A chromosome is a rod shaped structure within the animal cell which carries the stretches of DNA referred to as genes. They are located in pairs within the nucleus of the cell and determine the sex and the characteristics of the organism inherited from its parents. In the human species there are 46 chromosomes arranged in sets of 23 pairs, one set being donated by the mother and one by the father.
The sex cells are referred to as gametes; the male gamete being the spermatozoa, and the female gamete being the egg or ova. Commonly there is no problem associated with chromosomal crossing over during gamete formation but occasionally things go amiss and cause an alteration in the number of chromosomes in the nucleus or the internal gene structure within the chromosome becomes changed in some way.