Researchers have always wondered why the autistic gene affects more boys than girls. Now a new study has shed light on the matter. Girls simply tolerate neurodevelopmental mutations more than boys do.

The researcher analyzed about 16,000 DNA samples and sequencing data sets from people with neurodevelopmental disorders. This was done in collaboration with scientists from the University of Washington School of Medicine

The new study, published in the American Journal of Human Genetics, also analyzed 762 families that had a child with autism, and were able to identify two different kinds of mutation. One class of mutation is called copy number variation (CNV), which deals with deletions or duplications of a large chunk of genetic material. The second class of mutation is the single-nucleotide variation (SNV), which involves substitutions of a single nucleotide of DNA.

The researchers have found that the first class of mutation was three times more common in girls than in boys. Females diagnosed with any neurodevelopmental disorder had more harmful CNVs compared to males who were diagnosed with the same disorder. The second class of mutation was one third more common in girls as well.

This means that girls need more mutations before autism can take root in their system, compared to boys. This phenomenon is known as "female shielding." There is something that goes on in the brain development of females that protect them from getting autism spectrum disorder (ASD). However, scientists have yet to completely figure out what it is.

"There's a well-known disparity when it comes to developmental disorders between boys and girls, and it's been puzzling," said study author Sebastien Jacquemont, an assistant professor of genetic medicine at the University Hospital of Lausanne, in Switzerland. "And there have been quite a bit of papers trying to investigate this bias that we've seen in the clinic."

The study also showed that the mutations can either develop from the child or come from the parents, most likely from the mother. There could also be an explanation that lies in simple probability. A man with autism is more likely to have trouble forming and maintaining relationships, and is less likely, therefore, to have children, and thus less likely to pass on the mutation. A woman who bears the mutation but does not exhibit the autistic symptoms can lead a normal life, have children, and then pass the mutation on to her children.

The Centers for Disease Control and Prevention has stated that about 1 out of every 88 children are diagnosed with ASD. The disorder also affects 1 in every 54 boys, while in girls the rate is 1 in every 252.

Genetics is a complicated field. For autism alone, there are 500 different genes connected to it, so it's not easy to determine what affects the mutation of each gene.

"There's lots of different ways to create an autistic child," said Evan Eichler, a geneticist  for the University of Washington and the study's lead researcher. "But those genes mostly fit into about a dozen different pathways, suggesting different treatment approaches may be most effective for each subtype."

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