Autism genetic research has shown that it is one of the most heritable disorders. Despite an increasing amount of autism genetic studies being performed each year the researchers are still trying to discover the multiple genes believed to be the reason for autism.
There have been numerous studies, completed and ongoing, on twins and siblings. Identical twins, who share one hundred percent of their nuclear DNA, where one has autism, have about a sixty percent chance of the second child having autism and a ninety percent chance of the identical twin having some type of autistic spectrum disorder. With fraternal twins, where the genes shared number only fifty percent, the chance of the second twin being autistic lowers dramatically to four and half percent, just like it would with any sibling. For the average person the chance of having autism is .2%. These numbers speak strongly in favor of evidence that links autism with heritability.
Autism genetic studies of twins have been going on since the middle 1970s. Of nine studies conducted worldwide between 1977 and 2004 all of them came to relatively the same conclusion regarding identical twins and the chances of a second twin having autism. One study felt that the delivery highly influenced the chance of an autistic child, but overall the conclusions were very similar.
Many family studies have been conducted as well. A Danish study in 2005 looked at risk aspects of the entire family. If the autistic child had Asperger's syndrome the risk for other children to have some type of autism was believed to be 1.04% while if the child had other forms of autism the feeling was a risk factor of 1.76%. These risk factors were double if the mother had ever been diagnosed with any form of psychiatric disorder. These conclusions were formed after a thorough study of the entire family including collecting information on the parent's place of birth, their age, the location the child/children were born, and the family history for depression or psychiatric difficulties. Although paternal age seemed to be an issue, maternal age was not.
These studies have left almost as many questions unanswered as they have answered. If identical twins show such an increase in heritability why don't fraternal twins? Why are parents of autistic children almost always people who do not have autism if heritability is so definitely a factor in this disorder? Some researchers question if autism has become the word of its time and perhaps too many children are being lumped into this group for ease of diagnosis. All these unanswered questions will only be answered once more studies into the heritability of autism are conducted and results are known.
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