Genetics

Family Diseases: Common Heritability Myths Revealed

By Kendall K. Morgan

Randal Ford

With all the talk about the value of family history, it can sometimes be easy to see the mark of inheritance in places where it isn’t. Some conditions — leukemia, for example — aren’t really heritable at all. Even where genetics might play a role, sometimes there may be a better explanation. For instance, while twins do run in some families, fertility treatments have increased the incidence of twinning today. Hormonal changes in older mothers can make twins more likely, too.

Down syndrome may be the most commonly occurring genetic disease, but it almost never runs in families. The National Down Syndrome Society says the developmental condition is hereditary in just 1 percent of cases. Down syndrome occurs at random, with the risk increasing in mothers after the age of 35.

Sometimes people invent other kinds of complicated patterns of inheritance. A Google search for “Does schizophrenia skip a generation?” turns up several hits, but the National Institute of Mental Health’s overview on schizophrenia mentions no such phenomenon. According to the NIMH, an immediate relative with schizophrenia increases your odds from 1 percent for the general population to 10 percent. Second-degree relatives increase your odds of schizophrenia, too, but less so.

Julianne O’Daniel, a genetic counselor at the University of North Carolina, Chapel Hill, says sometimes even doctors can come up with unusual ways to explain what they see. (That’s one reason Lori Orlando, a primary care doctor at Duke University Medical Center, says she loves genetic counselors.) It might seem that a condition affects only people of a certain sex, people with a particular hair color, or people with freckles. People sometimes think (mistakenly) that children who favor an affected parent — either in terms of physical appearance or personality — might be more likely to carry that parent’s disease mutation, too.

In other instances, patients might conclude that their risk of cancer is elevated based on family history when it really isn’t. Maybe a couple of distant relatives had cancer, or their history is sprinkled with cancers of different kinds — lung, breast, prostate, bladder. Even if it seems like “everyone in your family has cancer,” your risk still might not be any higher than that of the general population.

The bottom line is this: Make note of your family health history as best you can, but don’t jump to conclusions. In some cases, it might even come as good news. In places where it doesn’t, it could be the push you need to get on track for a more healthful future.