HOW DOES TRISOMY MOSAICISM OCCUR?
In this section we begin with some background information on the early stages of growth of the zygote. At fertilization, the egg and the sperm fuse to form a zygote with 46 chromosomes. The single-celled zygote then enters a stage of growth called cleavage. Cleavage produces a rapidly increasing number of cells which get progressively smaller and smaller in size. The zygote divides through a process called mitosis. During mitosis the 46 chromosomes make an identical copy of themselves and each pair of replicated chromosomes pull apart from each other into separate daughter cells. The purpose of mitosis is to pass on a complete copy of genetic material to each daughter cell. The contents of the daughter cells are identical to the original cell. The diagram on the right illustrates typical mitosis. In the earliest stages of growth and development the zygote divides successively to create a ball of cells, called the morula. During these early cleavages, each new cell is called a blastomer
