Heredity
Dear Abbey,
My girlfriend tells me that there is no one like me? Do
you think that could be true. Hopeful in St. Peter.
Dear Hopeful,
I agree with your girlfriend, and it puts you in a select
category with 6 billion other people.
PRINCIPLES OF HEREDITARY
TRANSMISSION
The Germ Cells (produce sperm and ova)
•  Production of Gametes through Meiosis
–  Duplication of 46 chromosomes
–  Crossing-over: adjacent chromosomes break and exchange
segments of genes
–  Pairs of duplicated chromosomes segregate into two new cells
–  Cells divide, 23 single chromosomes
PRINCIPLES OF HEREDITARY
TRANSMISSION
The Germ Cells
•  Hereditary Uniqueness
–  Independent assortment – each chromosome pair segregates
independently, resulting in genetic uniqueness
Figure 3.2 Diagram of the meiosis of a male germ cell.
Important terms and concepts
Humans have 23 pairs of chromosomes
•  22 pairs of chromosomes are homologous (similar in shape and
size), called autosomes
•  23rd pair is sex chromosomes, X Y
Meiosis vs. Mitosis
Meiosis results in daughter cells containing 23
chromosomes (germ cells).
Mitosis results in daughter cells that are copies with a full
complement of 46 chromosomes.
"Crossing Over"
Figure 3.4 These karoytypes of a male (left) and a female (right) have been arranged so that the chromosomes could be displayed in
pairs. Note that the twenty-third pair of chromosomes for the male consists of one elongated X chromosome and a Y
chromosome that is noticeably smaller, whereas the twenty-third pair for the female consists of two X chromosomes.
Dominance
Recesssiveness
Homozygosity
Heterozygosity
Phenotype vs
Genotype
Gene interactions
Dominance, recessiveness
–  examples of dominant genes: Brown eyes, curly hair, double joints,
limb dwarfing
–  examples of recessive genes: baldness, normal fingers and toes
Co-dominance
•  Blood antigens A & B
Incomplete dominance
•  Sickle Cell
Pleiotropism - single gene has more than one effect
•  e.g. PKU, low intelligence and blond hair.
•  modifier genes
Can Mendelian genetics explain complex behavior?
•  Complex behavior polygenetic, but genetic effects accumulate according
to Mendel s laws.
•  Genes can be turned off and on
•  Some genes can direct a number or other genes in terms of their actions
Genetic Diseases
Tay-Sachs Disease – Recessive 1/3,600 European Jews
•  Central nervous system degeneration, with onset at about 6
months, leads to poor muscle tone, blindness, deafness, and
convulsions. No effective treatment. Usually die by 4.
Huntington s Chorea - Dominant 1/20,000
•  Central nervous system degeneration leads to muscular
coordination difficulties, mental deterioration, and personality
changes. Symptoms usually do not appear until age 35 or later.
Genes that cause problems often recessive, why is this one
dominant?
Genetic Diseases
Cystic fibrosis - Recessive 1/2,000 white, 1/16,000 black
•  Lungs, liver, and pancreas secrete large amounts of thick mucus,
leading to breathing and digestive difficulties.
Phenylketonuria (PKU) - Recessive 1/8,000
•  Inability to neutralize the harmful amino acid phenylalanine,
contained in many proteins, causes severe central nervous system
damage in the first year of life.
Sickle cell anemia - Recessive 1/500 black
•  Abnormal sickling of red blood cells causes oxygen deprivation,
pain, swelling, and tissue damage. Anemia and susceptibility to
infections, especially pneumonia, occur. Example of incomplete
dominance.
Sex Linked Genetic Diseases
Duchenne muscular dystrophy- Recessive 1/4,000 males
•  Degenerative muscle disease. Abnormal gait, loss of ability to
walk between 7 and 13 years of age.
Hemophilia - recessive 1/5,000 males
•  Blood fails to clot normally. Can lead to severe internal bleeding
and tissue damage.
Diabetes Insipidus - Recessive 1/2,500 males
•  A form of diabetes present at birth caused by insufficient
production of the hormone vasopressin. Results in excessive thirst
and urination. Dehydration
Figure 3.7 Sex-linked inheritance of red/green color blindness. In the example here, the mother can distinguish
reds from greens but is a carrier because one of her X chromosomes contains a color-blind allele. Notice that
her sons have a 50 percent chance of inheriting the color-blind allele and being color-blind, whereas none of
her daughters would display the trait. A girl can be color-blind only if her father is color blind and her
mother is at least a carrier of the color-blind gene.
