Familial Genetic Studies show that ADHD runs in families
Genetic Studies on Attention Deficit Disorder – ADHD show gene alterations that may contribute to ADD ADHD in some children. They are especially looking at the DRD4 dopamine receptor gene.
For example, a child with an older sibling with ADD ADHD is 300% to 500% more likely to himself have Attention Deficit Disorder than is a child without ADD ADHD siblings.
Twin studies and Adoption studies are also included.
Here are some facts from the NIMH on the genetics of ADHD, and some of the current research:
Epidemiology: In a large sample from the U.S. population, the prevalence of ADHD (male: female ratio) in school-age children was 6.7 percent (5.1:1). Depending on the use of adaptive functioning ratings to define definite maladjustment, prevalence estimates of 6.6 percent and 9.5 percent
Family Studies: Several studies demonstrate that ADHD aggregates in families. The rates in probands' sibs in three older studies ranged from 17 percent to 41 percent, with respective rates in controls' sibs ranging from zero to 8 percent. Rates of childhood ADHD in parents of hyperactive probands in several older studies ranged from 15 percent to 44 percent for fathers and 4 percent to 38 percent for mothers, although one study found no evidence of an increased rate of childhood ADHD in parents of ADHD probands
Twin Studies: Two small twin studies found that 4 of 4 and 3 of 3 MZ twins were concordant for ADHD. A larger twin study reported respective MZ and dizygotic (DZ) probandwise concordance rates of 51 percent and 33 percent, with a heritability estimate of 64 percent.
Adoption Studies: Increased rates of hyperactivity or a history of hyperactivity have been found among both adopted-away sibs of children with ADHD and the biological parents of hyperactive boys compared with controls.
Molecular Genetic Studies: The Tranmission Disequilibrium Test (TDT) was used in a family-based association study to identify an association between ADHD and a specific allele at the dopamine transporter locus on 5p (p = 0.006). Another population-based association study found an association between ADHD and an allele at the dopamine D4 receptor on 11p (p = 0.01).