Somatic vs. Gametic Cells
The somatic cell shown to the right is the type of cell that you will find in most parts of an animal's body, from its skin to its heart. The nucleus of a somatic cell contains a fixed number of chromosomes, usually present in pairs. The two members of a chromosome pair, which are referred to as homologous chromosomes, exist because one member of each pair has been derived from the maternal parent of the organism (shown in black) and the other from its paternal parent (shown in white). Cells such as this one that contain two copies of every chromosome are referred to as diploid.
Animal eggs and sperm are gametic cells. They contain only one set of chromosomes, consisting of one member of each homologous pair, as shown in the second frame of the animation. The nuclei of these cells are said to be haploid. These haploid gametes unite with others during fertilization to produce the diploid state of somatic cells.