The Y Chromosome

28 November 2014

Chromosomes carry the genetic code that determines the characteristics of a living thing. They are fascinating due to the varied factors they determine, the sometimes negative effects they can have and their complexity. Equally interesting are the stories of their discoveries. This series will explore the history of specific chromosomes and their impact on science.

Humans typically have 23 pairs of chromosomes. One of these is comprised of our sex-determining chromosomes, X and Y. Taryn Cain continues this series by looking at the Y chromosome.

The first mammals were tiny, shrew-like creatures that were still many millions of years away from being awoken by the melodic sound of an iPhone alarm or travelling to work in a huge piece of metal crammed with other mammals. While they carried on with their “simple” lives of eating and evading being eaten themselves, their DNA was also a fairly simple arrangement. All their chromosomes were autosomes and male/female differentiation was managed by genes on various autosomes rather than specific sex chromosomes.

Then a mutation occurred.

A gene called S0X3, normally associated with sex differences in fish, amphibians and reptiles, mutated into something new. A gene now known as the Sex Region Y (SRY) was born. This gene was dominant to all other sex determining genes, causing the chromosome which housed that gene to be shunned by its ancestral partner. Over time, that lonely chromosome became the shrunken, masculine body we know today. That chromosome was the Y.

 Test tube with jellybaby-like sweet inside.

Test tube with jellybaby-like sweet inside.

In 1906 a cytologist named Nettie Stevens was studying the newly discovered X chromosome in insects when she switched to studying mealworms and saw something strange. A single, tiny chromosome that appeared to pair with the much larger X. Nettie was lucky she found the Y, as being a female scientist at the turn of the 20th century, she has often been forgotten in the history books. Her discovery means we will always remember her name.

All mammals are expected to have an X and a Y chromosome. In fact, that is part of the very definition of being a mammal (along with nursing our young and possessing hair) among other things. The SRY gene is specific to a sub class of mammals called Theria (those that give birth to live young) which have been around for about 176 million years. Humans fit under the umbrella of Theria along with most mammals. The platypus, an egg laying mammal, does not. So while it does have a Y chromosome, it does not have a SRY gene.

Instead their sex determination system is more like that of birds. The mole vole and the Japanese spiny country rat are mammals that once had a Y, but lost it along with their SRY gene. Presumably now some other gene has taken over the job.

 Mole's foot amulet.

Mole’s foot amulet.

Throughout most of history humans had no idea how babies were made, much less how they were divided into male, female and other. Aristophanes believed that humans came in three genders: male, female and androgynous. Pythagoras believed that men impregnated women with fully formed, tiny humans.

In the 17th century, sperm were seen through the lens of a microscope for the first time, only to be declared in the 18th century that they had nothing to do with fertility and were instead some kind of parasite. The fusion between an egg and a sperm was finally seen in 1876 by a zoologist, thus solving the riddle of how babies were made once and for all.

After the X and Y were discovered, humans finally knew how boys and girls were decided biologically, but the question was: which did what? Which chromsome made boys and which made girls? The realisation that humans came in not only XX and XY karyotype, but also XXX, XXY and XXXY was an important clue.

By studying intersex patients in the 1950s it was determined the Y was male determining, which naturally created the myth of the Y encompassing everything masculine. A period of time followed where XYY and XXYY men were intensely studied in order to prove they were super males – taller, more aggressive, and “more male” than regular XY men. Eventually it was realised they were taller…and that was about it.

 XYY syndrome karyotype. This male has an extra Y chromosome.

XYY syndrome karyotype. This male has an extra Y chromosome.

Over the 80s and 90s the Y was sequenced by an American lab and the SRY gene was finally discovered, along with other genes unique to the Y chromosome (all to do with male fertility). For a long time it was believed that as the Y had been deteriorating for 200 million years, that it was possible that humans may go the way of the Mole Vole and lose it forever. That fear is now known to be unfounded.

Due to the Y’s habit of cloning its own DNA, as well as the migration of genes from autosomes, the Y has remained stable for the last 50 million years. The mystery of XX males was also solved, as the SRY was found to be placed very near to an area that is still shared by both the X and the Y. The SRY isn’t all powerful however, as the sex of a human depends on more than just genes. A gene on it’s own can do nothing, as it relies on proteins and hormones to relay its message.

One disorder where a person can be XY but still be developmentally female is Androgen Insensitivity disorder. Effectively the genes are saying ‘male’, but the cells are saying ‘nope, can’t hear you’, and making a female.

Now, we know we have X and Y chromosomes, but why? Why would a species need two distinct sexes to reproduce?

One reason could be to ensure all future generations have recombined their DNA in order to stay one step ahead of parasites. Another reason could be to reduce genetic competition. Rather than have individuals battle it out over who will contribute their DNA, who will pass on their mitochrondria and who will carry the offspring, evolution has sorted it out for us. Both parties pass on their DNA, while females pass on their mitochrondria and carry the babies. So life can continue on in peace, until the next mutation.

Taryn is a Visitor Experience Assistant at Wellcome Collection.