Down Syndrome

Question

How can non-disjunction lead to the production of an egg cell with an extra chromosome and, ultimately a child with Down Syndrome?

Answer 1

Hi! Simply put, nondisjunction is the failure of homologous chromosomes to separate in meiosis. Normally, all chromosomes are lined up with their homologues (same type of chromosome, but each from a different parent of the cell undergoing meiosis) in metaphase I. Then, in anaphase I, these chromosomes are separated and ultimately wind up in separate cells that undergo division again. In Down Syndrome (also called Trisomy 21) the 23rd chromosomes are not separated. As a result, one cell will have both homologues and one will have none. Both of these cells undergo division again (meiosis II).