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 starts this series off by looking at the X chromosome.

When cytologist Hermann Henking looked down his microscope in 1891, he was surprised to see that approximately half of his fire wasps had a spare chromosome floating around. Confused and intrigued, he named his lonely chromosome the “X element”.

By this point scientists already knew about cells; they knew that heritable material must come from these cells and they were aware that humans came from a sperm and an egg. What they didn’t know was how sex was determined while in the womb. Many theories have abounded throughout history.

Sperm of a mouse as seen under a microscope.

Sperm of a mouse as seen under a microscope.

Aristotle was sure that a hot and sweaty father during conception would bring forth a son. Galen believed a man’s right testicle would result in a son. Hippocrates thought it was the domination of a man’s semen over that of a woman’s that would bring him a son. In more modern times, Geddes and Thomson believed that good times would bring on a daughter whereas adversity would give you a son; a metabolic belief of sex determinism which persisted until the 1920s.

An unborn baby's genetic inheritance. (credit: Wellcome Library, London)

An unborn baby’s genetic inheritance. (credit: Wellcome Library, London)

After its discovery, the idea that the X chromosome was somehow responsible for sex determination grew quickly. However, it still took until the middle of the next century to be universally accepted. The X was briefly thought to be male determining until 1906 when this idea was put to rest by the same scientist who discovered the Y chromosome.

Part of the confusion came from the fact that the XX female and XY male combination aren’t standard amongst life on earth:

  • Henking’s fire wasps have XX females and XO males
  • Birds and butterflies have ZW females and ZZ males
  • A Platypus has 5X5X females and 5X5Y males
  • Roundworms have XX hermaphrodites and XO males

Of course, some species don’t rely on sex chromosomes for gender determination at all. Gender in humans isn’t purely determined by chromosomes alone either.

Illustration of a duck-billed platypus.

Illustration of a duck-billed platypus.

Over 200 million years ago the X and Y chromosomes were much like any other chromosome. As the first mammals were beginning to evolve, so were the X and Y chromosomes, drifting away from each other until they had little in common and barely spoke. The Y degenerated into the friendless chromosome we know today, being a third of the size of the X, whereas the X has remained relatively unchanged for the last 150 million years.

Human’s love to categorise things and we also like polarising opinions. This is why there are winners and losers, why you are a cat or a dog person and why your toilet roll must either face towards the back or the front. A similar thought process is at work with the X and Y chromosomes.

Ishihara charts for testing colour blindness.

Ishihara charts for testing colour blindness.

The X is often referred to as the ‘female’ chromosome; it is more accurately described as a ‘human’ chromosome. All of us, regardless of gender, have an active X chromosome in all of our cells. If you are female you were likely born with two Xs, so your cells will express one or the other. If you’re male, you likely only have one, so all your cells are the same.

This is the reason behind the higher incidence of autoimmune disease in women, the greater death rates of men and the male specific X chromosome disorders, such as colour blindness and Duchenne muscular dystrophy. That’s not to say that women are immune from these disorders. Usually when women are affected by them, it’s because they have inherited two mutant Xs, or one X chromosome has dominated the other during foetal development.

Meiosis. (credit: MRC NIMR, Wellcome Images)

Meiosis. (credit: MRC NIMR, Wellcome Images)

In 1992, the Human Genome Project (HGP) began sequencing each of our chromosomes. The X chromosome was sequenced by 250 scientists in 3 countries including the UK, with the nearly complete sequence being released in March 2005. The X was found to contain 155 million base pairs and 1100 genes, 50 of which are shared with the Y.

Along the way, some surprising things were discovered about the X. No one expected the X chromosome to contain genes for testis development, nor that the human X and dog X would be virtually identical. The HGP opened up a whole new door for exploring the path of human evolution.

The human genome library on display at Wellcome Collection.

The human genome library on display at Wellcome Collection.

In 2007, a complete printed library of the human genome, including the sex chromosomes, was given to Wellcome Collection, containing 118 books with over 3 billion characters. This genome library is still on display in our galleries today.

A look inside one of the books in the human genome library.

A look inside one of the books in the human genome library.

Taryn is a Visitor Experience Assistant at Wellcome Collection.

One thought on “The X chromosome

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