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. But if we examine the carriotype of an individual that has trisomy 21, let's suppose the individual is a male individual. This is what we're going to see. So notice, every single one of these homologous pairs of chromosomes consists of two individual chromosomes, except chromosome pair 21.