Chapter 3Chapter 3

Behavior Genetics

The Human Genome• Genetic Roadmap• Entire genetic code of the

human species

Genetic StructuresGenetic Structures• DNA - Dioxyribonucleic

Acid• Genetic Code• Genetic Alphabet = 4

Letters– A: Adenine– C: Cytosine– G: Guanine– T: Thymine

Genetic “Words” Genetic “Words” • Made up of two strands of

DNA molecules• Infinite in length (e.g..

ACGCTCTCGA)• Because DNA is a Double

Helix exact replication is possible

• Mutations result when “proofreading fails

ChromosomeChromosome• Continuous Strands of

DNA• Locations which have

functions are Genes• 46 Chromosomes in

humans• Located as 23 Pairs• 22 Pairs are Autosomes• 1 Pair are Sex

Chromosomes

Gene = Location on ChromosomeGene = Location on Chromosome

“Parsimony” of Genetics “Parsimony” of Genetics • 4 Bases (ACGT) produce

enormous diversity• Length & uniqueness of

ACTG which produces diversity

• Double helix of DNA allows exact replication during Mitosis

• Errors in mitosis allows further variability

Relative Size of Human Chromosomes

Genetic Functions• Mitosis: Cell & Genetic

Duplication• Meiosis: Biparental

Reproduction

Mitosis = Replication ProcessMitosis = Replication Process

Genetic Variablity• Random Assortment

(Meiosis)• Crossing over (Meiosis)• Mutations (Meiosis or

Mitosis)

Random AssortmentRandom Assortment

• Occurs during later stage of Meiosis, as gametes are being formed

• Pairs of chromosomes go to opposite ends of cell

• 1/2 of chromosomes (1 from each pair) go to new cell (gamete)

• Which individual of pair is random• Produces 1 in 64 trillion chance of

duplication• This is multiplied by incalculable effects of

mutation & crossing-over

Genetic VariabilityCrossing OverGenetic VariabilityCrossing Over•Occurs during Meiosis•Occurs prior to gamete formation•Result is “new” chromosome

Genetic Variability - MutationGenetic Variability - Mutation• Error during Mitosis

Genetic Variability -Bisexual ReproductionGenetic Variability -Bisexual Reproduction• Estimated that each

human has the capacity to produce 103000 different combinations in gametes

• Estimated that all the men who ever lived have produced only 1024 sperm

• Extremely unlikely that there has ever been a genetic duplicate

• Exception - Identical (Monozygotic) Twins

Genotype = Genetic Make-UpGenotype = Genetic Make-Up• Homozygous

– e.g.., BR BR

• Heterozygous– e.g.., BR bl

• One allele comes from each parent

Alleles - Alternate forms of GenesAlleles - Alternate forms of Genes

• Produce variability• Dominant Gene (allele)• Recessive Gene (allele)

Phenotype - Expression of CharacteristicsPhenotype - Expression of Characteristics

• Expression of– Genotype– Environment– Transaction

• Examples- – Down Syndrome (Life

expectancy of 12)– Height - since 1900-1

cm/decade– Age of Menarche

Age of MenarcheAge of Menarche• Norway

– 1840 - Reached at 17 years– 1990 - Reached at 13 years

• U. S.– 1890 - Reached at 14– 1990 - Reached at 12-13

Variation in PhenotypesDue to GenotypesVariation in PhenotypesDue to Genotypes•Dominant Alleles•Recessive Alleles•Homozygous•Heterozygous•Sex-Linked•Co-Dominance•Incomplete Dominance

Genotype = Both Parents Homozygous Dominant (BRBR - BRBR)

Genotype = Both Parents Homozygous Dominant (BRBR - BRBR)Genotype PhenotypeBR-BR BROWNBR-BR BROWNBR-BR BROWNBR-BR BROWN• All Genotypes are

homozygous BRBR• All Phenotypes are brown

(dominant)

Genotype=Parents Homozygous recessive (blbl - blbl)

Genotype=Parents Homozygous recessive (blbl - blbl)Genotype Phenotypeblbl blueblbl blueblbl blueblbl blue• All genotypes are

homozygous recessive• All phenotypes are blue

Genotype=1 Parent homozygous dominant (BRBR) 1 Parent homozygous recessive (blbl)

Genotype=1 Parent homozygous dominant (BRBR) 1 Parent homozygous recessive (blbl)

Genotype PhenotypeBRbl BrownBRbl BrownBRbl BrownBRbl Brown• All genotypes are heterozygous• All phenotypes are brown

Genotype= Both Parents Heterozygous (BRbl - BRbl)Genotype= Both Parents Heterozygous (BRbl - BRbl)• Both parent phenotypes are

BrownGenotype PhenotypeBRBR (Homozygous) BrownBRbl (Heterozygous) BrownblBR (Heterozygous) Brownblbl (Homozygous) blue• 3-in-4 chances phenotype will be

Brown• 1-in-4 chance phenotype will be

blue• How generation can be skipped

Sex-Linked CharacteristicsSex-Linked Characteristics• Characteristic is recessive• Location is on the Y

chromosome• Male who inherits that

recessive (MPB) has no dominant to mask recessive

• Female who inherits that recessive is likely to have a dominant on other Y

• These females are “carriers” who may pass Y based recessive to son.

Forget It!Forget It!• Human behavior is not

single gene• Mostly polygenic• Genes do not directly

produce behavior• Genes work through the

structures they enable• Human Behavior is

multifactorial

HeritabilityHeritability• Genotype• Shared Environmental

Effects• Nonshared Environmental

Effects• Measurement Error

Heritability - GenotypeHeritability - Genotype• Monozygotic Twins -

Identical Genes• Dizygotic Twins - Same as

siblings• Siblings - Share parent

gene pool• Unrelated children - share

broad gene pool

Shared Environmental EffectsShared Environmental Effects• All persons living in same

environment• Generally very low in

heritability estimates

Nonshared Environmental EffectsNonshared Environmental Effects• Effect on individual• No 2 persons (even living

together in same house) have same environment

• Determined by subtracting out Monozygotic twins living in same living situation (e.g..., NS=1.00-.76; NS=.24)– (.76 =genotype + Shared

Env. + error)

How Do Genes & Environment Interact?Genotype/Environment Effects

How Do Genes & Environment Interact?Genotype/Environment Effects

e.g..., height, weight, muscles, hair color(?), skin color

• Passive• Active• Evocative

A Behavioral View: Person/Environment Interactions A Behavioral View: Person/Environment Interactions e.g.., clothing, make-up,

behavior, skills, hair color?

• Passive• Evocative• Active

What Canalizes Behavior• Canalization – Increasing

restriction of outcome as development proceeds

• Genetic Canalization?• Experiential

(Environmental) Canalization?