It has long been clear that autism strikes males more often than females. But when males do get the condition, they tend to be at the severely affected end of the spectrum.

Now, a group of geneticists say they have found links between genetic alterations and an increase in autism for males..  In a study led by a team of Toronto scientists it was revealed that males who carry specific genetic alterations on their X-chromosome have an elevated risk for developing autism spectrum disorder, or ASD.

“The male gender bias in autism has intrigued us for years and now we have an indicator that starts to explain why this may be,” said co-principal investigator Stephen Scherer, director of the Center  for Applied Genomics at Toronto’s Hospital for Sick Children.

Males inherit one X-chromosome from their mother and one Y-chromosome from their father, explained Scherer.

“If a boy’s X-chromosome is missing the PTCHD1 gene or other nearby DNA sequences, they will be at high risk of developing ASD or intellectual disability,” said Scherer. “Girls are different in that, even if they are missing one PTCHD1 gene, by nature they always carry a second X-chromosome, shielding them from ASD. While these women are protected, autism could appear in future generations of boys in their families.”

The findings, in a study of more than 16,000 people, confirms that autism is not simply being missed in females.

“[I]t is actually occurring less often,” said Kevin Mitchell, a geneticist at Trinity College in Dublin.

There are about seven males with mild autism for every female, though the gender gap is much smaller at the more severe end of the spectrum. The study also showed that the mutations behind the autism are either new ones that develop in the child, or come from the parents – most likely from the mother.

A man who is severely affected by an autism-related genetic glitch is more likely to have trouble forming relationships and therefore less likely to have children, Mitchell said, so less likely to pass the mutation on to the next generation. A woman, who can have the glitch without noticing it, would be more likely to reproduce and therefore pass on the mutation.