Xlitningur

Xlitningur, in Icelandic usage referring to the X chromosome, is one of the two sex chromosomes in many species, including humans. It is inherited in a sex-linked manner and carries genes essential for development and physiology. In humans, females typically have two Xlitningur copies, while males have one Xlitningur and one Y litningur.

The Xlitningur is a large, linear chromosome with a short arm (p) and a long arm (q). It contains roughly 800 to 900 protein-coding genes and numerous regulatory elements. A portion of its sequence forms the pseudoautosomal regions that recombine with the Y chromosome during meiosis, while the rest forms a largely non-recombining body.

To balance gene expression between XX females and XY males, female mammals undergo X-inactivation, where one Xlitningur copy is silenced in each cell. This inactivation creates cellular mosaics and is mediated by the Barr body and noncoding RNAs such as XIST. Skewed inactivation can influence disease severity.

X-linked inheritance patterns are common for many disorders. Xlitningur genes include those associated with Duchenne muscular dystrophy, hemophilia A, and Rett syndrome. Because males have only one Xlitningur, deleterious variants often have direct phenotypic effects, while females may be carriers or variably affected.

The Xlitningur has been central to studies of genetics and development. The concept of X-inactivation, first proposed by Mary Lyon in 1961 and observed as Barr bodies, remains a key example of epigenetic regulation in mammals. Ongoing research on the Xlitningur informs understanding of gene regulation and X-linked diseases.