Sex Chromosome Abnormalities
Autosomal abnormality
XYY - Super Male - 1 in 1,000 male births
•  Typical characteristics are above-average height, large teeth, and
sometimes severe acne. Intelligence, development of male sexual
characteristics, and fertility are normal
XXX - Super Female 1 in 500 to 1,250
•  Impaired verbal intelligence. Affected girls are no different in appearance
or sexual development from normal age-mates,except for a greater
tendency toward tallness.
XXY - Klinefelter syndrome 1 in 500 to 1,000
•  Impaired verbal intelligence. Afflicted boys are unusually tall, have a
body fat distribution resembling females, and show incomplete
development of sex characteristics at puberty. They are usually sterile.
XO - Turner syndrome 1 in 2,500 to 8,000
•  Impaired spatial intelligence. Ovaries usually do not develop prenatally.
Incomplete development of sex characteristics at puberty. Other features
include short stature and webbed neck.
•  Trisomy 21. Down s Syndrome. Occurs more often in older
women, although 25% inherit problem from father.
•  Aging Ova, Hypothesis
–  Mother under 29, <1/1000 (Down s), 1/450 (any abnormal.)
–  Mother over 45 1/25 (Down s), 1/15 (any abnormal.)
•  Do older fathers have greater risks?
–  Fathers over 40 600% increase of having a child with autism,
900% for those over 50.
–  Fathers over 40 double the risk of a child with schizophrenia
–  Also related to increased risks of other birth defects.
–  Older mothers and fathers have increased over the last three
decades.
Nature vs. Nurture
Empiricists - John Locke (1632-1704).
•  Children born with a blank slate, experience determines
development.
•  Psychologist counterpart John Watson
Nativists - Jean Jacques Rousseau (1712-1778)
•  Children endowed with innate sense of right and wrong, and with
innate plan for orderly development.
•  Psychologist counterpart Jean Piaget
Amniocentesis, Performed 11-14 weeks, results back in three weeks.
Chorionic villi biopsy, Performed 6-8 weeks, Results back in 24 hours.
Methods for studying the effects of genes
Inbreeding - mating of related animals
Selective Breeding - selecting animals with desired
characteristics to breed
Behavioral Genetics
One of the problems in early studies was the assumption that
effects of genetics and environment can be separated in an
individual and studied. It cannot.
Modern Behavioral genetics uses groups that differ in
genetics and environments to estimate how much each of
these determine behavior.
•  In the case of groups we can divide them up into the amount of
genetic overlap and experiential overlap, examine how this affects
the behavioral variable in question.
Twin Studies
•  Identical twins – 100% genetic overlap and share environments
•  Fraternal twins and siblings - share 50% of genetic material and
share environments
•  Identical Twins raised in same home, Identical Twins raised apart
•  Siblings raised together and appart
Adoption studies
•  Adopted and Related siblings in same home have similar
environments
•  Adopted and Related siblings and relation to parents and each
other
•  Adopted children-share no genetic overlap with siblings
•  Adopted children share genes with natural parents but share
environments with adoptive parents
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
MZ twins
DZ twins
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
MZ twins
DZ twins
How similar would IQs be in siblings, siblings reared apart,
and unrelated siblings if it were due entirely to nurture?
Correlation
Correlation
How similar would IQs be in MZ and DZ Twins be if IQ is
due entirely to nature?
How similar would IQs be in MZ and DZ Twins if IQ is due
entirely to nurture?
Correlation
Modern Kinship Studies
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Siblings
Siblings Apart
Unrelated
r
.86
How similar would IQs be in siblings, siblings reared apart,
and unrelated siblings if it were due entirely to nature?
.72
Correlation
.60
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
.47
.24
.44
.24
.22
.32
Siblings
Siblings-Apart
.12
Unrelated
Heritability
Heritability estimate (0 to +1.0) - estimate of the percentage
of a trait s variation in a population that is determined by
genetics.
•  This is not the percentage applied to an individual, e.g. 90% of
your height genetic, but to a population.
Would the previous graph suggest to you that heritability is
very high or low?
Estimates of the Heritability of IQ
•  Identicals reared Apart +.72
–  Identical twins reared apart direct measure of heritability but
few in number, almost everyone agrees unreliable measure.
