Mosaicism Health Dictionary

This occurs in one in 2,500 live female births. It is caused by either the absence of or an abnormality in one of the two X CHROMOSOMES. Classical Turner’s syndrome is a complete deletion of one X so that the karyotype is 45XO. Half of the people with Turner’s syndrome have MOSAICISM with a mixture of Turner cells and normal cells, or other abnormalities of the X chromosome such as partial deletions or a ring X. They are females, both in appearance and sexually; clinical features are variable and include short stature, with ?nal height between 1·295 m and 1·575 m, and ovarian failure. Other clinical features may include a short neck, webbing of the neck, increased carrying angle at the elbow (cubitus valgus), widely spaced nipples, cardiovascular abnormalities (of which the commonest is coarctation of the aorta [about 10 per cent]), morphological abnormalities of the kidneys (including horseshoe kidney and abnormalities of the pelviureteric tracts), recurrent otitis media (see under EAR, DISEASES OF), squints, increased incidence of pigmented naevi (see NAEVUS), hypothyroidism (see under THYROID, DISEASES OF) and DIABETES MELLITUS. Intelligence is across the normal range, although there are speci?c learning defects which are related to hand-eye coordination and spatial awareness.

Patients with Turner’s syndrome may require therapeutic help throughout their life. In early childhood this may revolve around surgical correction of cardiovascular disease and treatment to improve growth. Usually, PUBERTY will need to be induced with oestrogen therapy (see OESTROGENS). In adult life, problems of oestrogen therapy, prevention of osteoporosis (see under BONE, DISORDERS OF), assessment and treatment of HYPERTENSION and assisted fertility predominate. For the address of the UK Turner Syndrome Society, see Appendix 2.... turner’s syndrome

A disorder caused by a chromosomal abnormality that only affects females. The abnormality may arise in 1 of 3 ways: affected females may have only 1 X chromosome instead of 2; they may have 1 normal and 1 defective X chromosome; or they may have a mixture of cells (see mosaicism), in which some of the cells are missing an X chromosome, some have extra chromosomes, and others have the normal complement of chromosomes. Turner’s syndrome causes short stature; webbing of the skin of the neck; absence or retarded development of sexual characteristics; amenorrhea, coarctation of the aorta, and abnormalities of the eyes and bones.

Treatment with growth hormone from infancy helps girls with Turner’s syndrome to achieve near normal height. Coarctation of the aorta is treated surgically. Treatment with oestrogen drugs induces menstruation, but it does not make affected girls fertile.... turner’s syndrome

(Down syndrome) a condition resulting from a chromosomal abnormality most commonly due to the presence of three copies of chromosome 21 (trisomy 21), which is most likely to occur with advanced maternal age. Down’s syndrome can also occur as a result of chromosomal rearrangement (*translocation) and as part of a *mosaicism, which are not related to maternal age. Affected individuals share certain clinical features, including a characteristic flat facial appearance with slanting eyes (as in Mongolian races, which gave the former name, mongolism, to the condition), broad hands with short fingers and a single crease across the palm, malformed ears, eyes with a speckled iris (Brushfield spots), short stature, and *hypotonia. Many individuals also have learning disabilities, although the range of ability is wide and some individuals are of normal intelligence. The incidence of congenital heart defects is 40–50%, and other structural malformations (e.g. *duodenal atresia) and associated abnormalities (e.g. deafness, squints, obesity, type 2 diabetes) may also be present.

*Prenatal diagnosis of Down’s syndrome can be obtained by *amniocentesis and *chorionic villus sampling, which are invasive procedures, and about 33% of structural abnormalities in fetuses with Down’s can be detected prenatally by *ultrasonography. *Prenatal screening tests (e.g. the *triple test) and soft *ultrasound markers (e.g. *nuchal translucency scanning) can estimate the risk of Down’s syndrome being present. [J. L. H. Down (1828–96), British physician]... down’s syndrome