Genetics
With heritability estimates of around 0.7, ADHD is considered
one of the most heritable psychiatric conditions (Faraone et al.
2005). However, no single genetic risk factor has yet been identified,
making it likely that ADHD develops as a result of the
interaction between multiple genetic risk variants, each of small
effect (Thapar et al. 2013). Research has thus far focused on
genes involved in dopaminergic transmission due to observed
dopamine deficiency in children with ADHD (Spencer et al.
2005) and the therapeutic benefits provided by methylphenidate,
a dopamine agonist.Ameta-analysis of commonly studied
candidate genes has revealed moderate associations between
ADHD and variants of the dopamine transporter (DAT1),
dopamine receptors (DRD4 and DRD5) and the serotonin
transporter (5HTT) (Gizer et al. 2009). Genome-wide searches
(GWAS) have yet to identify a single candidate gene through
‘hypothesis-free’ testing (Franke et al. 2009), although this is
probably due to insufficient sample sizes to date (Thapar et al.
2013).