Down’s syndrome is a chromosomal abnormality that can be identified in approximately 1 in 1000 live births. The condition has shown to be associated with the increasing maternal age and the likelihood of giving birth to a baby with Down’s syndrome rises steeply after the age of 35 years.
The underlying problem in Down’s syndrome would be the presence of an extra copy of the chromosome 21. Therefore, the condition is also known as Trisomy 21.
In normal human beings there are 46 chromosomes and according to its arrangement, a male will be having a 46,XY karyotype and a female will be having a 46,XX karyotype. In Down’s patients, the karyotype will read as having 47 chromosomes or both 46 and 47 chromosomes together.
The researchers have found several mechanisms that give rise to these karyotypes and following will be a short description of these mechanisms.
1. Non disjunction:
When the gametes (eggs or sperms) are made in the human body, each gamete will receive half of the chromosomes from the parent. Therefore, they will be having 23 chromosomes each. In non disjunction, these chromosomes would not divide equally and one gamete will receive 24 chromosomes instead of 23. The extra chromosome is more likely to be of the chromosome 21. When the gamete fertilizes to produce an embryo, the resulting cells will receive 23 chromosomes from one gamete and 24 from the other. This will result in a Trisomy state giving rise to the karyoptype (47,XX,+21) or (47,XY,+21).
2. Mosaic pattern
Mosaic pattern or mosaicism is thought to occur at the early embryogenesis. This is the time when the cell division takes place and the embryo grows in size and by the number of cells. In mosaic pattern, some of the cells will have the normal number of chromosomes where as the others will show an extra copy of the chromosome 21.
The more complicated mechanism of all, the translocation takes place between the long arm of the chromosome 21 and perhaps with the chromosome 14. Thus, a parent with such translocation would not show any signs but when the gametes are produced there is a possibility to produce one extra copy of the chromosome 21. Thus the resulting gamete will contribute to form a trisomy 21.
Apart from the above mentioned categories, there may be other ways of generating an extra copy of chromosome 21 and these are still under investigations.
Therefore, it is understandable that the Down’s syndrome is purely a chromosomal abnormality which seems not to have any environmental or behavioural relationship in its occurrence.