What are the causes of Downs Syndrome?
Normally, each parents provides one of each of the 23 chromosomes that defines the genetic endowment of the child. When a Down Syndrome (DS) child is conceived (more on this later), one parent accidentally gives the child *two* copies of chromosome #21. This is called a “trisomy” (“tri” meaning three). There are exceptions. Some children are conceived with the right number of chromosomes, but in cell division, gain an extra chromosome #21. This is called a “mosaic trisomy” because different cells have different genes–just like a mosaic is made up of different shapes and colors of stones. Depending on what percentage of the cells are correct and how many have the extra chromosome, the child may be less seriously affected by DS.
