Scientists have for the first time ‘switched off ’ the extra chromosome responsible for Down’s syndrome that affects between 23,000 and 29,000 children born in India every year – the highest in the world.

Scientists at UMass Medical School have successfully shown that a naturally occurring X chromosome "off switch" can be rerouted to neutralize the extra chromosome responsible for trisomy 21 or Down syndrome, a genetic disorder characterized by cognitive impairment.

This is the first such evidence that the cells causing the genetic defect can be suppressed. This paves the way for researchers to study the cell pathologies and identify genome–wide pathways implicated in the disorder.

Chromosomes are thread–like structures composed of DNA and other proteins. They are present in every cell of the body and carry the genetic information needed for that cell to develop. Human cells normally have 46 chromosomes that can be arranged in 23 pairs.

Of these 23, 22 are alike in males and females; these are called the "autosomes". The 23rd pair is the sex chromosome. Human cells divide in two ways.

The first is ordinary cell division by which the body grows. In this method, one cell becomes two cells that have the exact same number and type of chromosomes as the parent cell.

The second method of cell division occurs in the ovaries and testicles and consists of one cell splitting into two, with the resulting cells having half the number of chromosomes of the parent cell. So, normal eggs and sperm cells only have 23 chromosomes instead of 46.

People with Down syndrome are born with three (rather than two) copies of chromosome 21, and this "trisomy 21" causes cognitive disability, early–onset Alzheimer’s disease; and a greater risk of childhood leukemia, heart defects and immune and endocrine system dysfunction.