Study reveals why autism is more common in males

It has long been known that men have a greater risk for developing autism and other neurodevelopmental disorders, compared to women. While boys have a one in 52 chance of developing autism spectrum disorders (ASD), the risk is only one in 252 for girls, according to the U.S. Centers for Disease Control and Prevention (CDC).

Now, a new study published by the American Journal of Human Genetics reveals why so many more men are affected by these diseases.

Previously, researchers had speculated that mutations on the X chromosome may be to blame for the prevalence of ASD among men. However, study author Evan Eichler, a professor of genome sciences at the University of Washington School of Medicine, said that doesn't seem to be the case.

"Five percent of genes responsible for brain development map to the X chromosome," Eichler told FoxNews.com. "There are not enough brain development genes on the X chromosome to account for that big of a difference in terms of gender bias."

In an effort to puzzle out the gender disparity seen in autism and other disorders, Eichler and his colleague Sbastien Jacquemont, of the University Hospital of Lausanne in Switzerland, paired up to analyze DNA samples from nearly 16,000 people with neurodevelopmental disorders. They also analyzed additional samples from a separate cohort of 800 families affected by ASD.

Through their analyses, the researchers began to notice that despite the fact that more boys are affected by ASD, the serious genetic mutations responsible for these diseases were more likely to be passed to children through their mother's DNA, as opposed to from their father.

"We started to see this bias coming from mothers, who were supposed to be unaffected, that they were more likely to be transmitting mutations we thought were deleterious," Eichler said.

After analyzing the cohort of 16,000 individuals with neurodevelopmental disorders, Eichler and his colleagues also discovered that female children seemed to have a larger number of genetic mutations associated with neurodevelopmental disorders, compared to male children.

"If we divide the cohort into females and males, and look at really big mutations, do we see a difference between boys and girls in terms of frequency?" Eichler said. "The answer was, unequivocally, yes. Girls tend to have more of these than boys. Boys have fewer than females."

In analyzing the cohort of 800 families affected by ASD, the researchers also saw that girls had more major genetic deletions - and more small mutations - compared to boys.

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Study reveals why autism is more common in males