Down syndrome also known as “trisomy 21” is a genetic condition in which a child is born with an extra chromosone which causes certain features and delays in development. In sexual reproduction (meiosis) the new cell will have half of the mothers chromosones 23 and half of the fathers chromosones 23 to make a total of 46 (23 pairs) in the new cell (zygote). In a child with Down Syndrome they will have an extra chromosone 21 making a total of 47 chromosones. There is no specific reason why this extra chromosone is present but the older the mother is when giving birth the more likely that her baby will have Down syndrome.
Down Sydrome is named after John Langdon Down, the first person to describe the condition in 1866. He noted that some of his patients had very similar features and had the same intellectual characteristics despite having no connection. Initially he named the condition “mongoloid idiocy” as he believed the patients had features like a mongolian, although this name was not used for long. People disputed the fact as patients had very different features like hair or eye colour and some had blonde hair and pale skin unlike a mongolian.
"The number of idiots who have arranged themselves around the Mongolian type is so great and they present such a close resemblance to one another in mental power that I shall describe an idiot member of this racial division selected from the large number who have fallen under my observation. The face is flat and broad and destitute of prominence. The cheeks are roundish and extended laterally. The eyes are obliquely placed and the internal canthi more than normally distanced from one another. The palpebral fissure is very narrow. The tongue is long, thick and much roughened. The nose is small. The skin has a slight dirty yellowish texture and is deficient in elasticity, giving t he impression of being too large for the body."
(John Langdon Down, 1866)
Many tried to discover the cause of this condition but they did not have the facilities to study chromosones and as the 21st chromosone is one of the smallest present it was not possible to see any irregularities. It wasn't until 1959 when genetic research had come a long way, a Professor named Jerome Lejeune had discovered that Down Syndrome in fact is caused my a third chromosone 21 being present.
Types of Down Syndrome
There are three types of Down Syndrome, it is the most common cause of intellectual disability and is the most common genetic birth defect.
Trisomy 21 is the most common type of Down Syndrome as it accounts for 95% of cases. It is when the extra chromosone it present from fertilisation, either the egg or the sperm has two of the 21 chromosone and makes a total of three when combined with the other. This extra chromosone is present in every cell of the body and results in trisomy 21.
Translocated Down Syndrome accounts for about...