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Heredity and Behavior
Is it all in the genes? NARTURE

Behavioral Genetics- an
interdisciplinary field that studies the
influence of genetic factors on behavioral
traits
Chromosomes and Genes

Chromosomes are strands of DNA
molecules that carry genetic information
◦ Every cell in the human body contains 46
chromosomes (except sex cells) 23 pairs
(each parent)
 Zygote- a single cell formed by the union of a
sperm and an egg
 Genes are the DNA segments that serve as the
key functional units in the hereditary transmission
Why don’t family members look the
same?
Each parents 23 chromosomes can be
scrambled in over 3 million different ways,
yielding over 8 million configurations
 Homozygous condition –two genes in
a specific pair are the same
 Heterozygous condition – two genes
in a specific pair are different

Dominant Gene expressed when
paired genes are different
 Recessive Gene one that is masked
when paired genes are different
 Detached earlobe (D) Attached (R)

Genotype vs. Phenotype

Genotype- a person’s genetic makeup
◦ Determined at conception/fixed

Phenotype- how a person’s genotype is
manifested in observable characteristics
◦ may change over time/modified by
environmental factors
Polygenic Inheritance

Characteristics that are influenced by
more than one pair of genes
◦ 3/5 gene pairs determine skin color
Research Methods
We can’t manipulate and do selective breeding on humans

Family Studies- researchers assess
hereditary influence by examining blood
relatives to see how much they resemble
one another on a specific trait
◦ Schizophrenic studies
◦ Also indicate whether or not a trait runs in a
family

Twin Studies- researchers assess
hereditary influence by comparing the
resemblance of identical twins and fraternal
twins with respect to a trait
◦ Identical Twins (monozygotic twins) emerge
from 1 zygote that splits for unknown reasons
◦ Fraternal Twins (dizygotic twins) result when
two eggs are fertilized simultaneously by different
sperm cells, form two separate zygotes
◦ Useful because we can look at environmental
conditions

Adoption Studies- assess heredity
influence by examining the resemblance
between adopted children and their
adoptive parents
◦ Genetic/environmental influence on traits

Genetic Mapping- the process of
determining the location and chemical
sequence of specific genes on specific
chromosomes
◦ Human Genome project
◦ Researchers seek to determine things such as
the genetic basis for intelligence, extroversion,
schizophrenia, musical ability, and other
behavioral traits
 Virtually all behavioral traits tend to be polygenic
and are shaped by many genes
THE EVOLUTIONARY
BASES OF BEHAVIOR
Darwin
Natural selection
 Organisms vary in a variety of ways
 Some characteristics are heritable

◦ Variations in heredity are the basis of
evolution
◦ Fitness- refers to the reproductive success
of an individual organism relative to the
average reproductive success of the
population
 Variations in reproductive success are what actually
fuels evolutionary change

Natural Selection posits that heritable
characteristics that provide a survival or
reproductive advantage are more likely
than alternative characteristics to be
passed on to subsequent generations and
thus come to be ‘selected’ over time.
◦ Populations NOT individuals and a gradual
occurance
Refinements to Evolutionary Theory

Adaptation- an inherited characteristic
that increased in a population(through
natural selection) because it helped solve a
problem of survival or reproduction during
the time it emerged
◦ (preference for fatty foods)
◦ Inclusive fitness- sum of an individual’s own
reproductive success plus the effects the
organism has on the reproductive success of
related others
 Self-sacrifice for the good of your genes (so they can
survive)
Behaviors as Adaptive Traits

Behavioral traits as well as physical are
adaptive
◦ Behavioral maneuvers
 Hiding
 Trading sex for material goods that an aid survival