What causes a baby to have an extra chromosome as in Down syndrome?
Down syndrome is a genetic condition that is caused by an extra copy of the 21st chromosome. Before parents can make a baby, certain cells in the ovaries and sperm must divide so that the child does not get too much DNA. Each cell that will make the baby must have exactly 23 chromosomes. When the cell from the mother (the egg) merges with the cell from the father (the sperm), the baby will have its full 46 chromosomes. These parental cells must go through a very specific kind of division called meiosis where new cells have exactly half the genetic material. During this division, chromosomes that encode the same kinds of genes must line up next to each other. They then split away to the 2 new cells. Sometimes they “stick” together, leaving one cell with one extra and one cell with one fewer chromosome. This is called nondisjunction. Then the baby will have too many or not enough chromosomes when the sperm and egg merge. This is what happens in Down Syndrome. Chromosome 21 doesn’t separa
