Down's syndrome is a genetic condition caused by an extra copy of chromosome 21. It is also called trisomy 21, and affected people usually have 47 chromosomes in their body cells instead of 46.In most cases (around 95%) it results from non-disjunction of chromosome 21 during meiosis, usually during the formation of the egg. A gamete with two copies of chromosome 21 (24 chromosomes in total) fuses with a normal gamete (23), giving a zygote with three copies. Because the zygote then divides by mitosis, every cell of the body carries the extra chromosome.A small number of cases are caused by a translocation, in which part of chromosome 21 is attached to another chromosome, so the person has the extra genetic material even though they have 46 chromosomes.The extra copies of the genes on chromosome 21 change development. Common features include learning difficulties of varying degree, characteristic facial features, reduced muscle tone and an increased risk of heart defects. The severity differs greatly between individuals.The condition can be identified from a karyotype: an image of a person's chromosomes arranged in homologous pairs in order of size. A karyotype of someone with trisomy 21 shows three chromosomes at position 21.
The chance of having a baby with Down's syndrome increases with the mother's age: the risk is roughly 1 in 1500 at age 20 but about 1 in 100 at age 40. Screening and diagnostic tests such as amniocentesis are offered during pregnancy, which raises ethical questions about the choices parents may face.
The chance of having a baby with Down's syndrome increases with the mother's age: the risk is roughly 1 in 1500 at age 20 but about 1 in 100 at age 40. Screening and diagnostic tests such as amniocentesis are offered during pregnancy, which raises ethical questions about the choices parents may face.