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X (in genetics)
Although the letter X can be used as a symbol in various ways (such as with X-rays, the X-axis of a graph, etc.), "the X " in genetics and medicine today usually refers to the X chromosome.
The X is the sex chromosome that is found twice in chromosomally normal females and singly along with a Y chromosome in chromosomally normal males. The complete chromosome complement consisting of 46 chromosomes including the two sex chromosomes is thus conventionally written as 46,XX for chromosomally normal females and 46,XY for chromosomally normal males.
X chromosome
The sex chromosome found twice in normal females and singly, along with a Y chromosome, in normal males.
The complete chromosome complement (consisting of 46 chromosomes including the two sex chromosomes) is thus conventionally written as 46,XX for chromosomally normal females and 46,XY for chromosomally normal males.
Xanthelasma
Tiny (1-2 mm) yellowish plaques that are slightly raised on the skin surface of the upper or lower eyelids. Xanthelasma is caused by tiny deposits of fat in the skin and is often associated with abnormal blood fat levels (hyperlipidemia). Xanthelasma is a kind of minute harmless growth of tissue. Xanthelasma typically appears in or near the eyelids.
Under the microscope, Xanthelasma can be seen to be composed of lipid-laden foam cells. These cells, termed histiocytes, contain lipid material in their cytoplasm (the nonnuclear zone of the cell).
The word “xanthelasma” is made up of “xanth-“ from the Greek roots “xanthos” (yellow) and “elasma” (plate) = a yellow plate, so called because these are yellow plaques.
Xanthinuria
An inherited metabolic disorder in which there is deficiency of an enzyme needed to process xanthine, a substance found in caffeine, theobromine, theophylline, and related substances. Unchecked, xanthinuria can lead to kidney stone formation and urinary tract disease (due to xanthine stones) and to muscle disease (due to deposits of xanthine in the muscle). Treatment is by avoiding foods and drinks containing xanthine derivatives, such as coffee, tea, and colas.
The enzyme that is lacking in xanthinuria is xanthine dehydrogenase. The disorder is an autosomal recessive trait. The gene responsible for xanthinuria is on an autosome, a nonsex chromosome (chromosome 2p23-p22). Both parents of a child with xanthinuria are normal but carry one xanthinuria gene (the xanthinuria gene is recessive to the corresponding normal gene). Their affected child received both of their xanthinuria genes.
Xanthoma
Yellowish firm nodules in the skin frequently indicating underlying disease, such as diabetes, disorder of fats (lipid disorder or hyperlipidemia), or other conditions. A xanthoma is a kind of harmless growth of tissue.
Under the microscope, a xanthoma can be seen to be composed of lipid-laden foam cells. These cells, termed histiocytes, contain lipid material in their cytoplasm (the nonnuclear zone of the cell).
The word “xanthoma” is made up of “xanth-“ from the Greek roots “xanthos” (yellow) and “oma” (swelling) = a yellow swelling. A xanthoma is a circumscribed yellow swelling, a yellowish nodule.
Xanthoma tendinosum
Xanthoma that clusters around tendons, and is associated with lipid disorders.
Xanthoma tuberosum
Xanthoma that clusters near joints. It is associated with lipid disorders, cirrhosis of the liver, and thyroid disorders.
Xanthoma, diabetic
Xanthoma associated with poorly controlled diabetes mellitus. Treating the diabetes can cause the xanthomas to disappear.
Xanthoma, disseminatum
A type of xanthoma characterized by orange-to-brown nodules on the skin or mucus membranes.
Xanthoma, eruptive
Xanthoma linked to lipid disorders, and accompanied by a pink-to-red raised rash.
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