Chromosome X (human)

Human X chromosome
Human X chromosome (after G-banding).
X chromosome in human male karyogram.
Features
Length (bp)	156,040,895 bp (GRCh38)
No. of genes	825 (HGNC) 841 (Ensembl) 874 (NCBI)
Type	Allosome
Centromere position	Submetacentric (61.0 Mbp)
External map viewers
Ensembl	Chromosome X
Entrez	Chromosome X
NCBI	Chromosome X
UCSC	Chromosome X
Full DNA sequences
RefSeq	NC_000023 (FASTA)
GenBank	CM000685 (FASTA)

The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system. The X chromosome was named for its unique properties by early researchers, which resulted in the naming of its counterpart Y chromosome, for the next letter in the alphabet, after it was discovered later.

It was first noted that the X chromosome was special in 1890 by Hermann Henking in Leipzig. Henking was studying the testicles of Pyrrhocoris and noticed that one chromosome did not take part in meiosis. Chromosomes are so named because of their ability to take up staining. Although the X chromosome could be stained just as well as the others, Henking was unsure whether it was a different class of object and consequently named it X element, which later became X chromosome after it was established that it was indeed a chromosome.

The idea that the X chromosome was named after its similarity to the letter "X" is mistaken. All chromosomes normally appear as an amorphous blob under the microscope and only take on a well defined shape during mitosis. This shape is vaguely X-shaped for all chromosomes. It is entirely coincidental that the Y chromosome, during mitosis, has two very short branches which can look merged under the microscope and appear as the descender of a Y-shape.

It was first suggested that the X chromosome was involved in sex determination by Clarence Erwin McClung in 1901 after comparing his work on locusts with Henking's and others. McClung noted that only half the sperm received an X chromosome. He called this chromosome an accessory chromosome and insisted, correctly, that it was a proper chromosome, and theorized, incorrectly, that it was the male determining chromosome.

Luke Hutchison noticed that a number of possible ancestors on the X chromosome inheritance line at a given ancestral generation follows the Fibonacci sequence. A male individual has an X chromosome, which he received from his mother, and a Y chromosome, which he received from his father. The male counts as the "origin" of his own X chromosome (${\displaystyle F_{1}=1}$), and at his parents' generation, his X chromosome came from a single parent (${\displaystyle F_{2}=1}$). The male's mother received one X chromosome from her mother (the son's maternal grandmother), and one her father (the son's maternal grandfather), so two grandparents contributed to the male descendant's X chromosome (${\displaystyle F_{3}=2}$). The maternal grandfather received his X chromosome from his mother, and the maternal grandmother received X chromosomes from both of her parents, so three great-grandparents contributed to the male descendant's X chromosome (${\displaystyle F_{4}=3}$). Five great-great-grandparents contributed to the male descendant's X chromosome (${\displaystyle F_{5}=5}$), etc. (Note that this assumes that all ancestors of a given descendant are independent, but if any genealogy is traced far enough back in time, ancestors begin to appear on multiple lines of the genealogy, until eventually, a population founder appears on all lines of the genealogy.)