Posts Tagged ‘Genome’

I am but a prisoner of my genes

January 20, 2014

I’d like to fly but but my genes don’t agree

And they determine how tall I will be,

I am but a lowly prisoner of my genes,

My apparent freedom is not quite what it seems.

But thanks to my genes I’m not a chimpanzee.

All abnormal behaviour is illness, says DSM-five,

Just following precisely what our genes do contrive.

Our genomes hold us tightly captive,

As slaves in their battle to survive,

So it matters not how much we strive.

You are not just your genes, you are how they are expressed

December 8, 2013

As genetics advance it is becoming clear that an individual’s genes are only a part of the story. The same genes may be expressed in many different ways. And how a gene or a group of genes are expressed depends upon environmental and other triggers which are yet to be fully understood. Your genes may be your blueprint but you are what the manufacturer then produces depending upon the materials and resources available to him. In fact “blueprint” may not be the best analogy since a “blueprint” today may well even define the method of manufacture to be followed and the materials to be used. A set of genes being a “pattern” to follow may be a better representation. How the pattern is read and put into effect then determines the final product.

David Dobbs has an interesting article about how the simplistic view of the all-determining gene is changing.

… The grasshopper, he noted, sports long legs and wings, walks low and slow, and dines discreetly in solitude. The locust scurries hurriedly and hoggishly on short, crooked legs and joins hungrily with others to form swarms that darken the sky and descend to chew the farmer’s fields bare.

Related, yes, just as grasshoppers and crickets are. But even someone as insect-ignorant as I could see that the hopper and the locust were wildly different animals — different species, doubtless, possibly different genera. So I was quite amazed when Rogers told us that grasshopper and locust are in fact the same species, even the same animal, and that, as Jekyll is Hyde, one can morph into the other at alarmingly short notice. 

Not all grasshopper species, he explained (there are some 11,000), possess this morphing power; some always remain grasshoppers. But every locust was, and technically still is, a grasshopper — not a different species or subspecies, but a sort of hopper gone mad. If faced with clues that food might be scarce, such as hunger or crowding, certain grasshopper species can transform within days or even hours from their solitudinous hopper states to become part of a maniacally social locust scourge. They can also return quickly to their original form.

In the most infamous species, Schistocerca gregaria, the desert locust of Africa, the Middle East and Asia, these phase changes (as this morphing process is called) occur when crowding spurs a temporary spike in serotonin levels, which causes changes in gene expression so widespread and powerful they alter not just the hopper’s behaviour but its appearance and form. Legs and wings shrink. Subtle camo colouring turns conspicuously garish. The brain grows to manage the animal’s newly complicated social world, which includes the fact that, if a locust moves too slowly amid its million cousins, the cousins directly behind might eat it.

How does this happen? Does something happen to their genes? Yes, but — and here was the point of Rogers’s talk — their genes don’t actually change. That is, they don’t mutate or in any way alter the genetic sequence or DNA. Nothing gets rewritten. Instead, this bug’s DNA — the genetic book with millions of letters that form the instructions for building and operating a grasshopper — gets reread so that the very same book becomes the instructions for operating a locust. Even as one animal becomes the other, as Jekyll becomes Hyde, its genome stays unchanged. Same genome, same individual, but, I think we can all agree, quite a different beast. ….

…. Gene expression is what makes a gene meaningful, and it’s vital for distinguishing one species from another. We humans, for instance, share more than half our genomes with flatworms; about 60 per cent with fruit flies and chickens; 80 per cent with cows; and 99 per cent with chimps. Those genetic distinctions aren’t enough to create all our differences from those animals — what biologists call our particular phenotype, which is essentially the recognisable thing a genotype builds. This means that we are human, rather than wormlike, flylike, chickenlike, feline, bovine, or excessively simian, less because we carry different genes from those other species than because our cells read differently our remarkably similar genomes as we develop from zygote to adult. The writing varies — but hardly as much as the reading.

This raises a question: if merely reading a genome differently can change organisms so wildly, why bother rewriting the genome to evolve? How vital, really, are actual changes in the genetic code? Do we even need DNA changes to adapt to new environments? Is the importance of the gene as the driver of evolution being overplayed?

I think the idea that anything drives evolution is the wrong end of the stick. Evolution is a result of response to change. The resultant evolution is by deselection of those individuals who cannot survive the change – it is not a pro-active selection of desirable traits for some change yet to come.

So it seems to me that it is perfectly logical that a set of genes only describe and define an envelope of possibilities. It is gene expression which then – reacting to environmental or other triggers – determines the particular model from within the envelope that will materialise. But the set of genes are still critical in that they set the constraints – they define the envelope of possibilities. And no matter how creatively they are expressed, the constraints and the envelope still apply. I suspect that we have only just begun to understand the incredibly wide variation that gene expression permits with any given set of genes and how such expression can be triggered.

This variability is sufficiently wide that one twin can be a saint and the other can be a sinner but this variability is not so great that we can suddenly morph into chimpanzees.

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