So let's begin by focusing on trisomy 21. So the majority of the people in the world that have down syndrome have the tricemy 21 form of down syndrome. Now, what do we mean by tricemy 21? Well, trisomy means we have an extra copy of a chromosome and 21 refers to chromosome 21. So remember, the carreotype of any normal individual describes 23 pairs of homologous chromosomes and each pair normally consists of two individual chromosomes. So we have a total of 46 chromosomes in a normal carriotype.