Behavioral Genetics: Predicting Individual

DifferencesModule 14

Genes: Our Codes for Life• Chromosomes are long twisted strands

of deoxyribonucleic acid (DNA) found in the nucleus of the cell

• Humans have 23 pairs of chromosomes• DNA is the chemical basis of heredity and carries instructions• DNA code carried on each chromosome is arranged in

thousands of segments called genes. • Genes are the basic unit of heredity

– Complex traits are formed from different combinations of genes– Can be activated by environment

Expressing Genes: Dominant & Recessive

• Genotype—Set of genes you’re born with• Phenotype—traits that are expressed/displayed (influenced by genes &

environmental factors• Dominant genes—will always be expressed if present• Traits such as freckles, dark eyes, dark hair, and dimples are referred to as

dominant characteristics because they require only one member of a gene pair to be dominant for the trait to be displayed

• Recessive genes—will expressed only if paired with an identical recessive gene. Will not be expressed if paired with a dominant gene.• We inherit from our biological parents a genetic potential, the expression of

which can be influenced by environmental conditions.

Characteristic Dominant Recessive

Eye color Brown Grey, blue

Hair Dark Curly

Light Straight

Hands Extra fingers Limb dwarfing

5 fingers Normal limbs

Face Broad lips Dimples

Thin lips No dimples

Twin Studies

(Monozygotic)(Dizygotic)

Monozygotic Twins:

•Share 100% same genes but can have different phenotypes

Differences can be attributed to:

•Don’t always have same copy of genes

•If develop in different placentas

Dizygotic Twins:

•Genetically no more similar than ordinary siblings

•Share same fetal environment

Twin Studies & Family Influence Thomas Bouchard & the Minnesota Twins studies

• If trait genetic:• closely related more similar than less closely related

• Many close relatives share environments too• Types of studies to separate effects

• monozygotic twins reared together• monozygotic twins reared apart• siblings/dizygotic reared together• siblings/dizygotic reared apart• adoptive siblings reared together

• Personality characteristics does not seem to be heavily influenced by environment.• Environment does impact attitudes, values, manners, faith & politics• See the famous case of the Jim Twins (watch first 8 min)

Nature or Nurture?

Twin Studies & IQ

Molecular Genetics

• Seeks to identify specific genes that influence behavior• Hope is to predict the potential of a problem and then take steps to

prevent it.• Ethical concerns in regards to if this is a good idea?

Heritability

• How much of our differences can be attributed to our differing genes?• Refers to the variation between the group, NOT the influence of

nature or nurture.• Heritable trait is one that is capable of being passed down from

parent to child.• The more similar the environment, the more heritability can explain

differences between people.

Gene-Environment Interaction• Genes react to the environment.

- Example: nutrition affecting if you’ll reach your genetic potential for height- See Nature & Nurture: The Study of Twins (4 min) – Prenatal environmental differences can have long term effects but environment can help one reach their potential.• Environment acts in response to what genes have given us.

- Example: good looking people are typically treated more kindly by society• Selection Effects – We select environments that suit our nature.• Epigenetics – studies how environment can trigger or block genetic expression

• Diet, drugs, stress affect epigenetic molecules on your DNA• This explains why two identical twins could have the genetic precursor for a mental disorder but only one

ends up having it.