What causes chromosomal abnormalities?
Chromosomal abnormalities usually result from an error that occurred when an egg or sperm cell was developing. It is not known why these errors occur. As far as we know, nothing that a parent does or doesn’t do before or during pregnancy can cause a chromosomal abnormality in his or her child. But sometimes something goes wrong before fertilization. An egg or sperm cell may divide incorrectly, resulting in an egg or sperm cell with too many or too few chromosomes. When this cell with the wrong number of chromosomes joins with a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality. A common type of chromosomal abnormality is called a trisomy. This means that an individual has three copies, instead of two, of a specific chromosome. Down syndrome is an example of a trisomy. Individuals with Down syndrome generally have three copies of chromosome 21. Children with Down syndrome have varying degrees of mental retardation, characteristic facial features and, often, he
