Certain genetic diseases are widespread in the population; others, such as sickle-cell anemia and Cooley's anemia, primarily affect certain ethnic or racial groups. Cystic fibrosis is one of the most common genetic diseases among Americans because it largely affects whites of Northern and Western European ancestry. Tay-Sachs disease is a recessive-gene disorder that often strikes infants of Central and Eastern European Jewish descent.
The major diseases that strike American families today--heart disease, cancer and diabetes--all show a tendency to "run in families." That does not mean you are doomed to have one of them if a parent or relative suffered from the condition, but you should be aware of the risks where there is a precedent for the disease.
Early heart attacks do run in families. If you have a relative who had a heart attack before reaching 65, your risk of having one before that age is five to seven times the normal risk for your sex. Moreover, some of the leading factors known to contribute to coronary disease--high cholesterol levels, high blood pressure, diabetes and obesity--have been found to have a genetic link.
Several types of cancer are known to be directly inherited, such as retinoblastoma of the eye. There are common forms of cancer that tend to run in families--stomach, endometrium (uterine lining), lung, colon, bladder and breast cancer and malignant melanoma of the skin.
Researchers have established recently that you are likely to develop Type II, or non-insulin-dependent, diabetes, the most common form of this disease, if one or both parents have it.
What good does it do to know if you are at risk for a genetic disease? People who know about a predisposition toward a disease can do a lot to prevent that disease, either by changing high-risk behavior or by detecting health problems early. If you are alerted to the genetic liabilities you carry, you can develop corrective habits, or at least be treated at the earliest possible stage.
The March of Dimes Birth Defects Foundations recommends genetic counseling if you or family members have a disease with a genetic component; if you are a member of an ethnic group with a high risk of certain genetic diseases; if you have produced an affected child; if you are an expectant mother 35 years of age or older.
HOW RECESSIVE INHERITANCE WORKS
Both parents, usually unaffected, carry a normal gene (N) that takes precedence over its faulty recessive counterpart (r).
CARRIER FATHER: CARRIER MOTHER
The odds for each child are:
1. A 25% risk of inheriting a "double dose" of r genes which may cause serious birth defects.
2. A 25% chance of inheriting two Ns, thus being unaffected.
3. A 50% chance of being a carrier, as both parents are.
HOW X-LINKED INHERITANCE WORKS
In the most common form, the female sex chromosomes of an unaffected mother carry one faulty gene (X) and one normal gene (x). The father has normal x and y chromosome complement.
CARRIER MOTHER: NORMAL FATHER
The odds for each male child are:
1. A 50% risk of inheriting the faulty X and the disorder.
2. A 50% chance of inheriting normal x and y chromosomes.
The odds for each female child are:
1. A 50% risk of inheriting one faulty X, to be a carrier like the mother.
2. A 50% chance of inheriting no faulty gene.
HOW DOMINANT INHERITANCE WORKS
One affected parent has a single faulty gene (D) that dominates its normal counterpart (n).
AFFECTED FATHER: NORMAL MOTHER
Each child's chance of inheriting the D or the n is 50%.