Sunday 7 January 2001

Pic of the day: Cloned, by popular request. Because the world needs more of me... (Don't I wish.)

The OTHER twin paradox

I suspect every science geek and science fiction geek too has heard of the twin paradox: You send one twin off in a very fast rocket, and let the other remain on Earth. When the space twin returns, he is younger than his twin. Paradoxically, it is not really a paradox at all. The grandfather paradox is a paradox, but then again there is no known way to travel backward in time. Time dilation, on the other hand, is already measured. Though you need atomic clocks to measure it at the speeds we use for today's space travel. (Seems to be quite some room for improvement here!)

There is however another twin paradox, which is rarely spoken about. So of course I'll do that. I refer to the fact that identical twins are not identical.

***

Since the beginning of time, twins have been rare enough and common enough to scare the superstitious. It didn't improve by the discovery that sometimes twins were so alike that even their own family could not tell them apart, while other times they were no more similar than children born a year or two apart. In some remote tribes, twins were considered Bad Luck and one or both were set out in the woods. Even though this is rare, myths about twins abound. In particular the myth of the "evil twin", which is so common that it has become a part of the English language.

Today we know that twins come in two very different flavors. Dizygotic twins, popularly called fraternal twins, need not even have the same father. (Though it is certainly considered the decent thing.) Sometimes two ova - egg cells - set out to travel down the oviduct at nearly the same time. The surprised sperm cells suddenly get two chances at the price of one. This is almost certainly much more common than we know, because statistically many egg cells never develop further even though there are sperm aplenty. (To the surprise of many newlyweds, you don't always get pregnant at the first ovulation, even if both man and woman is otherwise healthy.)

The really intriguing twins are the others, monozygotic or identical twins. They are genetically identical, like clones. Research (mainly on animals) has shown that sometimes a small group of rapidly dividing cells will split in two, and if this happens before a certain stage (varying from species to species) each group will develop into a new individual. It need not be while there are still just two cells: For instance, 4 cells could go left and 4 go right. Or even 3 and 5. There will be no noticeable difference in size between the twins in the last case, as human eggs are not exactly like bird eggs. They're about 1/5 millimeters across, if I remember correctly. (I'll have to admit that I never saw one in real life. Female readers may want to study their tampons closely. Then again, they may not.)

Sometimes twins, although genetically identical, are mirror images of each other. That is, one of them has right and left sides switched. This causes no end of confusion if the mirrored one gets an inflammation of the appendix, for instance. Usually you can't see which one has his or her internal organs switched around, because outwardly our two sides seem very similar. (Not identical. Just very similar. This is explained later.)

It also happens that one twin dies early - so early that it can still be absorbed by the body - and only the other remains. Sometimes the other is the mirror, so don't feel too safe. Check today whether your heart sist just slightly to the right of center. If so, you probably once had a twin.

***

Twins are a treasure trove for science. If a disease is caused by genetic factors, then identical twins should either both have it or none of them. Actually it is rare to find an illness that follows this pattern exactly. Color blindness comes to mind, as does bleeder disease (the English name escape me - these guys have blood that does not coagulate, and may bleed to death from minor wounds unless treated).

Lately, it has also become acceptable again to study the genetic basis for personality traits. For instance, it would be interesting to know if both of the twins were homosexual. (They are most often not.) Intelligence, however, is strongly linked. The IQ of two identical twins raised apart is likely to be more similar than of fraternal twins raised together ... but not identical.

Overall, twin studies have showed that more of our personality and health are determined at conception than we thought a few decades ago. Then again, a few decades ago we had just won World War II and any idea that people were born superior in any way was considered nazist and as such forbidden by the thought police. Today we are more likely to try to make the best out of the raw materials that a child has, body and soul.

***

There are those who say that there are others who say that a human life begins at conception. I doubt there is actually anyone who says that a human life begins at conception, but there are those who are accused of saying so. That is a grave accusation. If the life of a human started at conception, then identical twins share a life. You could then, ethically speaking, punish one for the other's crimes. You could even kill one and still not have terminated a human life, if the other remained alive. (I'm sure some parents have thought this over carefully.)

While many, or even most, identical twins share a special bond, they clearly have one life each. One may die and the other survive, albeit in a psychologically reduced state, as they usually are very close. Apart from recurrent rumors of telepathy between twins, it seems that they have indeed one soul each. Already as toddlers they tend to have different personalities. Often one is more dominating, while the other is meeker. In fact, studies of identical twins that are raised apart show that they grow more similar the longer they live, while identical twins that grow up together diverge.

This then is the real twin paradox: That two people who were one, are two. That they were born with the same abilities, the same basic personality, and yet they grow up to be individuals.

The most drastic conclusion from this is that you cannot clone a person. You can clone a person's body; and if completely and utterly successful, it will be like an identical twin that has been in stasis for that many years. But the clone will grow up to become a unique individual, with its very own soul. There can be no "b-class" citizens of clones, because clones are simply people who have come into being in an unusal way. Artificial, time delayed twins. Exactly as if one of them had been sent on a voyage through space at relativistic speed ...

***

"The genetic code does not, and cannot, specify the nature and position of every capillary in the body or every neuron in the brain. What it can do is describe the underlying fractal pattern which creates them."
--Academician Prokhor Zakharov, Nonlinear Genetics.

Yes, it's one of our imaginary thinkers from Alpha Centauri again. But this time, he is saying what I am thinking.

Actually, the genetic code specifies proteins. At least as far as one has been able to decrypt, each triplet of bases in the genome codes for one particular amino acid. Sometimes different triplets give rise to the same acid. And sometimes the genes for one single protein are not even consecutive, but are broken up by "junk DNA", which means bases that don't make sense to us. (Computer geeks may think of a hard disk badly in need of defragmenting.) The order of the amino acids within a protein determines how it folds, and the shape of the protein determines its function and properties. So there is no "blueprint" for the human body. There is just a cluttered library of possible proteins, and certain triggers that activate or close off the production of various proteins.

The real enigma, then, does not apply only to twins but to every one of us: How and why do we become human at all? We share at least 98% of our genes with one or another of the great apes, and quite a few with mice, fish, even trees or yeast. How does the tiny little sphere of cells decide that it won't even try to become a juniper or an elephant, but go straigth for a human ... or two?