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Lecture 42 Prof Duncan Shaw Genetic variation & normal traits • Normal traits include height, IQ, blood pressure • These are influenced by many genes (called “polygenes”) and the environment • In a large population, they are distributed according to “normal distribution” • Genetic influence is apparent when trait is correlated in related individuals – Higher degree of relationship trait higher correlation for The Normal distribution SD = standard deviation Degrees of relationship Relationship Average % shared genes 1st degree MZ twins 100 DZ twins, sibs, parents/offspring 50 2nd degree Uncles/aunts/nephews/nieces 25 Grandparents/grandchildren 25 3rd degree 1st cousins 12.5 Great-grandparents/great-grandchildren 12.5 Genetic variation & disease • Many diseases are familial and have a genetic component - increased incidence in relatives of affected individuals • Suggests involvement of polygenes • Unlike normal traits, disease is a “discontinuous trait” - either you’ve got it or you haven’t • Described by the “liability threshold” model Some diseases with genetic influence • Congenital malformations – Cleft lip/palate, neural tube defects – Heart defects, hip dislocation • Adult onset – – – – Diabetes Epilepsy, migraine Cancer (many types) Manic depression, schizophrenia Consequences for patient’s relatives • In diseases with a genetic component, patient’s relatives are at greater risk than general population • For cleft lip/palate: – – – – Population risk 0.1% Risk to 1st degree relatives 4% Risk to 2nd degree relatives 1% Risk to 3rd degree relatives 0.5% • Risk is greater if patient is more severely affected • Risk is greater if >1 affected relative in family Heritability • The proportion of the trait (or disease) that is due to genetic factors • Estimated by comparing degree to which 1st, 2nd or 3rd degree relatives of patient are affected • Also estimated by comparing concordance rates in MZ and DZ twins • About 80% for schizophrenia, asthma; 65% for coronary artery disease; 35% for congenital heart disease Which genes are important? • Test “candidate genes” to see if they are “genetically associated” with the trait Gene A Gene B Allele: With trait Without With trait Without 1 97 100 80 100 2 103 100 120 100 • Example: diabetes and insulin gene