Hardy–Weinberg equilibrium analysis of the 48 bp VNTR in the III exon of the DRD4 gene in a sample of parents of ADHD cases

The aim of this study was to obtain the genotype and gene frequency from parents of children with attention-deficit/hyperactivity disorder (ADHD) and then assess the Hardy–Weinberg equilibrium of genotype frequency of the variable number tandem repeat (VNTR) III exon of the dopamine receptor D4 (DRD4) gene. The genotypes of the III exon of 48 bp VNTR repeats of the DRD4 gene were determined by polymerase chain reaction in a sample of 30 parents of ADHD cases. In the 60 chromosomes analyzed, the following frequencies of DRD4 gene polymorphisms were observed: six chromosomes (c) with two repeat alleles (r) (10%); 1c with 3r (1.5%); 36c with 4r (60%); 1c with 5r (1.5%); and 16c with 7r (27%). The genotypic distribution of the 30 parents was two parents (p) with 2r/2r (6.67%); 1p with 2r/4r (3.33%); 1p with 2r/5r (3.33%); 1p with 3r/4r (3.33%); 15p with 4r/4r (50%); 4p with 4r/7r (13.33); and 6p with 7r/7r (20%). A Hardy–Weinberg disequilibrium (χ(2)=13.03, P<0.01) was found due to an over-representation of the 7r/7r genotype. These results suggest that the 7r polymorphism of the DRD4 gene is associated with the ADHD condition in a Mexican population.