Chromosomal abnormalities describe changes in the normal number of chromosomes or structural problems within the chromosomes themselves. These abnormalities occur when an egg or sperm with an incorrect number of chromosomes, or a structurally faulty chromosome, unites with a normal egg or sperm during conception. Some chromosome abnormalities occur shortly after conception. In this case, the zygote, the cell formed during conception that eventually develops into an embryo, divides incorrectly.

Chromosomal abnormalities can cause serious mental or physical disabilities. Down syndrome, for instance, is caused by an extra chromosome 21. People with Down syndrome are mentally retarded and may have a host of physical abnormalities, including heart disorders. Other individuals, called Down syndrome mosaics, have a mixture of normal cells and cells...