•  Identical twins together (.86) – Fraternal twins together (.60) times
2 = .52
•  Siblings together (.47) – siblings apart (.24) times 2 = .46
•  Siblings apart (.24) times 2 = .48
Genes and the Environment
Do Genes limit development or provide opportunities for
growth?
Heritability
Can Heritability estimates change?
•  Height of Japanese after the war
•  What would happen if we created a country in which there was
equal opportunity for everyone? Would heritability for
intelligence go up or down?
•  Would heritability for IQ be higher or lower in the upper middle
class or in impoverished groups?
Range of Reaction
Comments on Range of Reaction
It may in some ways be a matter of semantics. In the case of
Freddie his is limited in the ability to develop intellectually
given a variety of environments, but in the case of Juan he
is given the potential to develop very high intellectual
ability. Would you say Juan is limited by his genes or
facilitated?
Berk
Others argue that heredity does not dictate children s
experiences or development in a rigid way. In one study,
boys with a genetic tendency toward antisocial behavior
based on the presence of a gene on the X chromosome
known to predispose both animals and humans to
aggression) were no more aggressive than boys without
this gene, unless they also had a history of severe child
abuse ( Caspi et al., 2002). Boys with and without the gene
did not differ in their experience of abuse, indicating that
the aggressive genotype did not increase exposure to
abuse. (p. 122)
Nature and Nurture
Canalization-the tendency of heredity to ensure the
development of some characteristics with little influence
from the environment.
•  Babbling-Infants will babble regardless of the linguistic
environment around 5 months.
•  Humans have a gene for using a language, apes do not. Humans
develop language even under circumstances that make it unlikely.
Are humans restricted or enabled in their development of language
through this gene?
Genes and Environments are Correlated
Passive Correlation
•  Parents provide children with genes and an environment which is
consonant with their genetic makeup. Hence there is a relation between
child's genes and the environment experienced.
Evocative Correlation
•  Children evoke responses from others because of their genetic makeup,
thus the environment experienced is affected by their genetics.
Active Correlation
•  People actively select experiences in their environments based on
preferences affected by their genetics, thus the environment is once again
influenced by genetics. Niche picking.
Genes create environmental effects and determine the way we develop
with age.
How would these correlations change with age?
Niche Picking?
Genetics and the Environment
Behavioral Genetics has allowed us to more carefully determine what
aspects of the environment are more responsible for traits.
100 % of Human differences in behavior = Genetics + Environment
If 50% of the variation in behavior is genetic then other half is due to the
environment.
However, the effects of environment is split into two parts.
•  Shared environments. Between family differences. Things that make
families different from one another such as SES, education, money,
religion, etc.
•  Non-shared environments. Within family differences. Differences in
persons within the family, based on the experiences unique to the
individual.
Estimates of the Environmental Effects
Estimate for IQ
•  Nonshared environment = 1.00 – r(identical twins together)
–  100 –.86 =14%
•  Shared Environment = 1.00 – (Heritability + NSE)
–  1.00 – (.52 +.14) = 34%
Estimate for Personality
•  Nonshared environment = 1.00 – r(identical twins together)
–  100 – .5 =50%
•  Shared Environment = 1.00 – (Heritability + NSE)
–  1.00 – (.40 +.50) = 10%
Shared or Non-Shared
Your family has an income of $150,000 per year.
You are born last in your family of 5?
Your father is an exercise nut and leads the family on a five
mile run on weekends.
Your math teacher takes you aside and tells you are a very
good student and should major in mathematics.
Your mother divorces your father, and moves you and your
siblings to a modest home in the suburbs.
In general which would have stronger effects on intelligence
and schooling? Personality?
Estimates of the Environmental Effects
Estimates for IQ
Nonshared environment = 14%
Shared Environment = 34%
Estimates for Personality
Nonshared environment = 50%
Shared Environment = 10%
Shared vs non-Shared Environmental Effects?
Studies of the resemblance of genetically unrelated children
who are adopted together show negligible correlations for
personality, psychopathy, and IQ (After childhood).
Since the effects of the shared environment small, it means
that the nonshared environment has a much larger effect.
The contribution to personality traits by:
•  Genetics--heritability often .40 +
•  Shared family influences-experiences you share with family
•  Non-shared family influences-experiences unique to you
Interesting conclusion. Family members resemble each other
primarily because of heredity and that the environment
(NSE) acts primarily to make children in the same family
different rather than similar.