What causes Trisomy 21?

The Chromosomes.

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 chromosomes ('X' and 'Y'). Human cells divide in two ways. The first is ordinary cell division ("mitosis"), 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 ("meiosis") 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. This is what a normal set of chromosomes looks like (fig.1). The test in which blood or skin samples are checked for the number and type of chromosomes is called a karyotype.

Fig. 1: normal set of chromosomes

DOWN SYNDROME
Down syndrome, also called Trisomy 21 is the most common cause of mental retardation and malformation in a newborn. It occurs because of the presence of an extra 21st chromosome. Each person inherits 23 chromosomes from their mother and 23 chromosomes from their father. Sometimes an accident occurs and one of the parents gives an extra chromosome. The extra chromosome could have come from either parent - so, neither the mother nor father should carry any 'blame' for their child's condition. When the extra chromosome happens to be chromosome number 21, Down syndrome occurs (fig.2).

Fig. 2: Chromosomal Alterations in Down syndrome