understanding sex is the key to understanding how to build a human All humans are built in bits, far too small to be seen with the naked eye. We're made up of cells. And inside every cell is exactly the same mix of genes - our DNA. Until recently, reproduction was regarded as pretty straightforward. To build a new human you first take the genes that make up a woman, then the genes that make up a man, and then enthusiastically mix them up. But things aren't quite that simple. We might have been doing it since the dawn of time, but scientists know very little about the basic processes. Reproductive physiologist Doctor Roy Levin has struggled with the technical limitations of studying sex for decades. We didn't really have the apparatus to allow us to do the measurements. And there was a long period of time when you only guessed what was happening from the external appearances of men and women in coitus. So that you couldn't really tell what was happening inside, because you just can't see. Our understanding of sex hasn't moved on much since Leonardo Da Vinci first started dissecting corpses and studying those over 500 years ago. The Queen holds this drawing by Leonardo in her very own private collection. Doctor Levin's life-long quest to reveal the secrets of sex is taking him to Germany. What I've come for is a machine. This is a machine that makes the body transparent. You can look inside and see movements and the secrets really of coitus and how people get sexually aroused. There are very few machines like this in the world, and one of them is here in a town called Krefeld, quite a few miles outside of Dusseldorf. And the machine Doctor Levin's come to see is this FMRl scanner. It's basically a camera which uses magnetic fields to penetrate human flesh. Today, in the interests of science, Michael DeGroot and his girlfriend Liz Leahy are going to attempt to have sex in its cramped confines. Well, this is the machine as you can see inside, it's got, like, two doughnuts, those are the very magnets, and in between is the space that you'll lie down in and have coitus. One theory is that it's a relic from our past, when sex was more commonly done on all fours and not face to face. Doctor Levin is aware these experiments might be seen as prurient or trivial. But he believes that understanding the mechanics has important practical applications. One possibility is that when we get a bank of normal images of functional the normal reproduction and the movements that occur, and we have a whole range of images, because this is just one couple that we've done, we can then possibly look at people who are abnormal, that they can't have children perhaps, and their -the dysfunctions that are there might be in the movements, might be in the motility that we can pick up, and then we'll be able to look at that and possibly have treatments for it. But that's really looking into the future. We're just starting out really, the first step this is. Understanding anatomy is important if you really want to understand what makes men and women tick. But there's also much that can be learnt by studying their cells. Down at this level, men and women are all very similar. Each cell holds a nucleus which, in turn, contains DNA, the substance that builds us. Each cell contains over two metres of DNA, divided and packaged neatly into chromosomes. There are 46 of these. The real difference between men and women is that women normally have two X sex chromosomes, while men have an X and a Y. But it's not always that straightforward. Look at Jan Johnson. She's an athletic, feminine 42 year old woman. She's also six foot three inches tall, a distinct advantage when you're playing volley-ball. The other thing that's different about Jan is that she's genetically male. There's a huge stigma attached to my condition. I think a lot of people don't want to talk about their personal conditions, and, you know, no one's perfect. People - lots of people have problems, and it just - it's a matter of facing your deepest, darkest secrets and bringing them out in the open. She had no idea that there was anything strange about her when she was growing up. The only thing that I remember is that l was just very active, and I just had a lot of energy. In terms of, like, feeling different no, I didn't. I just felt like a normal a normal little girl, and, you know, l had girlfriends, and I had all my dolls and, you know, everything was just normal. It wasn't until she went to college to study anatomy that she realized something was wrong. Quite simply, at the age of 19 she still hadn't gone through puberty. I felt self-conscious because l wasn't developing like all my girlfriends were developing. You know, it's painful to stand out and be different. And I wanted to find out why I wasn't starting my period. So I ended up going to the gynecologist for the first time. I came into class one day very upset, and - because I had gone to the doctor a few days before and they had very simply just written XY on a piece of paper, and gave that to me. And that was supposed to be the explanation of why I was the way I was. So why did Jan become a woman when her genes were those of a man? The answer lies in understanding how the male sex chromosome the Y chromosome -works. The most obvious thing about the Y is that it isn't very big. The Y has around 60 genes and the eggs around 3,000. The Y chromosome is the smallest chromosome, and when you look at it in detail only a very small part of it actually codes for anything. So most of it's a desert. Peter Goodfellow knows the Y chromosome intimately. He found the gene responsible for turning embryos into men. It's on the Y chromosome, and it's called the SRY. It's hugely influential in how men develop. And this is it. But being very important doesn't mean it's big. Imagine that your DNA was a piece of string, so you could see it. So we've magnified the DNA which is present in every cell in your body so it's visible. And that piece of string now would stretch from here to Moscow. Now, if we were to start a journey from here to Moscow, on foot, the Y chromosome would represent the first 20 miles of that journey. And the SRY gene would be the first step on the journey to Moscow. It takes 30,000 genes to build a human, but just one gene -the SRY to build a man. The SRY is a switch which clicks when the fetus is around six weeks old. Until then, the baby could become either male or female. When the SRY is switched on, it fires up the development of the male sex organs. There's a tiny genetic difference that genetic difference leads to one key difference and that one key difference is men have testes and women have ovaries. And it's what's produced by the testes and what's produced by the ovaries which is responsible for the differences you see between males and females. But Jan Johnson has a copy of the SRY gene, so why isn't she a man? Well, the SRY triggers development of the testes, and they produce a very versatile hormone called testosterone. Testosterone is a steroid hormone, and one of the things that steroid hormones are excellent at is going in lots of different places in the body and instigating lots of different changes, and testosterone's a beautiful example of that. Testosterone rampages through the male embryo building muscle and developing the cardio vascular system. It's this hormone which makes men so very different from women. To a remarkable extent, testosterone is responsible for the masculinisation of the human body. It's really, in some ways, it's kind of surprising that this one agent seems to start this cascade that has so many different effects. And it's testosterone that is the answer to the riddle of Jan Johnson's unusual genetic make up. Jan has a condition called AlS Androgen insensitivity Syndrome. Her genes make testosterone, but her body can't respond to it. If it can't react to testosterone the fetus assumes the default position, which is female. And that's exactly what happened to Jan Johnson. The discovery that she's genetically male was so traumatizing that she couldn't even tell her then boyfriend and future husband, Peter. I told him well, I'm unable to have children, and, you know, here's my situation, and I don't have my period. But I never really got into the nuts and bolts of it with him. Why? Because it was embarrassing for me. I didn't want to talk about it. And I didn't think anyone wanted to hear about it. The only person she felt would listen and understand was her anatomy professor Kris Blodget. And I said well, the first thing I want you to know is you're a normal girl, you're not a boy, you're definitely a girl, and you're a very feminine girl. This should not change your life.
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