Human beings, barring genetic anomalies, have 23 pairs of chromosomes in their genomes. One half of each pair comes from each parent. Twenty-two of these pairs of chromosomes are the same in all people. But the last set is different - they alone determine sex. You can have two possible combinations: XX and XY, and the Y is what makes a man male. It alone carries the genes for testes and sperm that separate the boys from the girls.
But the Y chromosome is behaving unlike any other chromosome in our bodies - it's shrinking. The X chromosome, Y's feminine partner, contains almost 1500 genes. It's believed that the Y chromosome, too, once contained around that many. But now, the lonely Y only contains 86 genes. It's lost almost 95% of its genetic material.
Why is the Y shrinking? The answer lies in how our cells replicate. When a cell divides, it has to make entire copies of its DNA to put into the new cell. But our cells aren't perfect - whenever they copy genes, they make a few mistakes. Some of these mistakes are caught by special enzymes which "check" the DNA, but not all of them. Those that make it past the initial checkpoints are usually fixed later by borrowing information from the partner chromosome. So if you have a flaw in a gene on your dad's copy of chromosome 18, the body replaces that gene with the corresponding one from your mom's copy of chromosome 18. But the Y chromosome has no exact partner - ever. The Xs get paired up to make girls in 50% of the population, giving the X chromosome time to repair its genes with the matching copy. But never, in healthy people, does the Y chromosome get another Y chromosome so it can fix mistakes. Over time, these mistakes accumulate, turning large portions of the DNA into junk, which is later removed - thus the chromosome slowly shrinks.
Possible trajectories for the degradation of the human Y chromosome
from 310 MYA (when mammals diverged from reptiles) up to the present
time (dashed vertical line), and prediction for extinction time.
(A) The rate of loss of active genes from the human Y assuming a
constant rate (blue), an exponential decline (green), a target size
that initially increases and then decreases (orange), or an exponential
decline slowed down in its final stages by positive selection (purple).
(B) A more realistic picture of the rate of gene loss from the human Y,
taking into consideration that at least three evolutionary blocks, the
ancient conserved layer 1, the older layer 2, and the Y added region
(YAR) were differentiated at different times, and each would have
presented an initially small target size for degradation.
But even this problem with matching doesn't explain the whole story. While the Y chromosome is shrinking in humans and other species which have the same sex determination, other animals which use a different but similar pattern don't have the same problem. In birds, males are the ones who are homogametic, or have two alike sex chromosomes: ZZ. The females, in turn are heterogametic: ZW. Based on the trouble with Y, you'd think that the W chromosome has shrunk over time, too. But it hasn't - not in all bird species. In the flightless birds, like ostriches and emus, the W is just as large and is very similar to the Z, while in others, like the chicken, the W is greatly reduced like the Y is in humans. The point is that even though it can't pair with a W to fix mistakes, the W chromosome in some birds is the same size as the Z chromosome, and shows no signs of degeneration. Why?
Some have said that the W doesn't shrink because both parents play equal, vital roles in raising the offspring. In humans, and many other mammals, this isn't the case - the man, while he might help out, isn't actually necessary for parenting. In many species, the man simply leaves the offspring to the mother. Thus men are only vital for one part of the reproductive process - mating. Because of this, fewer men are absolutely necessary for the continuation of the species. Most, then, are dispensable. Because of this strange selection pressure, the Y chromosome doesn't need to keep around its whole host of genes, just the few that allow it to be male. In other words, there's no real reason to invest a lot in the male chromosome when the men themselves are rather replaceable. But in birds, where the female is vital, the W has to stay in tact and fully functional.
Of course, behavior isn't the only possible explination. Others say that the reason the W hasn't degraded as much is because it's feminine. The W, thus, replicates and undergoes meiosis in the ovaries, whereas the Y does the same in the testes. The testes are a much harsher environment. Not only does a cell reproduce many more times to create the extremely high number of sperm (compared to the number of eggs), but the testes have a highly oxidative environment with few repair enzymes. In other words, there's a lot more chances for the DNA to make mistakes while replicating, and there are fewer of the usual ways to correct them. Because the W, in contrast, has to be copied much less and has the comfort of the ovary, it is less likely to end up with flaws in the first place.
