Excuse me for a moment, I'm just fiddling around with this onion, not very expertly as you can see. We'll come back to why I'm doing this in a moment, but I just have to get this going.

It's often the case in science that one has to sort of get things going early on, you can't sort of do it all in an instant like you saw with the Euglena. So we'll pop some of this onion in this blender and mash it up a bit. Onion molestation, it's one of my things I'm afraid. OK, that should be enough. Sieve it out. That's enough. We'll put a little bit in this tube, like that, and then add a couple of goodies. We have some detergent here which is going to break things open. That goes in there. Then there's an enzyme here that cuts up the protein bits. It's the same stuff that you find in your biological washing powder, if you use that at home. It digests proteins, blood and that sort of thing. Now we'll swirl that all up a bit, put its top on, yeah, it looks as though that's coming along nicely. We'll pop it in this water bath and that can carry on while we're talking about some other things.

So thinking about this business of passing characteristics down from one generation to another… We're going to play a little game of children and imposters here right now and we have a couple of parents in the audience, I think. We're going to have this row down here, come down. Now this is a mixture of children of these parents whom you can see on the screen, and some imposters who are pretending to be their children. What I want you to do is to sort them out and see if you can tell. We're just going to go through and take a little vote on whether you, number one, are a child of these parents or not.

Who thinks that she is a child of these parents? Who thinks she's not? Well, you seem to have a pretty big vote.

How about you, number two? Who thinks she is a child of these parents? Well, a few uncertain hands, anybody against? Yeah, mostly against, so you're not.

How about number three? Pretty evenly divided. You can come forward a little step.

They're not quite sure about you, number four. Who's for number four being a child of these parents, and who's against? OK, you stay where you are.

What about number five? Take a careful look now, you've got to find some children. There must be some, I promise you there's some! Who thinks that number five is a child of these parents? Ah, you've got one. You come right forward, you definitely got picked as a child.

Number six - for and against? Right now six and seven, for and against. Right, so you're forward as well.

Now let's see who was right! I want those who are the real children to come and stand beside me and the others to step back. Who is the real child of the parents? You are, aren't you. Well, that was great.

So there they are, they're the real ones. You could see some characteristics, and actually they're quite alike, but you're older - maybe that's why I find it easier with you, because you're growing up, you're getting more mature features like your parents. You're more obviously the same, you're less obviously the same - but the point is that the children aren't exactly the same. We're not certain, we know something is happening and yet we're not quite certain about it. Thank you very much to our volunteers for showing us that, thank you.

So how did it come about? People thought that maybe things were inherited and they got blended together. You know, that would be reasonable, like paint. Sometimes children do look a bit like that, you know, sort of halfway between their parents - and yet we know that sometimes particular characteristics are inherited, which is why that daughter was able to be picked out.

But the blending idea doesn't really work. You can carry things forward which are more specific than that. There are ideas that somehow the characteristics that you require during your lifetime are inherited. Think about the giraffe, for example. Rudyard Kipling wrote the 'Just So' stories about how the giraffe got its neck. The giraffe stretches to get those high leaves and in so doing maybe it lengthens its neck a bit - it really does, you can stretch your neck, some people do that by putting rings on or by strengthening themselves in the gym! Then they thought, maybe this long neck is inherited by the offspring - the giraffes were more successful, and passed that success on to their children.

But there are a number of problems with that. I mean, one problem is that the cells in the neck are nowhere near the ovaries and the testis that produce the cells to make children, so they would have to be transmitted in some way. But actually people were thinking about ways in which that might happen, and so we're going to just show you a little experiment with these rather lovely rabbits, a black one and a white one.

What they did was to take blood from a black rabbit and inject it into a white rabbit. The reason they did that was to test the idea that somehow all the organs of the rabbit were producing little corpuscles called Gemmules that circulated in the blood, took the information to the sex organs and allowed the inheritance of the characteristics that it had. So this chap with the black fur - if there were black Gemmules circulating in its blood, if you took some blood from the black rabbit and injected it into the white rabbit, the question is what sort of babies would the white rabbit have? Would they now be black because she's received this transfusion of black rabbit blood?

They actually did this experiment. Now the question is: what do you think the answer is? How many people think that she will now have white babies still, and how many think black? Quite a few people think black; others think white. They're not sure - so what is the answer? The answer is white. The blood does not carry Gemmules; it doesn't carry the information to do that. And yet it is a very appealing idea and indeed the phrase 'blood inheritance' is all over our language. The idea of things being in the blood has persisted.

What happened next, or at least in the parallel with all these goings-on, was the work of Mendel. Gregor Mendel was a monk who lived in Czechoslovakia - now the Czech Republic - in a monastery. He was a gardener. He tended pea plants in the monastery garden and they had lots of interesting variant peas in those days. We have rather standardised varieties now, but some of Mendel's varieties actually live on. These are the peas or descendants of the very peas that he used and they have lots of interesting different characteristics: great tallness, great shortness, different coloured flowers, yellow pods; they had wrinkled seeds inside some of them.

Let's just focus for a moment on one of the experiments he did as he uncovered the way in which inheritance happened. If he had a pure-bred tall pea plant and a pure-bred dwarf pea plant, that means to say that through many generations he just crossed tall with tall and short with short so they were kind of all homogenous. Then what would happen if he made a cross between the two? They all turned out to be tall - but when he did the experiment again, when he crossed those tall ones with each other, then what did he get? Does anybody know the answer to that? Somebody different, yes, behind there.

