The Human Heritage:Genes and the Environment

Nature/Nurture

• First coined by Richard Mulcaster (1582)

• Refers to the ‘twin forces on development’– Biological and environmental influences

• Nature– Inborn biological endowment

• Nurture– environment within which development occurs

• Interaction of Forces

Sexual Reproduction and Genetic Transmission

• Chromosomes– Single molecule of DNA, thousands of genes– 23 + 23 = zygote

• Genes– segments of DNA, double helix, ladderlike– blueprint for synthesis of protein molecule– units of heredity

Sexual Reproduction and Patterns of Heredity

• The basics– somatic cells hold genetic info on copies of 46

chromosomes inherited at conception– Chromosomes dispersed throughout the nucleus– 23 pairs of chromosomes

• 22 autosomes, 1 sex chromosome

• males xy

• females xx

Cell Division: Somatic Cells

• Mitosis– for somatic (body) cells– each cell replicates itself, then divides– result is two cells identical to original

Cell Division: Germ Cells

• Meiosis– for germ cells– replication with 3 processes to ensure resulting

cell is nonidentical to germ cells (genetic diversity: chances are 1/70 trillion)

• 2 steps of division

• crossing over

• random pairing during 2nd division

– monozygotic vs. dizygotic twins

Genotype/Phenotype

• Genotype– the set of genes you inherit, your unique

blueprint

• Phenotype– how that set of genes is expressed as a result of

the environmental stresses– the observable characteristics

Environment

• Genetic code is expressed in environments

• Multi-levels of influence possible (egs.)

– prenatal hormonal environment

– chemical environment of the cell

– nutrition of mother during pregnancy

– stress of mother

– opportunity for physical exercise

– birth order as it relates to attention from parent

Not Exactly Identical

Mendel’s Laws of Inheritance• Definitions of terms

– locus– homozygous– heterozygous– allele– dominant– recessive– intermediate expression of trait

Laws of Inheritance

• Discrete traits inherited on the basis of allele information from a single locus– Chin cleft– Blood type

• Polygenetic traits inherited as result of information from several genes– Verbal skill– shyness

Codominance: Result is distinct characteristics

Genetic Sex also Source of Variability

Sex Linked Genetic Effects:Red/Green Colorblindness

• recessive trait

• chances are 1/10 for males

• chances are 1/100 for females – gene carried on x chrom, males have no

complementary allele to compensate for the harmful effects

Gene-Environment Interactions

• Behavioral geneticists study how genes and environment combine to influence organism’s development– Variations in environment can have large effects on

development of phenotype

– Gene-environment interaction is a two-way process

– Genetic factors play a role in the environments that individuals inhabit and how they shape their environments

Range of Reaction: charting the change in phenotype as environment changes

Canalization: some characteristics restricted to narrow range despite wide

variation in environment

Heritability: The study of genetic influences on behavior

• Attempt to assess the degree to which variation among individuals on a given trait can be attributed to genetic as compared to environmental differences– e.g., weight - a physical trait– e.g., intelligence - a psychological trait

• can factors in the environment influence weight or intelligence?

Heritability, cont’d• Degree to which variation among

individuals on a particular trait can be attributed to genetic differences

• h2 = variation due to genes• total variation• applies to groups and not to individuals• for clones, any variation has to be due to

environment, thus h2=0• h2=1- variance accounted for by environment• heritability increases as genetic diversity

increases and decreases as genetic diversity decreases

Heritability: Infant Temperament

• This video from the laboratory of Hill Goldsmith shows part of a study about infant temperament. An object of desire is set in motion, first for each infant in a pair of fraternal twins and then for each infant in a pair of identical twins. Note how the twins react when prevented from reaching the desired object.

Heritability of Intelligence: History

• Francis Galton’s Eminent Persons Study– tested idea that differences in intellectual

achievement was due to genetic differences– basic flaw in logic is that eminent people also shared

similar environment as well as gene pool– first to use adoptive method in study of boys

adopted by RC Popes; found little evidence that environment affected eminence

– Recommendations made for selective breeding, as they were to be made later by Hitler

– Interesting aside: Darwin’s cousin

Heritability of Intelligence: Kinship Studies

• Twin Methods– correlations among IQs of pairs of people

related to different degrees, e.g., identical twins, dizygotic twins, siblings,

• Adoptive Methods– correlations among pairs of people who

share an environment but not genes, e.g., adopted child and biological child

Intelligence: The Findings

• Identical twins raised apart .72• Identical twins raised together .86• Same sex twins raised together .62• Diff sex twins raised together .57• Siblings .47• Unrelated siblings .30

Heritability of Schizophrenia

• Adoption Studies– e.g., Kety et al (1976) found strong evidence of

heritability• only biological relatives of adoptee who is

schizophrenic show high incidence of schizophrenia

Heritablity of Schizophrenia• Twin Studies

– High concordance for identical twins, this concordance deceases as genetic relatedness decreases

• Identical twins 48% concordance

• Fraternal twins 17%

• Siblings 9%

• Half-sibling 6%

• Fisrt cousin 2%

• Child of two sch. Parents 46%

• Child of one sch. Parent 13%

Mutations and Genetic Abnormalities

• Mutations are errors in the process of gene replication; can be positive but usually negative

– Down Syndrome (chromosomal error)– Phenylketonuria (PKU- defective recessive

gene)– Klinefelter syndrome (sex-linked

chromosomal abnormality)– Sickle-cell anemia (recessive gene)

Chromosomal Errors• Most incompatible with survival

• Variations in number sex chromosomes– Turner’s XO– Klinefelter’s XXY– XYY

• Variations in number of other chromosomes– Down’s Syndrome, Trisomy 21– Edward’s Syndrome; Trisomy 17

PKU

• Recessive gene disorder

• causes excessive buildup of phenylalaline causing brain damage

• environmental intervention (diet) can reduce effects

Huntington’s Disease• dominant gene disorder• 50% chance of offspring inherit the

disorder• 1/15,000 incidence• charcterized by severe mental and

physical disorder• lag in onset of symptoms (35-45 yrs)

accounts for continued occurrence• can be detected through genetic testing

Biology and Culture

• Cultural evolution can also account for existence of skills in a population– Innovations passed onto succeeding generations– Through processes of direct instruction,

language, and demonstration– Little evidence of cultural evolution in other

species– Meme: basic unit of cultural evolution

Cultural and Survival

• Some cultural tools may ensure survival– Hunting, gathering methods– Language– Child rearing practices (Quechuan babies)

Coevolution

• Two forms of evolution (cultural and biological) influence each other

• Evidence that culture has influenced biology through the impact it has on reproductive advantage (e.g., tool use)

• The problems of sorting out the nature-nurture influence