that totally new trait come to be? Or am I fundamentally misunderstanding how mutations work?

Hopefully that helps to make my question clearer. Thanks again.

--Phil
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Postby herb386 » Sun Jul 09, 2006 3:38 pm

OK, I think I understand your question now.

You would be correct to think that most traits do not come about through switching on an already present gene as non functional genes are quickly lost (or their functions changed) from populations.

Mutations can work in many different ways so the steps involved in producing a new structure like antennae can be large or small but whichever path evolution takes to get to the end result it will (nearly) always begin with something that is already present. The evolution of antennae would start with some environmental factor that would make antennae useful. Once this happens, any mutation that brings humans closer to having antennae will spread through the population and become "fixed". It is the build up of these small changes that will eventually lead to having antennae.

For example, if you consider that hairs are in part sensory structures, the first step towards having antennae might be that some hairs on the head would become longer than others and thicker so that they convey more sensory information. Many small mutations would probably occur in a number of different genes, each of which make these hairs slightly better at receiving sensory information. They would gradually get bigger and gain more neural connections until eventually they would be unrecognisable as hairs and would be antennae.

Probably the most important thing in thinking about how evolution works to produce new traits is to realise that there is no drive towards a mechanism or structure. Changes are random and if they are better than the original then they are kept.

A series of small changes is probably the normal mechanism of evolving an entirely new structure but other changes can occur very rapidly with a single mutation. For example, there is a mutation in Drosophila which causes it to grow legs on its head. This is a single mutation. This type of change requires that the mechanism for building the structure is already present though.
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Postby simphilip » Sun Jul 09, 2006 5:52 pm

Ok, thanks very much...that was basically what I was asking.

I guess I am just perplexed (and impressed) by nature's ability and flexibility when it comes to coming up with (although I know it's not really a conscious or driven process, it helps conceptually to think of it as such sometimes) totally new ideas for organisms.

--Phil
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Postby mith » Tue Jul 11, 2006 12:18 pm

n the essay, Gould complained that textbooks liked to illustrate evolution with examples of optimal design, such as insects that exquisitely mimicked a dead leaf. "Odd arrangements and funny solutions are the proof of evolution," he argued. Darwin himself spent a lot of effort uncovering the strange contortions by which organs changes shape and took on new functions. And to Gould, the thumb of the giant panda was the epitome of a funny solution.

Before a giant panda eats a piece of bamboo—its favorite meal—it grabs the shoot between its flexible thumb and finger and strips off the leaves. But this useful thumb is not really a thumb at all, at least in our sense of the word. Pandas descend from carnivorous mammal ancestors, as reflected in the many traits they share with bears, dogs, and other relatives. For one thing, its true thumb is lined up with its other fingers. What looks like its "thumb" is actually a wrist bone (the sesamoid) that evolved until it was so big that it stuck out to one side of its forepaw. The muscles that control the bone have become rearranged so that now it can move much like our own opposable thumb. Gould pointed out that the corresponding bone in the panda ankle is somewhat oversized, which he suggested was the result of genes that controlled the growth of the sesamoid in all its limbs. The large size of its ankle bones serve no function. Instead they're merely the byproduct of natural selection acting on other parts of the panda body.

http://www.corante.com/loom/archives/20 ... _thumb.php
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