circadian clock An internal time-keeping system in all organisms. Changes in the external environment, particularly in the light–dark cycle, train this biologic clock. When environmental conditions are constant, rhythms driven by the circadian clock follow a nearly perfect 24-hour pattern. The human circadian clock regulates many daily activities, such as sleep and waking. When a…
circinate balanitis See balanitis, circinate.
chromosome, acentric A fragment of a chromosome that lacks a centromere, so that the chromosome is lost when the cell divides.
chromosome, acrocentric A chromosome that has its centromere located near one end of the chromosome. Humans have five pairs of acrocentric chromosomes. Down syndrome is due to an extra acrocentric chromosome (chromosome 21).
chromosome, autosomal Any chromosome other than a sex chromosome (X or Y chromosome). Also known as an autosome.
chromosome, dicentric A chromosome that is abnormal in that it has two centromeres rather than one. Because the centromere is essential for chromosome division, a dicentric chromosome is pulled in opposite directions when the cell divides. This causes the chromosome to form a bridge and then break and be unstable.
chromosome, marker An abnormal chromosome that is distinctive in appearance but not fully identified. A marker chromosome is not necessarily a marker for a specific disease or abnormality, but it can be distinguished under the microscope from all the normal human chromosomes. For example, the fragile X (FRAXA) chromosome was once called the marker X.
chromosome, metaphase A chromosome at the stage in the cell cycle at which it is most condensed, easiest to see by itself, and therefore easiest to study. Metaphase chromosomes are often chosen for karyotyping and chromosome analysis.
chromosome, prophase A chromosome at a stage before metaphase in the cell cycle, when the chromosomes are long and often tangled like a ball of twine. Prophase chromosomes may be selected for analysis via resolution chromosome banding when it is important to detect minute details.
chromosome, sex The X or Y chromosome in humans. (Some other species have other sex chromosomes.)
chromosome, X The sex chromosome found twice in normal females and once, along with a Y chromosome, in normal males. The complete chromosome complement (consisting of 46 chromosomes, including the 2 sex chromosomes) is thus conventionally written as 46,XX for chromosomally normal females and 46,XY for chromosomally normal males. The X chromosome not only determines…
chromosome, Y The sex chromosome found in normal males, together with an X chromosome. Once thought to be a genetic wasteland, the Y chromosome is now known to contain at least 20 genes. Some of these genes are unique to the Y chromosome, including the male-determining gene and male fitness genes that are active only…