Down Syndrome: A Genetic Disease

Down syndrome is a genetic disorder, often referred to as trisomy 21 because the cells for chromosome 21 are trisomic, meaning an individual with Down syndrome would have 47 chromosomes in all of their body cells. Down syndrome is a genetic disorder, there are 3 different types of chromosomal changes which can lead to Down syndrome: complete trisomy 21, whereby the nondisjunction occurs during the formation of the egg or sperm cells meaning all body cells will have 47 chromosomes. This is the most common causality of Down syndrome, accounting for 95%. The second being mosaic trisomy 21, this occurs in early development after fertilisation of the egg cell. In this case most of the body cells have 47 chromosomes but not all. This is rarer and only accounts for 1%. The third being translocation trisomy 21, where only a small section of an extra chromosome 21 is present in the cells that gets stuck to and copied to other cells during meiosis (National Institute of Child Health and Human Development 2015). The frequency of Down syndrome is seen to correlate with the age of the mother. Children to born to mothers under 30, the risk is 0.04%, however children born to mothers aged 40 the risk increases to 0.92% (Campbell et al 2015).
Within the UK, all women are offered prenatal screening tests for Down syndrome for within the first trimester. This usually consists of a blood test and an ultrasound that’s used to detect for fluid in the back of the foetus’ neck, as this can be an indicator for Down syndrome. For those who are too late to test in the first trimester, a blood test will be conducted in the second trimester. This is used to detect certain protein levels that are indicative of Down syndrome (National Institute of Child Health and Human Development 2015). Women who show high risk positive results will then have a prenatal diagnostic test.