What are the causes of Down’s syndrome?

Down’s syndrome is fairly uncommon. In the UK the condition affects around 1 in every 1000 live births and around 750 babies are born with condition in the UK each year.

Down’s syndrome is caused by an additional chromosome; the chromosomes are like building blocks in the body and contain the DNA. Chromosomes control everything from eye and hair colour to growth and development. A healthy baby will inherit 23 chromosomes from their mother and 23 chromosomes from their father, meaning they have a total of 46 chromosomes.

Down’s syndrome occurs as a result of having an extra chromosome 21; the type of abnormality determines the type of Down’s syndrome; there are three different types of Down’s syndrome:

  • Full trisomy 21 Down’s syndrome: this is the most common form of Down’s syndrome. Full trisomy 21 affects 94 percent of all cases of Down’s syndrome. This form occurs as a result of every cell having an additional chromosome 21.
  • Mosaicism Down’s syndrome: this form of Down’s syndrome only accounts for 2.5 percent of cases; it is the rarest form of Down’s syndrome. Mosaicism Down’s syndrome occurs when some cells have an extra chromosome 21 and some don’t.
  • Translocation Down’s syndrome: this form of Down’s syndrome is also rare; it occurs as a result of a portion of a chromosome 21 cell attaching to a different chromosome in the cell. Translocation Down’s syndrome can be passed on from parent to child; some people do not have symptoms of translocation Down’s syndrome but they may be carriers of the condition, which means this could be passed into their children. The chances of passing the condition on are 1 in 35 for male carriers and 1 in 8 for females.

The reason why some babies are born with chromosome disorders is unknown, but there are a number of factors which may increase the risk of Down’s syndrome; these include:

Age of the mother: the risk of having a baby with Down’s syndrome increase as the mother gets older, as follows:

  • 15-29 years old: 1 in 1,500
  • 30-34 years old: 1 in 800
  • 35-39 years old: 1 in 385
  • 40-44 years old: 1 in 106
  • 45 years old: 1 in 30

Environmental influences: research suggests that there are certain areas where there is a higher than average number of cases of Down’s syndrome; this is known as clustering. Possible environmental factors include:

  • Exposure to radioactive waste or radiation
  • Exposure to pesticides
  • Proximity to waste disposal sites or landfill sites
  • Exposure to smoke (especially if the mother smokes during pregnancy)
  • Exposure to infective microorganisms and bodies (including viruses)

It is unknown why exposure to certain chemicals and environments contributes to Down’s syndrome and further research needs to be carried out in this area.

A more comprehensive guide to Down’s syndrome is given below, and concentrates on the following areas:

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