Question:

I may be adopting a newborn who has a sibling with autism. Would different birth fathers have any outcome on this baby being autistic?

Answer:

The answer to this question depends partly on how much genetics contributes to autism. The current thinking is that the genetic contribution is substantial, possibly through conferring vulnerability to environmental stressors. Genetic vulnerability would come half from each parent if it depends on nuclear DNA. Thus, we would expect that half-siblings would have about half the risk of full siblings of inheriting the genes that contributed to the autism of the first child.

However, there are several complications. One is that not all DNA is in the nucleus. There is DNA in the cytoplasm of the cell, within mitochondria, which is critical for normal metabolism. There is a suggestion that for some cases, this could be implicated in autism. All mitochondrial DNA comes from the mother.

If the particular case of autism happens to be one of the rare ones associated with mitochondrial DNA abnormality, then the risk would be the same for half-sibs and full sibs with the same mother. Additionally, if the autism was somehow triggered by brain damage from some kind of intrauterine insult that is likely to be repeated (maternal immune dysfunction, substance use, hypoxic uterus), that could also tip the scales toward the importance of the maternal contribution, in this case nongenetic.

On the other hand, sperm are more vulnerable to mutations than are eggs, and this risk increases with age of the father, so that if the father of the child with autism was very old and the father of the second child young, that would increase the hope that the second child would not have autism.

Another consideration is that for unexplained reasons, boys are about 4 times as likely as girls to have autism, so a sister of a child with autism would theoretically have 1/4 the chance of a brother to also have autism.

Finally, in assessing the odds, remember that although we are having an epidemic of autism, it is still less than 1% of births, and although one child with autism in a family increases the chance of a sib having it also, there are also many families that have a single child with autism and normal sibs.

For more information:

Go to the Autism health topic.