Heredity:
Our Genetic Background
Psychology: Chapter 3, Section 4
Heredity: Our Genetic Background
• Heredity is the transmission of characteristics
from parents to offspring
• The traits we inherit help shape our behavior
• Shyness, leadership ability, aggressiveness, even
an interest in arts and crafts are influenced by
heredity (while environment also plays a big role)
• Psychological disorders, such as depression,
schizophrenia, bipolar disorder and alcoholism
are also affected by heredity
This chart seems to
indicate that the closer
someone is related to
someone with
schizophrenia, the more
likely that person is to
develop a similar
diagnosis
Genes and Chromosomes
• Genes are the basic building blocks of heredity
• Genes match up in pairs, one gene in each
pair from each parent. As these genes pair up,
traits are determined.
Genes and Chromosomes
• Genes are found in threadlike structures called
chromosomes
• Chromosomes are composed of deoxyribonucleic
acid (DNA)
• DNA looks like a twisting rope ladder (called a
double helix)
Genes are the building blocks of chromosomes
DNA: The Genetic Code
•The threadlike molecules of DNA that make up chromosomes
contain the codes for the development of particular traits.
•Each chromosome has more than 1000 genes.
•The rungs in the ladder form the genetic code that causes
the organism to develop certain traits, and can be the source
of some mutations
Genes and Chromosomes
• Most normal human cells have 46
chromosomes, organized into 23 pairs
• In each of the 23 pairs, one chromosome comes
from the father and one from the mother
• These chromosomes contain instructions for
traits.
• That mixing of chromosomes is why you may
look and act a little like your mother and also
like your father
Genes and Chromosomes
• 22 out of 23 of the chromosomes are the same for
both males and females
• The 23rd pair is the sex chromosome, determining
whether we are to be male or female
• In females, both chromosomes are in the shape of
an X, so they are both called X chromosomes (XX)
• In males, one of the chromosomes looks like a Y,
so the males are said to have XY
• This X or Y is contributed by the father; he
determines the sex of the child
In the case of these 23 chromosome pairs, this offspring is a
boy, as he has the larger x chromosome matched with the
smaller y chromosome in the 23rd pair
Genes and Chromosomes
• Occasionally, there is a mutation where there
are not 46 chromosomes in each cell
• If there is an extra chromosome on the 21st
pair, then the baby will be born with Down
syndrome.
• Down syndrome usually means there will be
some level of mental retardation, with some
heart and respiratory problems
People with Down syndrome have some
similar facial features
The Nature- Nurture Debate
• For many years, people have debated about the
role of biology in determining who we are as
people
• This is often called the Nature-Nurture debate
• Nature refers to what people inherit biologically
from their parents
• Nurture refers to how a person is raised– living
conditions, parental involvement, educational
opportunities, etc.
The Nature- Nurture Debate
• Most psychologists agree that influences from both
nature and nurture determine our psychological
traits
• There may be someone who has the genetic
potential to write a brilliant novel, but won’t
because she never gets the chance to read or write
• Heredity is not destiny, but it is a predisposition
within an environment
Nature vs. Nurture
We inherit our
physical,
intellectual, and
emotional
characteristics from
our ancestors, but
these things are
also greatly
affected by where/
how we live.
Kinship Studies
• The most common way to figure out the roles of
heredity and environment is to do kinship studies
• Kinship is based on degree people are related
– Identical twins share 100% of their genes
– A parent and child shares 50% of their genes (1/2)
– Brothers and sisters also share 50% (1/2)
– Aunts/ uncles share 25% with their nieces and nephews
(1/4)
– First cousins share 12.5% (1/8)
Kinship Studies
• Psychologists can look at traits in individuals
and see how they compare with relatives
• Two common types of kinship studies are twin
studies and adoptee studies
Identical twins share 100% of their genetic code, whereas
fraternal twins are basically just like any other siblings
Twin Studies
• Studying identical and fraternal twins can show a
lot about nature vs. nurture, as identical twins
share 100% of the same genetic makeup
• If identical twins are more similar on a certain
trait than fraternal twins, then that could be an
indication that that trait is influenced by genetics
Twin Studies
• Some traits that do seem to be more similar
between identical than fraternal twins are
shyness, activity level, irritability, sociability,
and overall happiness
• Autism is highly linked by genetics– if one
identical twin has autism, there is a 96% chance
the other has autism. In fraternal twins, there is
a 24% chance.
Adoptee Studies
• One problem with twin studies is that identical
twins tend to also be raised in the same
environment and treated similarly, so that
complicates the nature/ nurture discussion
• But if twins were adopted, then they would have
different environments but still the same genetics
Twins Reared Apart
• It can therefore be useful to study twins who were
raised apart from each other
• In such studies, researchers have found that
intelligence, following rules, risk avoidance,
aggression, and leadership traits were very similar
in twins, even when reared apart
• They also may sit or stand very similarly, and have
similar tastes in clothing
• Twins: Is it All in the Genes?
https://youtube.com/watch?v=Wd5Y3-F79LY
Assessment
• Complete the three questions on page 72