Turner's Syndrome

Turner's syndrome is a condition affecting females in which one X chromosome is missing. Affected individuals have 45 chromosomes: 44 autosomes and a single X chromosome, written 45,X (or XO). It is an example of monosomy.It usually arises from non-disjunction of the sex chromosomes during meiosis in one of the parents. For example, if the X and Y chromosomes fail to separate in anaphase I during sperm formation, some sperm receive both X and Y and some receive no sex chromosome at all. A sperm with no sex chromosome (22 autosomes only) fertilising a normal egg (22 autosomes + X) gives a zygote with 45 chromosomes and a single X.Monosomy of any autosome is lethal, but a single X chromosome can be survived. This is because in normal females one X chromosome in each cell is largely inactivated, so body cells already work mainly with one active X. However, a few genes on the second X are needed, so most 45,X embryos do not survive, and those that do show characteristic features.Common features include short stature, underdeveloped ovaries leading to infertility and no natural puberty, a broad chest, a webbed neck and an increased risk of heart and kidney problems. Intelligence is usually in the normal range. Treatment with growth hormone and oestrogen can help with height and development at puberty.Worked example: a karyotype shows 44 autosomes, one X chromosome and no Y. The person is female, has 45 chromosomes in total, and has Turner's syndrome.