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Disorders of Childhood and Adolescence- ADHD (4).docx

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McMaster University
Richard B Day

September 26 , 2012 Psych 2AP3: Abnormal Psychology – Major Disorders Disorders of Childhood and Adolescence: ADHD (4) Twins - monozygotic (identical) twins: born from single fertilized egg. Not genetically identical. Single nucleotide polymorphisms or copy number variations. Epigenome. In terms of genetic activity, they are not identical. Develop from a single fertilized egg. Share 100% of genes and 100% of gene effects - dizygotic (non-identical) twins: can be of opposite sexes. On average the members of a dizygotic twin pair share 50% of genes. 50% additive, 25% non-additive effects - how do we use twins to study disorders: if any phenotypic character is completely dependent on genes then there is a 100% change that monozygotic twins will share this characteristic. We look at the concordance rate. If concordance rate is high, then we are confident in saying it is heavily genetic. If it is low we know there is an impact of experience on this character. We expect the same thing in dizygotic twins. - Most mental disorders are relatively rare to find in monozygotic twins. Heritability Coefficient (h ) - proportion of variability in phenotype accounted for by genetic not environmental variability in the population - not about any single individual, it is a statement about group variability 2 - h =2(r MZ )DZ - binary variable - ranges from 0.00to 1.00 (0-100%) - refers to populations, not individuals - not a constant: because relative variability produced by genetic and environmental aspects. Variability changes over time. Heritability coefficient
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