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)
Sex
 Genes
Autosomes
Chromosomes
 DNA segments; key functional
units in hereditary transmission
 Can be active (expressed) or inactive
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; may change over time
**Different genotypes can yield the same phenotype
 **Genes are pliable

 Polygenic Inheritance
 Characteristics that are influenced by more than one
pair of genes
Basics: Genes – Our Code for Life
 Genetic & Chromosomal Problems
 Cystic Fibrosis


Sickle Cell Anemia


Two recessive genes; blood disorder
Tay-Sachs Disorder


Two recessive genes; respiratory & digestive tract
Two recessive genes; fatal neurological disorder
Phenylketonuria

Two recessive genes; difficulty digesting a certain amino
acid – phenylalanine
Basics: Genes – Our Code for Life
 Genetic & Chromosomal
Problems

Down Syndrome


Klinefelter’s Syndrome


Extra chromosome
(21st pair)
Extra X chromosome;
reduced masculine characteristics (e.g. enlarged breasts,
obesity, etc.)
Turner’s Syndrome

Missing X chromosome (e.g. infertility, sexually
underdeveloped, etc.)
The Biological Bases:
Heritability
PART TWO
The ultimate natureversus-nurture laboratory
What is Heritability?
 The extent to which the
differences among people
are attributable to their
differing genes

What percentage of the difference
among height can be attributed
to genes?



90%
Individual differences v.
group differences?
Nature v. Nurture?
Heritability: Twin Studies
Monozygotic
Dizygotic
Heritability: Twin Studies
Used to determine the heritability of a trait
Heritability: Jim Lewis
 Middle class
 Wife: Betty
 Son: James Alan
 Dog: Toy
 Hobby: Woodworking
 Bad Habits: Chain smoker, fingernail biter…
 Other: Drove a Chevy, watched stock-car
racing & drank Miller…
 Conditions: High blood pressure &
migraines…
Heritability: Jim Springer
 In 1980, Jim Springer called
Jim Lewis (37 year separation)

Everything down to their dog’s
name was the same (except sons
James Allan & James Alan)
 When listening to each other's voices, they
would mistake themselves for the other
 First in Thomas Bouchard’s “Minnesota
Twin Study”
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?


YES!
It has an effect on
biologically related and
unrelated children alike
Effects attitudes, values,
manners, beliefs, faith,
etc.
 Different from personality…
 “Heredity deals the cards, environment plays the
hand…”

The Biological Bases:
Molecular Genetics
PART THREE
DEFINITION: A branch
extension of behavior genetics
that asks the question, “Do
specific genes influence
behavior?”
GOAL: To find some of the many genes that influence
normal human traits (e.g. body weight, sexual
orientation, extraversion)