A Short History of the Y Chromosome

Among the many chromosomes in a man’s body, the smallest one with the largest personality has to be the Y chromosome. With it, you are a male; without it, you are a female, with few exceptions. More than any other chromosome, it really defines who you are.

This tiny chromosome also controls other traits as well: hairy ears, tooth enamel, and stature to name a few. Because the Y chromosome is passed on to a son exclusively by a father, all changes in it are, by default, dominant since the mother does not have a Y chromosome to balance it out. For the longest time, the Y chromosome was also considered home to a lot of “junk DNA” that we thought had no purpose. We now know that much of this DNA has a purpose and that the Y is the home of many important male fertility genes.

Before its association with male fertility, the Y chromosome was widely considered a genetic black hole, a chromosome that evolved as a broken remnant of the X chromosome. We knew that the “maleness” gene was on the Y and a few other genes. However, since the Y chromosome has been fully undressed as a result of the Human Genome Project in the 1990s, we now know that it is very unique, even special, and that it evolves in its own special way to keep men men.

The Y chromosome, and its neighbor the X chromosome, evolved into “sex” chromosomes hundreds of millions of years ago. Remarkably, to this day many species do not have a chromosome for each sex like we do. Some species become male or female based simply on the environment in which they find themselves. For example, the sex of turtles and crocodiles is influenced by the temperature of the egg after fertilization. Imagine that! A boy in the Artic but a girl in the Caribbean. At first, the original sex chromosomes probably evolved as a pair of two X chromosomes. Then, 150 million years ago, the Y chromosome made its break from the X chromosome. Kind of hte opposite of Eve coming from Adam’s rib. Basically, it stopped associating with it and this led to our current X-Y system of sex determination.

Some males are born with an extra Y chromosome. XYY syndrome is a genetic condition that affects about one out of 1,000 male births. Many men who have an extra chromosome go their whole lives without knowing they have this genetic difference. That is because they lead normal, healthy lives. However, they are more likely than the average guy to be tall, experience acne outbreaks and get diagnosed with learning disorders like ADHD or autism. They are also likely to have a lower than normal sperm count.

As it works now, the single Y chromosome has no partner with which to swap genes when sperm are made (at a normal rate of 1200 sperm/heartbeat!) This “swap meet” of genes that occurs when new sperm are formed is an important repair process for the 22 other chromosomes and is critical for our evolution as a species. In fact, this is the source of our evolution. So, now that the Y chromosome has become isolated and less of a team player, is it doomed to extinction? More importantly, are men are doomed to extinction?

So how does the Y chromosome survive and repair itself, living alone in isolation while the world is changing around it? Well, we now know that it manages very well on its own, thank you. And this has probably been true for about 5 million years. Although it no longer swaps genes with the X chromosome, from which it came, the human Y chromosome can swap genes with itself to discard bad genes. Research suggests that the Y chromosome in gorillas and chimpanzees developed the same ability 5 million years ago. This discarding is called gene conversion and no other chromosome does it. Just the Y. How uniquely male.

Basically, essential Y chromosome genes are arranged in a series of eight “palindromes,” or mirror image sequences, each of which folds like a hairpin in which its two arms come together. Then the “DNA checkers” compare the two arms for any differences and convert a mutation back to the correct sequence, thus saving the Y’s genes from mutational decay. So, the older “junk DNA” thought to exist on the Y chromosome is now known to represent DNA that it critical for its survival. One man’s junk is another man’s treasure. And so it goes, the Y lives on, and men do too.