Monosomy X (Turner syndrome)

Monosomy X (karyotype 45,XO) is a frequent
chromosomal aberration, representing about
5% of those in humans at conception. However,
of 40 zygotes with monosomy X, only one will
develop to birth. The phenotypic spectrum is
very wide. During the fetal stage, (1) lymphedema
of the head and neck result in cystic hygroma,
large multilocular thin-walled lymphatic
cysts. Congenital cardiovascular defects,
especially involving the aorta, and kidney malformations
are frequent. An important component
of the disease is the absence of ovaries,
which develop only as connective tissue (streak
gonads). Small stature is always a feature (average
adult height about 150 cm). In newborns,
webbing of the neck (pterygium colli) may be
present as a residual of the lymphedema (clinical
picture of Ullrich–Turner syndrome). On
the other hand, the manifestations may bemild
(2). Very frequently, pure monosomy is not
present, but rather chromosomal mosaicism
with normal cells (45,XO/46,XX) or a structurally
altered X chromosome (deletion of the
short arm, isochromosome of the long or short
arm, ring chromosome, or other).