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◦Characteristics like eye color, height, IQ

Genotype: genetic makeup a person inherits (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

Genetic similarity◦Degree of trait similarity

Shared environmental influence◦Living in the same home

Non-shared environmental influences◦Unique experiences

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