Webster's New World Medical Dictionary

chromosome, acentric
A fragment of a chro-
mosome that lacks a centromere, so that the chro-
mosome 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 chromo-
some). 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 chro-
mosome 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 chromo-
some 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 con-
densed, 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 chro-
mosomes.)
chromosome, X
The sex chromosome found
twice in normal females and once, along with a Y
chromosome, in normal males. The complete chro-
mosome complement (consisting of 46 chromo-
somes, including the 2 sex chromosomes) is thus
conventionally written as 46,XX for chromosomally
normal females and 46,XY for chromosomally nor-
mal males. The X chromosome not only determines
gender but also carries the genetic code for many
essential functions in both males and females.
chromosome, Y
The sex chromosome found in
normal males, together with an X chromosome.
Once thought to be a genetic wasteland, the Y chro-
mosome is now known to contain at least 20 genes.
Some of these genes are unique to the Y chromo-
some, including the male-determining gene and
male fitness genes that are active only in the testis
and that are thought to be responsible for the for-
mation of sperm. Other genes on the Y chromosome
have counterparts on the X chromosome, are active
in many body tissues, and play crucial “housekeep-
ing” roles within cells.