Steve Jones Q&A: Sex and genetics

Professor Steve Jones

Why are identical twins not completely identical, when they have exactly the same DNA?

Identical twins may be genetically identical but have experienced a different environment. That might even be before birth, when perhaps one gets more nutrition from the placenta than the other.

Or there might be mutations during development that may make them slightly different.

Will we eventually end up with more genes, and evolve into a new species?

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The interesting thing is how little evolution there has been since modern humans appeared.

If a Cro-Magnon man were to sit next to me on the tube I probably wouldn't notice. He might be covered in mud and wearing skins but physically he would be almost indistinguishable from me.

My guess is that the same will be true for the foreseeable future and we won't get more genes.

Is there a link between wealth and genes for attractiveness?

Cash and sexual success are closely related, but there are no genes for bank balance, so I guess the answer is no.

Are we more or less genetically diverse than chimpanzees?

We are much less diverse than chimps, which is a surprise. It may be because until recently we have been rare and they have been common.

Could a human mate successfully with a chimp?

I'll leave it to you to find out…

There have been several claims that human sperm have been used to fertilise chimp eggs - and in principle I would not be surprised if that worked. However, there is something called ethics.

DNA studies suggest that Homo Sapiens and Neanderthals produced viable offspring. So why are we considered different species?

That's an excellent question. The definition of a species is more theology than biology. If we could interbreed with Neanderthals then, in some sense, we were the same species. Maybe what's wrong is the word 'species'. I wish I knew the answer to your question.

Do the rules of natural selection apply at the scale of individual cells?

Natural selection is the inherited differences in the ability to reproduce. More copies means more success.

Cancer cells are just the same. They divide faster than others and can kill their owners. They are natural selection gone wrong.

Are we are closer to finding a cure for cancer, thanks to our understanding of genetics?

The word cancer is misleading - in fact there are many, many different cancers. We can already deal with certain cancers, like some forms of leukaemia, quite well. Others are still recalcitrant.

Very recently it has become possible to target particular genetic changes in cancer cells with drugs and that will no doubt become more common.

Perhaps the most important contribution of genetics isn't treatment but diagnosis. We can pick up the damaged genes long before a tumour appears and start treatment.

How many genetic diseases could be treated more effectively, as a result of the Human Genome Project?

What's interesting is that genetics has made the environment seem much more important. If everyone smoked, lung cancer would be a genetic disease.

What the Human Genome Project (HGP) has done is tell us what external agents, from tobacco to cheeseburgers, are most dangerous for each one of us.

In a few cases, the HGP will help treat disease. In many more it will remind us that our health is not in our genes but in our behaviour.

Can allergies be inherited?

The allergies themselves cannot be inherited but there is a lot of genetic variation in the effectiveness and sensitivity of the immune system which means that some people are more susceptible to developing allergies than others.

Is there any evidence to back up the idea that our immune system is a symbiotic relationship between humans and microbes?

Yes, certainly. Only one tenth of your cells are human. The rest are bacteria. Without them, your immune system is not primed to work, which might explain the new wave of allergies.

It also turns out that the obesity epidemic may come, in part, from destroying gut bacteria with antibiotics.

What does having a condition called 'mosaic DNA' mean?

It means having two different kinds of DNA in your body. Most probably there has been a mutation early in development, so that some of your cells have the DNA that has changed and others do not.

How strong a microscope would you need to have a proper look at DNA?

To see the actual double helix, you would need an electron microscope, which can magnify by hundreds of thousands of times.

Where would we be without mutation?

Nowhere!

The last questions for Steve are about epigenetics - the way that we inherit characteristics without our DNA having to change.

Epigenetic changes happen through the interaction of molecules called methyl groups with our DNA.

What are methyl groups and what do they do?

A methyl group is a small organic molecule which can attach to DNA, to one of the four letters of the genetic code: A, G, C or T. The methyl group modifies the activity of the DNA.

Can methyl groups be copied along with DNA? If not, how are epigenetic changes passed down from parent to offspring?

It's not the methyl groups that are copied. It is the imprint on the DNA that is in fact inherited. That alters the activity of genes, and the extent to which messenger RNA is made.

Can stress affect the epigenome?

Yes, certainly. For example, people under stress often have changed hormone levels - and hormone levels are controlled epigenetically.

Steve Jones is a professor of genetics at University College, London. He was answering questions submitted by viewers of Dara O Briain's Science Club, on 6 November 2012.

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