It is a genetic condition caused when an unusual cell division result in an extra full or even partial copy of chromosome 21.

Table 1: Chromosome Counts in the Somatic Cells of Several Type A and Type B Species As illustrated in Table 1, the somatic cells from Type A males had one less chromosome than the somatic cells ...

In most cases, females possess two X chromosomes, while males have one X and one Y. However, there is some evidence that the Y chromosome is being phased out of the evolutionary pipeline ...

Males typically have one copy of the Y chromosome and one copy of the X chromosome ... but which genes are the most important and how they work is less clear. In research published today in ...