Introductory Psychology:
Biological Bases of Behavior
AP PSYCHOLOGY: UNIT II
Topic: Behavior Genetics
“We share half of our genes with a banana…” – Robert May
What is Behavior Genetics?
 Behavior Genetics
 The
study of the
relative power &
limits of genetics
and environmental
influences on
behavior
 Nature
v. nurture
The Biological Bases:
Basic Genetic Principles
PART ONE
Basics: Genes – Our Code for Life
 Chromosomes
 Strands of DNA
(deoxyribonucleic acid) that
carry genetic information;
located in the nucleus of every
human cell

Humans: 46 chromosomes (23/23)
 Genes
 Segments of DNA molecules


Contain code for production of
proteins
Key functional units in hereditary
transmission (biological
development)
Autosomes
Sex
Chromosomes
Basics: Genes –
Our Code for Life
Like chromosomes, genes
operate in pairs, with one
gene in each pair coming
from each parent
 Homozygous
Condition
Paired genes are the same
 Heterozygous Condition
Paired genes are different
In this case, one gene in the pair masks the
other
Basics: Genes – Our Code for Life
 Genotype
 A person’s genetic makeup; determined at
conception and fixed forever
 Phenotype
 The ways in which a person’s genotype is
manifested in observable characteristics
 Person’s actual physical characteristics
 Height, eye color, disease history, general
disposition
• Different genotypes can yield the same phenotype

May change over time
Basics: Genes – Our Code for Life
 Genetic & Chromosomal Problems

Cystic Fibrosis
 Two recessive genes;
 Disorder that affects respiratory & digestive tract

Sickle Cell Anemia
 Two recessive genes; blood disorder
 Abnormality in shape of red blood cells

Tay-Sachs Disorder
 Two recessive genes; fatal neurological disorder
 Destroys nerve cells in the brain and spinal cord
Basics: Genes – Our Code for Life
Genetic & Chromosomal Problems

Down Syndrome
 Extra chromosome
(21st pair)
 Causes lifelong
intellectual disability,
developmental delays,
and health problems

Turner Syndrome
 Missing X chromosome
(e.g. infertility, sexually
underdeveloped, etc.)
The Biological Bases:
Heritability
PART TWO
Twin studies:
The ultimate natureversus-nurture laboratory
What is Heritability?
 The extent to which the
differences among people are
attributable to their genes
 Studies
focus on:
 Individual
differences v. Group differences?
 Obesity: (2010)
Americans – 28%
Japanese – 3%
 Nature
v. Nurture?
 We all have the capacity to learn language, but
we do not all speak the exact same
Heritability: Twin Studies
Monozygotic
Dizygotic
Heritability: Jim Lewis & Jim Springer
 Middle class
 Wife: Betty
 Son: James Alan (Allan)
 Dog: Toy
 Hobby: Woodworking
 Bad Habits: Chain smoker, fingernail biting
 Other: Drove a Chevy, watched stock-car racing
 Health conditions: High blood pressure &
occasional migraines
Identical twins
Gerald Levey and
Mark Newman
Idealized View of an Adoption Study
 Parent-offspring
 Genetic and environmental
resemblance can arise for
both genetic and
environmental reasons
Bio and
Rearing
Parent
Environmental
Genetic
Offspring
contributions to parentoffspring; Resemblance are in
principle separate
Birth
Parent
Adoptive
Parent
Genetic
Environmental
Offspring
Heritability: Twin Studies
Used to determine the heritability of a trait
https://www.youtube.com/watch?v=IyT2AlzzzQs
Heritability: Adoption Studies
 Compare children's
traits with those of their
biological and adopted
parents

Studies suggest that
adoptees tend to be more
similar to their biological parents

Two adopted children in the same household
bear no more resemblance to each other than two
biological siblings - in terms of personality
Heritability: Adoption Studies
 S0 does parenting
even have an effect?


It has an effect on
biologically related and
unrelated children alike
Early stimulation is key
to proper development
 Effects
attitudes, values, manners, beliefs, faith, etc.
 Different from temperament and personality…
“Heredity deals the cards, environment plays the hand…”
“Mutation and natural
selection are the engines
of evolution”
Genetic mutations
• What causes a genetic mutation?
• How often do mutations occur?
• How have mutations been of
benefit to the human population?
https://www.youtube.com/watch?v=qVmusHZtQms
NATURE VS. NURTURE
AND
GENDER
Sexual Birth Defects
Ambiguous genitalia
 Androgen insensitivity syndrome (AIS)
 Fetal tissue insensitivity to male hormones
 Affects genital development
 Congenital adrenal hyperplasia (CAH)
 Inherited condition – affects hormone production
 Excessive manufacture of male hormones
 Sex chromosome disorders
 Baby has mixture of XX and XY sex chromosomes
 Y chromosome may be inactive; one of X chromosomes may
have a tine Y segment attached
 Maternal factors
 Other factors during pregnancy that impact hormone production
Understanding Gender
 Gender roles
 Our expectations about the way
men and women behave

Influenced by culture
 Gender identity
 Sense of being male or female
 Gender-typing
 Exhibiting traditional gender
traits and interests
 Transgender
 Persons whose gender identity,
gender expression or behavior
does not conform to sex
assigned at birth
 Social learning theory
 Children learn gender-linked
behavior by observing and
imitating and being rewarded or
punished
 Gender schema theory
 Concept of what it means to be
male or female


Gender becomes lens through
which we view our experiences
Based on social learning theory