X chromosome
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The X chromosome is one of the two sex chromosomes found in many organisms, including mammals. It is present in both males and females and is part of the XY and XO sex-determination systems. Early researchers named the X chromosome for its unique properties, which also led to naming its counterpart, the Y chromosome, after the following letter in the alphabet, following its later discovery.
Discovery
In 1890, Hermann Henking in Leipzig was the first to identify the X chromosome as unique. While studying the testicles of Pyrrhocoris, Henking observed that one chromosome did not participate in meiosis. Chromosomes are named for their ability to absorb stains (chroma in Greek means color). Although the X chromosome could be stained just as well as the others, Henking was uncertain whether it belonged to a different class and thus called it the X element, which later became known as the X chromosome once it was confirmed to be a chromosome.
The belief that the X chromosome was named due to its resemblance to the letter "X" is incorrect. Chromosomes typically appear as amorphous blobs under the microscope, taking on a distinct shape only during mitosis. This shape is vaguely X-shaped for all chromosomes. It is completely coincidental that the Y chromosome, during mitosis, has two very short branches that can appear merged under the microscope and resemble the descender of a Y-shape.
In 1901, Clarence Erwin McClung first suggested the involvement of the X chromosome in sex determination. After comparing his work on locusts with that of Henking and others, McClung noted that only half of the sperm carried an X chromosome. He referred to this chromosome as an accessory chromosome and correctly insisted that it was a proper chromosome while incorrectly theorizing that it served as the male-determining chromosome.
- More information is available at [ Wikipedia:X_chromosome ]
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