Down’s Syndrome

Down’s syndrome is a learning disability caused by an extra copy of chromosome 21, making total chromosome count 47 instead of the ‘regular’ human count of 46. Physical characteristics are very recognisable of a broader, flatter face, smaller ears and nose and more space between the eyes whilst an associated medical issue is an increased risk of heart problems.

There are three ways in which Down’s syndrome can occur; through non-disjunction of chromosome 21 (94% of cases), by translocation (4% of cases) or mosaicism (2% of cases). Non-disjunction of chromosome 21 is when chromosomes do not separate properly during the anaphase stage of the cell cycle, translocation is when a part of a chromosome breaks off and attaches to another chromosome, and mosaicism is when people’s cells vary in number of specific chromosomes.

Translocation occurs specifically with the breaking off of part of chromosome 21 and it attaching to a different chromosome in the parent. After receiving two copies of chromosome 21 (one from mother and one from father) and an extra chromosome 21 attached to another chromosome by translocation, total count of chromosome 21 equals three in the offspring, causing Down’s syndrome. People with translocation do not always show Down’s syndrome characteristics but do pass on translocation, giving their child the extra chromosome 21. Translocation can also occur during meiosis, when the parents do not have translocation themselves.

Mosaicism occurs either by the fertilised ovum having three copies of chromosome 21 and losing one during mitosis, or when the ovum originally has cells with two copies of chromosome 21 and during mitosis gains an extra copy of this chromosome by mistake. If the ratio of cells containing three chromosome 21s to cells containing two chromosome 21s is large then Down’s syndrome is likely, and if the ratio is small, Down’s syndrome is less affective, shown by the person having fewer of the characteristics linked to Down’s syndrome.

The biggest risk factor of conceiving a child with Down’s syndrome is a women’s age with risk increasing with age. Figures by the NHS currently stage that maternal age of 20 gives you a risk factor of 1 in 1500, age 30 of 1 in 800, age 35 of 1 in 270 and age 40 of 1 in 100. Risk is also increased if the person has already conceived a baby with Down’s syndrome and further increased (up to 1 in 10) if one of the parents has translocated genes.

In today’s NHS, foetuses are tested for Down’s syndrome by amniocentesis and ultrasounds. Amniocentesis is the removal of some amniotic fluid which is taken to a laboratory to be stained and looked at under a microscope to look for extra copies of chromosome 21. However, some women choose not to have this test as it increases chance of miscarriage so other less invasive tests are taken first such as ultrasounds and blood tests, before referral for amniocentesis.

Ultrasound is high-frequency sound waves to create a moving image in order to see thickness of fluid between skin and foetal neck (taken between weeks 14 and 22), with a thickness greater than 3mm likely to imply chromosome abnormality. Taking a blood sample from a pregnant mother is another less invasive and therefore less risky test, from which alpha-fetoprotein concentration can be measured. If the mother’s concentration is 25% less than normal, it is significantly more likely that their child will have Down’s syndrome so will be referred for an amniocentesis for a more reliable test.

A test that may be available in the future is the analysis of a foetus’s DNA from their mother’s placenta by week 10 of pregnancy. It is currently 99% accurate at detection which is not considered high enough to be widely available on the NHS.

Test results give people a choice; to terminate or continue with their pregnancy – an extremely difficult decision for many.


Emily Buchanan

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