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Zygote: Union of sperm & ovum at conception Contains 23 pairs of chromosomes One pair from each parent Each pair influences a characteristic Chromosomes: thousands of genes containing DNA Meiosis: process producing sperm, ova Mitosis: cell-division process creating all other cells throughout life Massive genome analysis projects 999/1000 human base chemicals: identical 1/1000 accounts for differences between us Humans/Chimps share 96% genetic material Gene variants evolved in recent centuries ◦ Adaptations to food sources, diseases,etc. Findings also useful to identify genes associated with disease, drug treatments Monozygotic (MZ) twins: 100% related ◦ Single zygote divides ◦ 2 genetically identical individuals Dizygotic (DZ) twins: 50% on average ◦ 2 ova fertilized by 2 sperm Siblings: 50% on average Parent & Child: 50% related, shared Males: XY; Females: XX Genes: instructions for development Genotype: genetic makeup a person inherits ◦ Characteristics like eye color, height, IQ (potential) e.g., genes for tallness Phenotype: actual/expressed trait (height) Regulator genes turn gene pairs on/off at different times ◦ Turned on for adolescent growth spurt ◦ Turned off in adulthood Always influenced by environmental factors also Single gene-pair inheritance ◦ Dominant gene = dominant trait ◦ Recessive genes Trait expressed if paired with a similar gene (Homozygous) Trait not expressed if paired with dissimilar gene (Heterozygous) ◦ Recessive traits: homozygous recessive ◦ Dominant traits: hetero or homozygous gene pair About 9% affected in US ◦ Homozygous recessive Heterozygous are “carriers” ◦ Can transmit gene to offspring ◦ If both parents carriers: 25% chance Example of incomplete dominance ◦ Offspring may have sickling episodes Single genes located on sex chromosomes Actually X-linked Males - no counterpart on Y chromosome ◦ Only needs one to be color-blind Females - counterpart on 2nd X chromosome ◦ Usually for normal color-vision (dominant) ◦ Must inherit on both to be color-blind Also Hemophilia, Duchene MS, others X-Linked Inheritance For most important human characteristics ◦ Height, intelligence, temperament, etc. Trait influenced by multiple pairs of genes These traits are normally distributed ◦ I.e., found in the same proportion in all populations A change in gene structure/arrangement Produces a new phenotype More likely in sperm than in ova May be harmful or beneficial Can be inherited by offspring Errors in chromosome division during meiosis ◦ Too many or too few chromosomes result ◦ Most spontaneously aborted Down Syndrome: Trisomy 21 ◦ Physical characteristics ◦ Mental retardation ◦ Related to age of both parents ◦ Often develop Alzheimer’s in middle age The rate of Down syndrome births increases steeply as the mother’s age increases. Turner’s syndrome: 1/3000 females ◦ Single X chromosome (XO) : small, unable to reproduce, stubby fingers, webbed neck Klinefelter syndrome: 1/200 males ◦ XXY: Sterility, feminine traits Fragile X syndrome: one arm on X is fragile (Leg of X barely connected) ◦ Usually males (sex-linked inheritance) ◦ Most common heredity cause of MR Helps people understand and adapt Prenatal diagnosis: techniques include ◦ Amniocentisis, preimplantation Human genome project yielded much info Tay-Sachs disease ◦ Cause: recessive gene pair (European Jews/French Canadians) Huntington’s disease ◦ Deterioration of nervous system ◦ Single dominant gene ◦ One affected parent = 50% chance in offspring Genetic/environmental cause of traits Heritibility estimates proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. Experimental and selective breeding ◦ Tryon’s maze-bright rats Twin, adoption, family studies ◦ Reared together or apart ◦ Concordance rates--twins are concordant if they both display a trait of interest Genetic similarity ◦ Degree of trait similarity Shared environmental influence ◦ Living in the same home Non-shared environmental influences ◦ Unique experiences Analysis of genes and their effects ◦ May compare humans with other animals eg. Alzheimer’s disease ◦ Most common form of old age dementia ◦ Twin studies show heritability ◦ Possible genetic links being tested ◦ Environmental factors also being tested High cholesterol, head injury Correlations highest in identical twins ◦ Genetic factors determine trait Correlations higher if twins reared together ◦ Environmental factors ◦ Non-shared experiences influential Identical twins more alike with age Temperament Correlations ◦ Identical twins = .50 to .60 ◦ Fraternal twins = 0 (even reared together!) Personality Correlations Similar ◦ Shared environment unimportant ◦ Genetic inheritance important ◦ Non-shared experiences important for differences Correlations between the traits of identical twins raised apart in Minnesota Twin Study. Schizophrenia concordance rates ◦ ID twins: 48% ◦ Fraternal twins: 17% ◦ Affected parent increases risk even if adopted at birth Inherited predisposition ◦ Environmental factors (triggers) ◦ Prenatal exposure to infection suspected