Down syndrome is caused by a person having three copies of chromosome 21 instead of two copies. This is why Down syndrome is also referred to as Trisomy 21. It is named after John Langdon Down, the British physician who described the syndrome in 1866.
What are Chromosomes?
Chromosomes are structures that hold crucial nuclear material called genes. There are thousands of genes on each chromosome that tell our bodies how to develop and function. Items as basic as whether the eyes are brown, blue or green, whether you have brown, black or blond hair, curly or straight. The genes determine the color of your skin, your athletic ability and temperament and much more.
Where Are the Chromosomes?
The body has many different kinds of cells, such as liver cells, blood cells, heart cells, skin cells, reproductive cells. In the center of each cell is where the chromosomes are found. A typical human has 46 (23 pairs) of chromosomes in each cell, and an estimated 25,000 genes. One set of 23 chromosomes is inherited from the mother (egg-cell), and the other set is inherited from the father (sperm-cell).
Cells split and divide over and over again. This process is called Mitosis.
Mitosis is cell division that starts with one cell and ends with two identical cells. The starting cell is called the parent cell and the ending cells are called the daughter cells. In mitosis, the daughter cells are not only identical to each other, but they are also identical to the parent cell. We use mitosis all over our bodies for repair after injury, replacement of cells as some get worn and die, and growth. Mitosis is also used for embryonic development from the single, fertilized egg to the multicellular fetus.
However, when the reproductive cells divide, they result in egg- and sperm-cells, each containing half the number of chromosomes (23 chromosomes each). This process is called Meiosis.
Errors occur when the reproductive cells and chromosomes divide or when they are being duplicated. Picture this: There are 23 pairs of chromosomes in a reproductive cell holding hands. When the cell divides into 2 cells, the pairs have to let go of their partner, they stop holding hands. One partner of each chromosome goes in one cell, and the other partner has to go into the other cell. Here is where the mistake happens. The chromosome partners numbers 21 do not let go of each other’s hands, they “stick” together, and so they both end up in the same cell. This is called nondisjunction, because the chromosomes have failed to split up. The result is that one cell now has 24 chromosomes, because it has an extra copy of chromosome 21, it has 2 instead of the normal 1. The other cell is missing chromosome 21 and therefore cannot survive and will die off. This mistake usually happens in the egg-cell. So now there is a fertile egg with 2 copies of chromosome 21, and when this egg unites with a fertile sperm cell containing the normal 23 copies of chromosomes, the end result will be a cell with 47 chromosomes instead of the usual 46. The new human now has 3 copies of chromosome 21 instead of the usual 2. This causes the person to have Down syndrome or Trisomy 21. All the other chromosomes are normal. This abnormal fertilized cell now duplicates itself over and over by the process of Mitosis, forming the human body. Consequently each cell of the new human will have the extra copy of chromosome 21.