yawn Involuntary opening of the mouth, accompanied by slowly breathing inward and then outward. Repeated yawning can be a sign of drowsiness, boredom, or depression. The yawn helps to open tiny air sacs (alveoli) in the lungs that can collapse during shallow breathing.
yard A measurement of length that is equal to 0.9 meters, 3 feet, or 36 inches.
Y map The array of genes on the Y chromosome.
Y chromosome sex-determining region The region on the Y chromosome that decides the sex of the individual. Abbreviated SRY. SRY is necessary for male determination. It encodes the testis-determining factor. Mutations in SRY are responsible for XY females with gonadal dysgenesis who appear to be normal females at birth but at puberty do not develop…
Y chromosome The sex chromosome that is found, together with an X chromosome, in most normal males. The Y chromosome contains the unique male-determining gene and the male fitness genes that are active only in the testis and are responsible for the formation of sperm. Other genes on the Y chromosome have counterparts on the…
Y 1 In chemistry, the symbol for the element yttrium, an extremely rare metal that has been used in certain nuclear medicine scans. 2 In genetics, the Y chromosome.
XYY syndrome A chromosomal disorder that affects about 1 in 1,000 males 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 disturbances. 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. See also X chromosome; Y chromosome.
XXX syndrome A chromosome condition that is present in 1 in 1,000 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 mental retardation. Also known as triple X and triplo-X.
XX The sex chromosome complement of a normal human female. See also X chromosome.
X-ray therapy The use of X-ray radiation to treat cancer. X-rays may be used inside or outside the body, depending on the type of tumor involved. See also radiation therapy.