burrow and murder their way into the uterine mucosa will never become an actual part of your body. The ones that will become you sit hidden within the cell vesicle. One week after conception, you consist of a bunch of stem cells that can form any body part – they could become heart muscle cells, nerve cells, liver cells or anything else. At this stage they are still so flexible that they can even create more than one body. If the cells were to detach from one another, and form two separate cell clusters instead of one, they might develop into two complete people. This is the most common way that identical twins occur, and since the placenta is already being formed, the twins will have to share it. Alternatively, the cells could have fallen apart a few days earlier, when they resembled a microscopic raspberry, in which case two vesicles will attach to the uterus and two embryos, each with their own placenta, will be created. About one third of identical twins begin like this.
Since identical, or monozygotic, twins originate from the same cell, they possess exactly the same DNA strands – they are natural clones. If one of the twins commits a crime, investigators will be unable to distinguish between them using a DNA analysis. However, if their fingerprints were examined, then the culprit would be revealed. This is because fingerprint patterns are in part shaped by the environment in the womb. The two twins occupy different spaces, and therefore experience different streams and pressures against their fingertips. In addition, because the supply of nutrients from the placenta is not evenly distributed, one can grow slightly faster than the other. This means there will be small variations in the twins, even though their genes are exactly the same.
Twins can also result if the mother releases two eggs instead of one, and each becomes fertilised by its own sperm cell. These are called non-identical, or dizygotic, twins, and their DNA strands are no more similar than in normal siblings. But they are not exactly normal siblings. It appears that twins can exchange cells while in the womb, just as cells can be transferred to mothers. In this way, for example, they may end up having two blood groups – one that comes from themselves and another that comes from the twin sibling.
I have no twin that I know of, but perhaps I had one that I was never able to meet. On rare occasions, the two cell clusters recombine before two bodies can be formed. If this happens with non-identical twins, then the child will grow up with two sets of DNA, a so-called chimera. Instead of all the cells having the same DNA strands, some of them will carry the DNA strands of the ‘twin’. Usually it never comes to light, but the phenomenon can sometimes lead to quite absurd situations. In 2002 Lydia Fairchild, from the US state of Washington, was expecting her third child and had applied for child support. The authorities required that she and her ex-boyfriend take a DNA test to prove they were the parents. As expected, the results showed that the ex-boyfriend was the father. The only problem was that, according to the DNA test, she was not the mother. Fairchild was suspected of fraud and feared that her children would be taken from her. The court summoned a witness who was present at the third child’s birth. More blood tests were done. The DNA analysis still showed the impossible: without a shadow of doubt, she could not be the mother of the child she had just given birth to.
How was this possible? Were the tests flawed? Only after taking samples from different parts of Fairchild’s body was the mystery solved. The blood and skin samples taken previously matched one another, but the cells they obtained from the cervix were different – they carried a second DNA profile. Fairchild was a chimera. Before she was born, her cells had merged with a twin in the womb. When this happens, instead of each twin making its own complete body, the recombined cells become woven together and share the tasks between them. In this case, the cells that made the skin came from one twin; those that made the egg cells and cervix came from the other. Fairchild’s body was created by twin sisters – which made her the child’s mother and aunt at the same time.
Unless you have an identical twin, there’s not one person on the planet who has exactly the same DNA as you. When the sperm and egg cell merged during your conception, a unique code emerged. But the areas where your uniqueness is manifested are very small – most of the recipe is the same in all people, and these days it’s possible to look it up online. Through the Human Genome Project, researchers have mapped the entire human DNA – all three billion letters of it. The mapping was not of any one individual’s DNA; many anonymous donors contributed different sections of the code. It was a vast project that took many years to complete, and cost hundreds of millions of dollars. It shows how fast technology has evolved that, today, it costs around $1,500 to do the same thing as an individual (even less if you’re content with a rough survey). A laboratory can take some of your spit and provide you with your exact rows of A, T, C and G just a few days later. Altogether, the formula would fill more than a hundred thick books. If you were to read one letter per second, it would take you ninety-five years to finish.
It’s unlikely you’d have learned anything more about yourself, either. Imagine a book written without a single full stop, comma or space, and that in some places it’s written backwards without warning. The whole thing would be nothing more than page after page of incomprehensible gibberish. Your DNA is something like that – and it’s among this chaos, this ocean of apparently random letters, that researchers are now searching for words and sentences that make sense. One of the first things they found was that they had made a serious miscalculation when estimating that humans contained about 100,000 genes. We’re not even close to that number. Human beings – the inventors of the computer, founders of civilisations and cities – have only about 20,500 genes each. That’s roughly the same amount as the tiny roundworm C. elegans. Even the maize plant beats the socks off us with 33,000. In fact, your genes account for less than 2 per cent of your DNA. So what do they actually do?
THE CONTOURS OF A BODY
AT THE START of the third week, the cell cluster which is soon to become you flattens and spreads out across the vesicle. Right now there’s nothing that even vaguely resembles a body – you look more like a little round plate. On each side of the plate are two fluid-filled sacs. One of them becomes the foetal sac, which will enclose you and the little pool in which you’ll live for the next few months. The other will become the yolk sac, a round balloon with its cord fastened inside your stomach. The yolk sac creates your first blood cells; a job that your liver, spleen and your bone marrow will eventually take over. When it’s no longer needed, the yolk sac will shrivel up and become part of your intestines.
With birds and other egg-laying animals, the most important role for the yolk sac is to provide nutrition – they have no placenta to feed on, of course – so it’s packed with vitamins, minerals, fats and proteins. If you crack open a hen’s egg, in addition to the yellow yolk sac you may also notice some thin white threads keeping the yolk attached to the centre of the egg. In a fertilised egg, a chick slowly emerges from a thin white plate on the surface of the sac. At first, it’s a barely visible speck, but after a few days red blood vessels coil themselves around the yolk. Soon after that, the yolk sac shrivels up and a living creature gradually emerges. Three weeks later the egg hatches and a new chick is ready to meet the world.
Things go a little bit slower for us humans. But at the start of the third week you take at least one important step forward from the plate stage. Over the course of a few crucial hours, you are given a front, a back, a top and bottom, and right and left sides. It’s one of the most critical periods in your whole development. Had something gone wrong, then you wouldn’t be reading this book now, with your intestines safely packed behind your skin and your heart beating reliably on the left side of your chest.
The first sign of this dramatic change is that the round plate becomes more of an oval shape. At the same time a thin strip appears. This is the beginning of your back, and it extends from the edge and towards the centre of the oval plate, where your head will pop up later on. If we were to zoom in on this strip, we’d see all the cells wandering down it towards a small pit at its centre. The cells dive into it, forming a new layer under the topmost one. Soon you will consist of two cell plates stacked one
