The heritability of Attention-Deficit/Hyperactivity Disorder (ADHD) is exceptionally high for a complex psychiatric condition, with twin studies consistently estimating it between 74% and 80%. Despite this strong genetic signal, pinpointing the exact causal variants has proven challenging, leading to the familiar problem of "missing heritability." The genetic architecture of ADHD is not a story of single-gene causation but a complex tapestry woven from thousands of genetic variants, each contributing a small, additive effect.

This architecture is primarily understood through two main categories of genetic risk: common variants and rare variants. The common variant hypothesis posits that the bulk of heritability is driven by single nucleotide polymorphisms (SNPs) that are relatively frequent in the population (Minor Allele Frequency > 1%). In contrast, rare variants, such as large copy number variants (CNVs), are individually more penetrant but occur in a much smaller fraction of individuals. While CNVs affecting neurodevelopmental pathways contribute to a subset of ADHD cases, large-scale studies confirm that common variants explain the majority of the risk at a population level.

A landmark meta-analysis from the Psychiatric Genomics Consortium (PGC) dramatically clarified the polygenic nature of ADHD. By combining data from numerous genome-wide association studies (GWAS), the study amassed a sample size powerful enough to detect the small effect sizes typical of common variants. This work successfully identified 27 genome-wide significant loci, providing the first robust set of common variant associations for ADHD.

These findings solidify the view that ADHD is not a monogenic disorder. Instead, it operates on a liability threshold model, where an individual's risk is a cumulative sum of thousands of genetic variants. When this polygenic load crosses a certain threshold, influenced by environmental factors, the clinical phenotype of ADHD manifests. This model explains the continuous distribution of ADHD traits seen in the general population.

Among the identified loci are genes with plausible links to neurobiology. For instance, FOXP2 has a well-established role in speech and language development. Variants in SORCS3 are implicated in synaptic plasticity and neuronal sorting. DUSP6 is involved in the MAPK/ERK signaling pathway, a critical cascade for neuronal proliferation, differentiation, and survival. While these associations provide tantalizing clues, the precise mechanisms by which they contribute to ADHD pathophysiology are still under intense investigation.

While twin studies estimate heritability at ~75-80%, SNP-based heritability ($h^2_{SNP}$) provides a different perspective. This metric quantifies the proportion of phenotypic variance explained by all common variants assayed on a SNP chip. For ADHD, the SNP-based heritability is estimated to be around 22%. This figure confirms that common variants collectively account for a substantial portion of the genetic risk, even if it doesn't capture the entire heritability signal seen in family studies. The gap between these two estimates is attributed to factors like rare variants, structural variations, and gene-environment interactions not captured by standard GWAS.

Crucially, enrichment analyses reveal where these risk variants are most active. ADHD-associated loci are significantly enriched in genes expressed in the brain, as one would expect. More specifically, they show enrichment in evolutionarily conserved genomic regions. This conservation implies that these regions are under purifying selection, suggesting they have critical biological functions that have been maintained across species. The fact that ADHD risk variants are concentrated in these important, brain-relevant areas reinforces their biological significance.

Understanding the complex interplay of these numerous small-effect variants is the current frontier in ADHD genetics, moving the field beyond simple models and toward a more nuanced, network-level view of the disorder.