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My husband and his ex-wife have 3 sons - two teenage boys with autism and mental retardation and a healthy 7-year-old boy. He and I have one healthy daughter, almost two. The risk of us having an autistic son is something like 4-12% (based on 2 siblings and a recurrence risk of 2-6% for 1 sibling which is what I found on the internet). What is the risk for a daughter? (I know it's less) And since siblings share 1/2 their genes and half-sibling share 1/4 of their genes, wouldn't our risk be slightly less than that 4-12%?

-A curious couple from the United Kingdom

January 28, 2005

(We will need to update this soon as the latest incidence of autism is more like 1 in 133 instead of 1 in 1000.  The increased risk for second degree relatives will be similar but the absolute percentages will change.)

The genetics of autism are very complicated. There are probably lots of genes involved, most of them unknown.

Even without knowing the genes, scientists do know that it runs in families. By looking at something called twin studies (see below), scientists can figure out how likely it is that someone will get autism.

Half-siblings are called second-degree relatives. They share a quarter of their genes. The chance of having autism when a second-degree relative has autism is 0.18%. The chances of anyone in the general population having an autistic child are about 1 in 1000 or 0.1%. So, while the risk is real, the chances of you and your husband having an autistic child are still very low.

As I said, autism most likely involves lots of genes. Because of this, the recurrence risk is harder to figure out than you might think.

Recurrence risk is just the chance that something will happen again in the same family. If autism were caused by one gene, the recurrence risk would be the same whether you have one or more children with the disease.

The recurrence risk would be 2-6% for every child regardless of how many children you have with autism. But, because lots of genes are probably involved in autism, you need to get more than one from each of your parents.

If a couple has 2 autistic children, this suggests a "perfect storm" of autism genes. In other words, the parents have many of the same autism genes that they can pass on. This is why the recurrence risk is actually 35% for the 3rd child rather than the 4-12% you might think.

To get such a high chance of having an autistic child, you need both parents involved. If only one of these parents is involved the chances are much, much less.

In the general population 1 in 1000 people are autistic. Boys are 3 to 4 times more likely to have autism than girls. Second-degree relatives, like half-siblings, aunts, uncles, grandparents and grandchildren have a 0.18% chance of recurrence of autism. Third-degree relatives, like cousins, have a 0.12% recurrence of autism.

So why is the genetics of autism so complicated? Part of it has to do with the complexity of the disease.

Autism is a brain disorder that causes problems in development. It can range from mild to disabling. People with autism have trouble talking and relating to other people. They also tend to repeat movements and have obsessive interests. The range in symptoms makes it likely that there are several genes involved in autism.

The most recent estimate is that a person with autism could have mutations in five to ten genes. This means that two autistic people could have mutations in two different sets of genes. You can imagine that this makes it very difficult to study the disorder.

So how can we tell if a complicated mental disorder is genetic? The best test is through twin studies.

Twin studies calculate the chances of fraternal and identical twins having the same disorder. Identical twins came from the same embryo and have the same DNA. Fraternal twins came from two embryos and only share 50% of their DNA, like siblings who are not twins.

Twin studies of autism show that there is a 36-91% chance of two identical twins being autistic. Fraternal twins only have a 2-4% chance of both being autistic. Since identical twins have the same DNA and they have a much greater chance of both being autistic, the disorder must be partly genetic.

Scientists have known for some time that autism is a genetic disorder caused by many genes. Over the past ten years scientists from all over the world have worked together to find the autism genes. Data has been collected from hundreds of families with autism. Old and new tools have been put to work to find the autism genes.

A recent study found one gene that could be involved in autism. This gene is mutated more in people with autism. The gene codes for a protein that works in the process that makes ATP. ATP is the fuel or energy that cells need to function. Brain cells need a lot of energy to do their work. It's easy to imagine that if this gene were not working right, the brain cells would also not function normally. These types of results have encouraged scientists to continue their search for autism genes. The hope is that one day we may not only know your chances of having an autistic child, but also be able to cure it.