Genes responsible for a number of autism's characteristics come in two varieties, which could help explain not only the condition's diversity, but also how it's inherited.

A new study on the genetics behind the disorder has revealed the kinds of mutations associated with lower IQ are also linked with impeded motor skills.

What's more, the severity of these mutations might also explain why many aspects of autism spectrum disorder (ASD) are more pronounced in some than in others.

In most cases, a diagnosis for ASD is based on a mix of social and physical behaviours, including difficulties in communicating and sensory processing, repetitive movements, and impeded motor functions.

"Diminished motor skills appear to be an almost universal property of children with autism," says molecular geneticist Michael Wigler from Cold Spring Harbor Laboratory.

Yet there are also traits that commonly coincide with an ASD diagnosis, without necessarily being considered as a defining part of the condition.

For example, intellectual and learning disabilities can affect a significant proportion of individuals with ASD, with just under half of those with a diagnosis also having an IQ lower than 70.

But it's still something of a mystery as to what really gives rise to this rich spectrum of characteristics.

There are strong implications that a number of genes are involved, and that numerous epigenetic changes can be responsible for switching them on and off.

Many of these genetic changes are known to be inherited, but past research has linked so-called de novo mutations – coding differences that aren't found in the parents – with learning difficulties among those with ASD.

Another recent study also concluded that de novo mutations happening after fertilisation could account for as much as 2 percent of all autism diagnoses.

Researchers have now used a database consisting of over 2,700 families who had only one child affected by ASD to discover that de novo mutations in key genes can also accurately predict the presence and severity of reduced motor skills.

The same link wasn't found for other components of ASD, such as social skills and challenges in communication, meaning the genes for those autism traits are more likely to be passed on from the parents.

The discovery adds much needed detail to what is an incredibly complex condition, and could even help explain how it persists in our population.

One possibility is that some of the more challenging aspects of ASD could be offset by having a higher intelligence.

There is already speculation that there is a strong interplay between IQ and autism's core characteristics – where social skills make it difficult for researchers to accurately predict cognitive abilities.

By the same token, the researchers suggest having strong cognitive and motor skills could in turn affect how other autism behaviours are expressed.

The end result could mean we're more likely to pass on genes that impact on social and communication skills so long as they're not accompanied by genes that also affect IQ or interfere with movements.

In other words, those genes giving rise to learning difficulties or affecting motor control are more likely to occur through de-novo mutations rather than inheritance.

On a more practical level, the research has implications for studying the inheritance of ASD, as well as diagnosing the disorder.

The scientists suggest a new way to classify ASD based on its genetic foundations – mild, with little impairment of either motor skills or IQ, moderate impairment mainly to motor skills, and severe impairment, affecting both.

They also emphasise the importance of evaluating IQ and motor skills when forming a diagnosis and designing therapies.

"As such, objective assessment of cognitive function should be a facet of any clinical evaluation of the patient," says Wigler.

This research was published in the Proceedings of the National Academy of Sciences.