Two children, both now teenagers, have developed symptoms and are at risk for sudden cardiac death, the report says. It's the second documented instance of a genetic condition being inherited through sperm donation.This case highlights the importance of thoroughly screening sperm donors.

The sperm bank involved now gives all donors electrocardiogram tests to weed out men with genetic heart problems; the study authors recommend that other sperm banks follow suit.

Voluntary sperm bank guidelines say donors should be required to provide a complete medical history to rule out those with infectious diseases or a family history of inherited diseases. Many also do testing but for genetic diseases that are less common than the heart problem, according to co-author Dr. Barry Maron of the Minneapolis Heart Institute, a leading authority on the condition called hypertrophic cardiomyopathy.

Hypertrophic cardiomyopathy thickens the heart and makes it harder to pump blood. It affects about one in 500 people; many more likely have the genetic defect without symptoms, said study co-author Heidi Rehm of Harvard Medical School.

Neither the sperm bank nor the donor were identified. The donor, now 42, had no symptoms of genetic heart disease and no obvious family history when he donated sperm in the early 1990s. His own condition wasn't diagnosed until after a child born through sperm donation was diagnosed. Maron declined to provide more details on the donor's health, citing privacy concerns.

The children are now ages 7 to 16. Nine, including one born to the donor's own wife, tested positive for the heart mutation. One born through sperm donation died; two others have developed symptoms, with one getting a defibrillator. The remaining children are at increased risk for problems, which often don't show up until adolescence, Maron said.