Saturday 28 September 2002

Pic of the day: Thank [Cosmic Designer of your choice] that there are only two sexes. It can certainly be complicated enough. (Screenshot The Sims.)

Haploid, diploid, triploid

You have certainly heard about chromosomes, the tightly packed spirals of DNA, carrying our genes from generation to generation. People have them, animals have them, plants have them, many microscopic organisms have them. In some simple single-celled organisms such as bacteria, the DNA just floats around. But in us who have many cells, the DNA is packed in chromosomes in the nucleus of the cell. And almost without exception, there is 2 of every chromosome.

A creature with 2 sets of chromosomes, like us, is said to be diploid. If we had only one set, we would be haploid, and if we had 3, we would be triploid. There are some plants with 4 chromosomes, these are called tetraploid. They tend to grow faster, but because each cell is larger, wood from tetraploid trees is not as hard as normal.

Sexual reproduction, which is so popular in both animals and plants, is only possible when there is an even number of chromosomes. The body of animals and flowers produces special cells with only one set of chromosomes (or two, if the plant is tetraploid). In animals, such cells are sperm and ova (egg cells), which I am sure you have seen lots of times in pictures. When these cells are made, the chromosomes are pulled apart, one from each part in opposite directions. But they don't follow each other as a set. You know you have one set from your father and one from your mother? Well, they don't go their separate ways, your father's chromosomes to one cell and your mother's to another. No, they are thoroughly mixed. It is even common for the chromosomes to cuddle up to one another first and exchange parts, so that even one single chromosome can have parts from both parents.

If the chromosomes had traveled through the generations as sets, things would have been very different. For instance, many siblings would be identical like clones. If two brothers inherited the chromosomes from their mother's mother, they would have exactly the same DNA (since, being brothers, they already inherited their father's father). In the real world, this is not so. We are a thoroughly mixed bunch.

***

In some species of insects, and a few lizards, children are actually like clones. They have done away with the pesky males and reproduce alone. They lay eggs that don't need to be fertilized ... they grow into new individuals with the exact same genes as their mother. But even these are diploid. Their ancestors reproduced sexually, but then they stopped. Usually these species are only a few million years old, but there is one small type of animal that has kept up since the dinosaurs were young. And why not?

Indeed, why have two sets of chromosomes in the first place? They contain the same information. If you have one healthy set of chromosomes, you are all set. Why import another set which may be of inferior quality to your own? Why take the risk of diluting your genes?

Mutation happens. In fact, we are all mutants: In a long-lived species such as ours, there are a few mutations per generation. Usually these are completely unimportant, as most of our genes are not in use. But even if the mutation happens in an active gene, it only has an effect if it changes the structure of the protein that is coded from that gene. (Almost all active genes code for protein, and the shape of the protein is the important thing. Some parts of a protein are less sensitive to what amino acid is used, others are more critical. It depends on the way the protein is folded.)

Even when there is a protein-changing mutation, it doesn't necessarily have any effect. Since we have two of each gene, a healthy gene will usually overrule a defective one. Also in things like hair color some genes are stronger than others, which is why we are seeing fewer and fewer natural blondes as the blond populations interbreed with other groups. But the genes don't die from being suppressed. They remain, and some day they team up with another like themselves, or one that is even easier to suppress. Then they become active. These recessive genes cause many inherited illnesses, such as sickle-cell anemia or color blindness. But some mutations can also be good. For instance, thousands of years ago someone got a mutation that let them drink milk without getting sick, even after early childhood. Over time, the offspring of this lucky individual became dairy farmers and colonized places such as northern Europe, where cows and goats could graze on land that was not fit for grain and vegetables. Without that gene, grazing animals would only give food once, when slaughtered. But because some unknown ancestor was a mutant, we can milk that mutation and the cows for all they are worth.

***

In a haploid creature, a mutation would take effect immediately. It would not slosh around unnoticed for several generations before suddenly showing up. If it was good, the haplo would have the benefits at once. If it was bad, the haplo would be in trouble and the gene with it. You would expect evolution to work faster and more precisely. So why, oh why, do we need this elaborate dance of the genes? And why do we need the opposite sex? Why can't we just lay our own eggs and be done with it?

(The voices in my head insist that I write a science fiction story about a human society where we are all haploid and lay eggs. It will have to wait, though.)

The theory currently in vogue is that sex is all about parasites, but in a good way. Because with the mixing of genes, children have a slightly different set of proteins from each other and from their parents. This makes them a moving target for parasites ... if a virus specializes too much, it will not infect so many individuals since they are different; but if it specializes too little, it will not be very effective at what it does.

The irony is, of course, that sex is one of the most effective way of exchanging parasites. Several dangerous diseases spread mostly that way. But overall, the variation makes us more robust.

Another way of thinking says that sexual reproduction spreads a reserve of genes in the herd, which may not be useful now and therefore is mostly dormant. But when the environment changes drastically, the reserve is already there and the few individuals that express it gain a benefit. For instance when humans waded north into the fog and winter cold of northern Europe, black skin and black hair wasn't so smart anymore, as they radiate heat into the arctic night. The blond, light- skinned freaks suddenly were the best fit, and inherited the land. In an asexual species, they would have been eliminated by natural selection long ago.

In short, a haploid species would allow evolution to work faster and more precisely, but would not have a reserve of variation in the species.

***

Then why aren't we triploid? That would allow even more combinations of recessive genes, a dramatically larger storage chest of genes. A chair with three legs is much more stable than one with two. So why is there not a single triploid species on Earth?

One possible explanation is that it was never invented. The internal mechanisms in the cell to separate three sets of chromosomes would have to be in place exactly at the time when the third set of chromosomes were added. The likelihood that such a thing would happen even once (with us diploids) is vanishingly small, and triploids would be more complex. To have it happen again might take another few billion years. By now, the niches are pretty much taken already by us diploids. Also, you wouldn't reap the full benefits of it until you had 3 sexes. As if 2 were not complicated enough ...

Finally, there is the possibility that duality is an intrinsic part of nature. We have electrons and protons, we have matter and antimatter, even the quarks all come with a complement particle. They all come in pairs. Perhaps the Intelligent Designer of our universe is giving us a hint, but a hint about what? Its own nature? It's certainly not a very precise message, if any.