Genetic and environmental factors discrimination in attention deficit hyperactivity disorder. Resumen Con el fin de elucidar los componentes geneticos y ambientales implicados en la susceptibilidad a desarrollar por deficit de atencion con hiperactividad (TDAH ), se llevo a cabo un analisis de segregacion compleja en familias nucleares (n = 53 ) obtenidas de individuos afectados pertenecientes a Medellin , en el Estado de Antioquia , Colombia. Metodos y resultados. Los modelos de efecto cohorte ( noninheritance), gen multifactorial , mayor recesivo, nonmajor componente genetico y nontransmission de gen mayor fueron rechazadas. Por el contrario, los principales modelos dominantes y codominantes de genes y componentes nonmultifactorial no podia ser rechazada. Por lo tanto , el modelo de mejor ajuste de los datos era el del gen principal (dominante / codominante) . Este gen principal explica mas del 99 , 99 % de la varianza fenotipica TDAH ( valor de heredabilidad en el modelo mixto igual a 0,007 %), que permiten suponer un aporte bajo del componente ambiental para el TDAH fenotipo . F recuencia del gen mayor fue del 3% en la poblacion general de Antioquia y su penetrancia fue cerrado a 30 %. Palabras clave: deficit de atencion e hiperactividad, genetica del comportamiento, analisis de segregacion compleja, la genetica, el gen principal, modelo multifactorial Abstract In order to elucidate the genetic and environmental components involved in the susceptibility to develop attention deficit hyperactivity disorder (ADHD), a complex segregation analysis on nuclear families (n= 53) ascertained from affected probands belonging to Medellin, in the Antioquian State, Colombia, was performed. Methods and results. Models of cohort effect (noninheritance), multifactorial, recessive major gene, nonmajor gene component and nontransmission of major gene were rejected. Contrarily, dominant and codominant major gene models and nonmultifactorial component could not be rejected. Thus, the better model fitting the data was that of the major gene (dominant/codominant). This major gene explains more than 99, 99% of the ADHD phenotypic variance (value of heritability in the mixed model equal to 0.007%), which permit to assume a low aport of the environmental component to the phenotype ADHD. Gene frequency of the major gene was 3% in the general population of Antioquia and its penetrance was closed to 30%. Conclusion. Some cautions and aspects related to the bias of the interview and diagnosis of the parents are discussed [REV NEUROL 1999; 28: 660-4]. Key words: Attention deficit hyperactivity disorder, behavioral genetics, complex segregation analysis, genetics, major gene, multifactoral model.