No doubt you have heard several times that genes play an important part in ADHD. But what exactly does that mean? Well, it means that different types of genetics studies have identified links between ADHD and certain genes. There is also important research underway concerning the role of environmental factors and their interplay with genes, but more of that later.
Behaviour genetics investigates the contribution of genes and the environment to individual differences in behaviour. It examines familiality (Does ADHD run in families?) and heritability (Does ADHD get passed on?) And many studies confirm the highly familial nature of ADHD, with first-degree relatives displaying a four to five fold risk of ADHD compared to the general population.
Twin studies and adoption studies are also valuable. Adopted children share genes but not the environment with their biological parents. Any similarity between these adopted children and biological parents could therefore be attributed to their shared genes. In the same way, similarities between adopted children and their adoptive parents could be attributed to the environment they share.
- Adoption studies have found that adoptive parents of children with ADHD display less behaviours of hyperactivity, and do better on standardized tests of attention than the biological parents. This supports the belief that ADHD has a genetic component.
- Twin studies have also investigated the genetic components of ADHD. When conducting twin studies, researchers consider behaviours which are highly common to identical twins to be influenced by genetics, with those common to non-identical twins to be caused by environmental factors. In reviewing the numerous twin studies on the heritability of ADHD, researchers have concluded that up to 90% of the cause of ADHD can be attributed to genetic factors.
Molecular genetics research of ADHD includes linkage and association studies.
- Linkage studies aim to pinpoint the location of individual genes responsible for a particular disease. This has not been successful with ADHD, due to the multiple genes involved.
- However association studies in ADHD have followed a candidate gene approach, which has been very successful in identifying groups of risk genes for ADHD. Focusing on the genes of neurotransmitter systems such as dopamine, serotonin and noradrenalin, it has identified DRD4, DRD5 and DAT1 as risk genes for ADHD.
Researchers in the field of genetics are presently developing methods of combining behaviour genetics with molecular genetics research. These combined studies could, in the future, assist in the identification of specific genes implicated in ADHD, assisting the processes of diagnosis and treatment.
(Ref: Hawi, Z., & Lowe, N. (2007). Molecular genetic aspects of attention deficit hyperactivity disorder. In M. Fitzgerald, M. Bellgrove & M. Gill (Eds.), Handbook of attention deficit hyperactivity disorder (pp. 129-149). West Sussex: John Wiley and Sons.)