Chromosomal conditions or anomalies occur when there is an abnormal amount of chromosomal material. The usual chromosome complement in each cell is 46. There are 22 pairs of autosomes numbered 1 to 22, with each pair having the same genes. The 23rd pair is called the sex chromosomes, with females having two “X” chromosomes and a male having an “X” and a “Y” chromosome. The “X” and “Y” chromosomes, unlike autosomes, have different genes. There is sometimes an abnormal total number of chromosomes – sometimes an extra chromosome. For example, a child with Down syndrome has three copies of chromosome 21, rather than the usual two, so the condition is called “trisomy 21.”