What will happen if the Y chromosome disappears? No one's entirely sure. In truth, no one's sure that it can be lost entirely. Some believe that the important "male" genes will attach to the X chromosome or another chromosome, which has happened in some species of mole vole. These animals have either two X chromosome paired together or only one, unpaired X and still have two distinct sexes. In humans, this can happen, though rarely. It's known as XX male syndrome, where somehow a few of the Y's genes have attached to an X, rendering the person still "male" despite a lack of a Y chromosome. However, in this case, the X chromosomes tend to feminize the person as well, leading to small testes, sterility and effeminate characteristics. Others believe that another chromosome will attach to the Y, giving it more genes and allowing it to continue onward. Still others believe that it will never fully disappear, and will remain indefinitely as a husk of a chromosome containing only the one or two genes absolutely necessary for manhood.
Why is it so important to have men around? After all, plenty of species breed asexually or without sex distinctions. There are even species of lizard which are entirely female. Instead of mating with men and exchanging genetic material, they hump each other and simply clone themselves. The process of reproducing without fertilization in vertebrates is called parthenogenesis. It has been documented in some species of shark, too. But never, not once, has it been found in humans. Other than a rumor of some woman named Mary about 2,000 years ago, no one has ever had a virgin birth.
The truth is, we have sex for a reason - you know, other than the fun part of it. Sex allows people to mix their genes, giving their offspring a head start against disease, parasites, and changing environmental conditions. This variation is so beneficial, scientists argue, that it outweighs the costs that come along with sex. After all, to have sex, you have to find another member of your species. Not only that, but your genes take a hit in the next generation - only 1/2 of them move onward. And even more importantly, in sexual species, only one member can bear offspring, which slows the reproductive rate dramatically. Sex is the reason that, even in the friskiest animal species, reproductive rates are nowhere close to the asexual animals. All these potential negatives, though, are overwhelmingly outweighed by the ability to create genetic variety.
It's this genetic variety, as well as sex in general, that is facing extinction if the Y chromosome disappears. Even if human beings were to be suddenly able to undergo parthenogenesis, we would lose a vital component of our reproductive process. Without that variety, we'll be far more susceptible to the onslaught of disease and parasites which evolve far more rapidly than we do.
The Y chromosome isn't taking its losses lying down, however. Scientists have found that to protect its vital genes, the Y chromosome has evolved an entirely unique way of fixing mistakes - it swaps genes with itself. The vital genes on the Y chromosome are actually palindromes (where they read the same forwards as backwards). The first half is the gene itself - the second half is the gene's copy, but backwards. Thus if something goes wrong with the functional portion of the gene, it can be repaired using the backwards template.
Even still, men are quickly becoming a dying breed. In the second half of the 20th century, it has been estimated that the average sperm count has dropped a staggering 40%. While this figure is up for debate, scientists have found that, overall, men's semen quality is dropping. The most likely culprit is the recent rise of certain chemicals in our water. Some chemicals are so similar in structure to hormones that they can fool the body. These mimics, called pseudo-estrogens, can cause men to become more feminized. In animal species, males have been found with female tissues and a lack of testes. In humans, exposure to these chemicals has been shown to have a big impact on male traits, particularly when the person is exposed to them as a fetus in the womb. So, if it's not bad enough that the Ys are shrinking, the men, in general, are losing their manliness due to environmental effects.
Most scientists believe that a true loss of the Y, and thus men, would spell the end of our species. Unlike the infamous lesbian lizards, humans cannot breed asexually. There are several, vital genes, they argue, that have to come from the male. And clearly, our reproductive process is currently dependent upon men. Even if we did evolve a way around the lack of males, the loss of genetic diversity is likely to be devastating. We'll be at the mercy of fast-evolving parasites and viruses. But then again, who really knows? Even if the Y disappears, perhaps the next few million years will not spell the end of the age of humans, but solely the end of the age of man - with the age of woman just beginning.
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