How DNA is inherited
Each of your parents gave you half your DNA, and you'll give your children half of yours. Which half? That's completely unpredictable. In each child, the chromosomes are assembled slightly differently. Except for identical twins, no two people get exactly the same mix of genetic code. So, right from conception, we differ from each other in the structure and function of our bodies – and the health predispositions we carry.
Single-gene diseases: Some diseases – cystic fibrosis, sickle cell anemia and Huntington's disease, for instance – are caused by a variation in just one gene. And if you have the genetic variant that causes this type of disease, you will get the disease. (Navigenics does not offer genetic tests for these single-gene disorders, though some other companies do.)
Disease caused by multiple factors: Complex conditions like diabetes and heart disease are different. They are caused by multiple markers on or near multiple genes (along with non-genetic factors), and chances are that you won't inherit all of those markers from your parents.
Just because one of your parents has heart disease, for instance, doesn't mean you will automatically inherit the markers that contributed to it. The outcome for your parents, children and siblings is complicated by the number of genetic variations and environmental factors involved in complex diseases like diabetes and cancer.
Often, specific combinations of these markers are necessary to strongly predispose you to disease, which further clouds the ability to determine whether your family members are at increased risk. For example, if you have a marker that raises your risk of colon cancer:
- At least one of your parents has that marker and gave it to you (though your parent may never have had the condition itself).
- Your siblings might, but might not, have inherited that marker from your parents.
- Your children might, but might not, inherit that marker from you.