Correct, that's right! And this was the immediate indication. Now he worked it out in great detail and in a way, the fascination about this was that nobody knew what he was doing. His work had to be rediscovered nearly 50 years after he published it in Czechoslovakia but it illuminated all of biology when he did that because the point was that the characteristics came down without necessarily being expressed.

So the factors were there inside that led to the characteristics - but they didn't necessarily show themselves. Now to make that a little more graphic, you I think have some dark glasses to put on, there they are, if you'd like to pop your glasses on,, and I have some other glasses here. But you also have some cards. Now the point is that Mendel discovered these factors, the idea that there was something in there which was discrete and being inherited. He didn't know what they were. He didn't actually have a better name for them, factors, although later on, when his work was rediscovered, people invented the word 'genes' - and that's where the word 'gene', which is so familiar to us today, comes from. Its from Mendel's work. Although he didn't call it that, he was the one who recognised this absolute all or none nature of them.

Here's some people who are going to play at being parents of these people. Now, when you're a parent of somebody, what happens is that you contribute a gene for all the various characteristics. You contribute one set of all your genes to each of your children and the other parent contributes one set of all their genes. And then the children in turn pass these on.

It's important that we all carry two sets of genes, you see, one from our mother and one from our father. We're just going to think about one gene and you have to imagine that expanded to all the others.

So now you're going to reproduce and you're each going to hand on a gene to her as your child. OK, you're going to go there first - whichever way it doesn't matter, good. Pass those on and then the next one, see they're handing out one gene; no not you, you have to take both, that's right, that's right, you take those two and then you take the next one, that's it. One from each - you see how the genes are being combined in the children.

Now let's see what those genes are. If you open up these booklets then you'll find inside some pictures of glasses, so you've got two brown genes. Now what that means is that your eyes are going to be brown so you can put those glasses on. You can fold the booklets up once we've seen them. OK, let's look at yours. Can you open them both or shall I? I'll hold one. OK, that's right, side by side, very good, thank you. Brown and blue! Now does anybody know what the inheritance is if you have a brown and a blue? Up there.

Brown - that's quite correct, the brown is the dominant gene. It corresponds to Mendel's tall pea plant. And so you also get brown glasses. OK, you fold those up, and the next person please. Can you hold them up side by side if possible? That's right, get your neighbour to help - that's a good idea - that's it, another brown and blue, so we've already done that one, there you are, you have your brown glasses. Let's see the last one, what have you got? Ah blue, yes, and another blue. Now this is the case where the blue gene is there twice. This is the one that only shows itself if there aren't any browns around, so you're the one who gets these blue glasses - there you go. Now of course that tells us about the parents, doesn't it? What must have been the parents' genes that they inherited? Yes, can you tell us?

Well, the parents must each have had a blue-eyed parent and a brown-eyed parent and you get the blue eye and the brown eye genes coming together which creates a brown eye, but they still have the blues eyes in them.

And then when blue eyed children, blue-eyed genes randomly come together to create other blue-eyed children.

And brown eyes - you can get either two blue eyes, a blue eye and a brown eye or two browns.

That's right. Both those parents must have had a brown gene copy and a blue gene copy because the only way she could get two blue gene copies was that they both had them. But on the other hand they also both had brown gene copies - otherwise they would not have brown eyes. So you see we can deduce your genes from them which is interesting, isn't it? Genes run in the family. OK, about a century ago perhaps, people had got this sorted out and it was going very well; people really had got a grip on this sort of notion of inheritance.

But what was the thing that was being inherited? That took a while to discover and in fact it's when microscopes really became important. If we can see the view from the Zeiss microscope - there we are … this is a section through an onion root tip. If we go in and magnify it a bit more and a little bit more ... this is a section to an onion root tip which is dividing very fast and I think you can see there these little rods? Over here it's rather lovely. You can see the cell dividing, because like all larger creatures like us, the onion is divided into many, many cells. Here's one getting ready for cell division and this has just divided and its moved these rods apart.There is a clue there in these strange processes with these little rod things appearing in the centres of the cells. So people began to develop the chromosomal theory of inheritance, and in fact they turned out to be right.

And we're going to jump ahead a little bit to what they found in those nuclei. This extract of onion I made here... I'm going to see if we can do another little experiment and take it on a bit further. It's become very sticky in there, so I'm cutting the tip off this so it's nice and big. We're extracting some of it and I'll try and put some into one of these tubes. I'll take some out and see how this goes. Yeah, it's got very sticky. And maybe you can see that if it's magnified on the screen? As I drip it into the tube it's really quite a stringy, viscose sort of stuff. And now, to look at it more closely, I'm going to pour some alcohol over, very gently so as not to stir it up too much, so I can get it right.

Now what we have is onion juice in the bottom and the whole tube filled with alcohol up above. I'm going to dip into the tube at the bottom and see what we can do. As I pull the onion up through the alcohol you should be able to see the fibres being pulled out. Can you see that? The fibres are so strong that I'm actually rocking the liquid as I pull them out and they come out and out and out. It's like spinning cotton out of the cotton seed heads. I can pull it up - if I'm lucky - to the top of the tube and trap it there. Can you see perhaps on the screen - if they're showing you - the fibres? This is the information to make life. And it took life four billion years of evolution to get to the point where it could look inside its own cells and find out what was in there! This is truly the secret of life: it's the instructions to make life itself - and next time we'll find out what it is.