Men with multiple sclerosis are more than twice as likely than women with the illness to pass it on to their children, U.S. researchers report.

"When we looked at a large population of MS patients, when there was a parent and a child who had MS in a family, the child with MS got the disease twice as often from the father rather than the mother," researcher Dr. Brian Weinshenker, a Mayo Clinic neurologist, said in a prepared statement.

Reporting in the July 25 issue of Neurology, Weinshenker and his colleagues theorized that this may be because men may have a greater "genetic load" of MS genes compared to women.

"The hypothesis of this study is that men are more resistant to MS, so they need stronger or a larger number of genes in order to develop MS, and then pass these genes to their children," study author Dr. Orhun Kantarci, a Mayo Clinic neurologist, said in a prepared statement.

The fact that men are more likely to pass MS to their children is not easily explained by hormonal differences between women and men or by genes on the sex chromosomes, Kantarci said.

The findings shouldn't affect how men with MS are counseled about the risk to their children, the researchers said. A child with an affected parent has about a 20-fold increased risk of MS. But the additional risk of having a father with the disease is not enough to change current patient counseling methods.

"The over-transmission by men is primarily of interest to scientists studying the mechanisms of genetic transmission of MS susceptibility," Kantarci said. The finding "may indicate that nontraditional, or so-called epigenetic factors, play some role in the transmission of MS," he theorized.

Eighty-five percent of MS cases have no known cause. Among 15 percent of MS patients, a family member within a generation is also affected by the disease. In familial cases, no single gene has been identified that strongly predisposes a person to MS.