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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 […]

X chromosome

The sex chromosome that is found twice in normal females and singly, along with a Y chromosome, in normal males. The complete chromosome complement consists of 46 chromosomes, including the 2 sex chromosomes, and is thus conventionally written as 46,XX for chromosomally normal females and 46,XY for chromosomally normal males.

XYY syndrome

A chromosomal disorder that affects males only, caused by the presence of an extra Y chromosome. Symptoms may include increased height, speech delays, learning disabilities, mild to moderate mental retardation, and behavioral disturbance. Also known as polysomy Y syndrome.

Xylitol

A sweetener that is found in plants and used as a substitute for sugar. Xylitol is called a nutritive sweetener because it provides calories, just like sugar. However, it is less likely than sugar to contribute to dental caries.

XY

The most frequent sex chromosome complement in human males.

XXXXX syndrome

A chromosome disorder in females, caused by the presence of five X chromosomes rather than the usual two. It is characterized by decreased intelligence (IQ 20 to 75) and multiple malformations including upward-slanting palpebral fissures (eyeslits), heart defect, and small hands with incurving fifth fingers. It is the most severe extra-X syndrome in females. Also […]

XXXX syndrome

A chromosome disorder in females caused by the presence of four X chromosomes rather than the usual two. The disorder is characterized by decreased intelligence (IQ 30 to 80; average 55), speech and behavioral problems, tall stature and reduced fertility. Also known as tetra-X syndrome.

XXX syndrome

A chromosome condition present in 1 in 1000 females, due to the presence of three X chromosomes rather than the usual two. The condition is associated with increased height but no malformations. Intelligence ranges from above normal to mild retardation. The average IQ is 85 to 90. Learning problems may be helped by early special […]

XX

The sex chromosome complement of a normal human female. See also X chromosome.

Xuezhikang

One of the three major preparations of red yeast rice, a tradition Chinese medicine now used to lower cholesterol.