Other kinds of geneticically based disorders
Chromosomal disorders arise from errors in an entire chromosome rather than mutations in a single gene. These errors occur when the egg and sperm are forming and may be inherited from either the father or the mother.
Perhaps the most familiar chromosomal disorder is Down's Syndrome, which occurs when a child receives three copies of a certain chromosome instead of two. Other birth defects result when chromosomes are broken, missing, or malformed.
Unlike single-gene diseases, chromosomal disorders cannot be predicted by screening parents before pregnancy. Prenatal disgnostic tests (CVS or amniocentesis) will reveal whether your baby's chromosomes are normal. Because the risk of chromosome abnormalities increases as women age, doctors will typically recommend prenatal testing for mothers who will be over 35 at the time of birth.
A great number of diseases, such as diabetes, congenital heart defects, spina bifida, and cancer, are now known to have a hereditary component. These are called "multifactorial" diseases because they are caused not by a single gene mutation, but by a combination of genetic and environmental factors working together in ways that aren't yet fully understood.
Genetic screening for mutations associated with multifactorial diseases can only tell you about your risk. It cannot predict whether you will develop the disease. For instance, a woman who inherits an alteration in the BRCA2 gene is more likely than other women to develop breast cancer, but she may also remain disease-free. The altered gene is only one risk factor among many. Lifestyle, environment, and other other biological factors also play a part.
If you are concerned about a health problem that seems to run in your family, and want to know whether your genes put you or your children at increased risk, you may want to talk to a genetic counselor.