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THE INFLUENCE OF
HEREDITY: THE NATURE OF
NATURE
OT 500
Spring 2016
WHAT DO WE MEAN BY HEREDITY?
Biological transmission of traits/characteristics from
one generation to another
 What we inherit makes certain behaviors possible,
and limits us
 Genetics is the field of science that studies heredity
 Physical traits are inherited (height, hair color and
texture, eye color)
 Genetics also plays a role in intelligence, and many
personality traits (sociability, anxiety) and interests
 Genetics plays an important role in many diseases,
and mental disorders (schizophrenia, autism,
substance abuse)

CHROMOSOMES
AND
GENES ARE THE
FUNDAMENTAL UNITS OF HEREDITY

Chromosomes



Found in cells; humans have 23 pairs of rod-shaped
structures; 46 chromosomes
Genes

Segments within chromosomes; 20,000 to 25,000 genes in
every cell

Regulate development of traits which may be transmitted
by single gene, or many genes (polygenic)
DNA

Large strands, in double spirals (helix) that make up genes

Composed of phosphate(P), simple sugar (S)

Base pairs adenine with thymine (A – T) or cytosine with
guanine (C – G)
DOUBLE HELIX OF DNA
Figure 2.1
CELL DIVISION

Mitosis

Cell division by which growth occurs; strands of DNA
break apart, duplicate and are rebuilt resulting in identical
copies of DNA strand (mutations can occur)
CELL DIVISION

Meiosis (reduction division)
 Cell division by which sperm and ova are
produced; 23 chromosome pairs divide
 Result is a new cell with only 23 chromosomes
 22 pairs are autosomes
 23rd pair are sex chromosomes
 determines sex: X from mother and X or Y
from father
FIGURE 2.5: MITOSIS AND MEIOSIS
THE 23 PAIRS OF HUMAN CHROMOSOMES
Figure 2.3
HUMAN DIVERSITY
•
•
•
Fertilization: new cell (zygote) receives 23
chromosomes from the father’s sperm cell, and 23
from the mothers ovum
Gene is a segment of chromosomes that control
particular aspect of production of specific protein
23 pairs of chromosomes = genotype; while your
observable characteristics are your phenotype
TWINS
Monozygotic Twins (MZ); Identical twins, same
genes
 Derived from a single zygote that has split in two
 Dizygotic Twins (DZ) (about 2/3s of twins)
 Derived from two zygotes, and share 50% of
genetic material
 Probability of twins increases with maternal age, use
of fertility drugs; and dizygotic twins run in families

CHROMOSOMAL
OR GENETIC
ABNORMALITIES






Chromosomal abnormalities occur when children do not
have the normal complement of 46 chromosomes
Risk increases with the age of the parents
Most common example, Down’s syndrome; trisomy 21
Sex-linked Chromosomal Abnormalities (abnormal # of
sex chromosomes (eg. XYY; XXY-Klinefelter syndrome;
X-Turner syndrome)
Genetic Abnormalities: PKU (recessive); Huntingtons
(dominant); Cystic Fibrosis (recessive)
Sex-linked Abnormalities: only carried on the X sex
chromosome eg. Hemophilia (recessive); more likely to
affect sons of female carriers; Duchenne’s muscular
dystrophy; color blindness
DOWN SYNDROME/ TRISOMY 21
GENETIC COUNSELING AND PRENATAL TESTING

Amniocentesis
Routine among American women over age 35
 Used to detect over 100 chromosomal and genetic abnormalities
 Indicates the sex of the baby
 Some risk of miscarriage

AMNIOCENTESIS
Figure 2.6
GENETIC COUNSELING AND PRENATAL TESTING



Chorionic Villus Sampling (CVS)
 Can diagnosis abnormalities earlier than amniocentesis
(done between the 9th-12th weeks)
 Slightly greater risk of spontaneous abortion
Ultrasound
 Sonogram “picture” of fetus
 Beneficial in determining position of fetus
Blood Tests
 Used to reveal presence of recessive genes in parents (eg.
cystic fibrosis)
 Alpha-Fetoprotein (AFP) assay (detects neural tube
defects)
SONOGRAM
Figure 2.7
PRENATAL ASSESSMENT
CVS – Chorionic Villus Sampling
BACK TO NATURE VS NURTURE:
ENVIRONMENTAL EFFECTS ON GENE
EXPRESSION
Factors such as temperature, light, stress,
nutrients, and other chemicals affect proteins
produced by body…which may affect genes
 What is the difference between our genotype and
our phenotype and what accounts for those
differences??? Why identical twins are NOT
identical in every way!!!
 Canalization: growth rates that are mostly
genetically determined; When affected by
environmental factors the rates have a tendency
to return back to the genetically determined
pattern

THE PART OF DEVELOPMENT THAT IS
NATURE
 Heritability


Degree to which different traits are influenced by
genetic factors
We have learned a lot from twin studies, adoption
studies, and other family
relatedness/generational studies
 Darwin’s


Influence: Theory of evolution
Survival of the fittest and natural selection
Epigenesis: A gradual process of increasing
complexity due to interaction between heredity
and the environment
HOW GENES DETERMINE TRAITS
•
•
•
•
•
Traits are determined by pairs of genes; each pair
is an allele; when both alleles for a trait are the
same, the person is homozygous for that trait;
when they differ, they are said to be heterozygous
Dominant traits are expressed; recessive traits
are not expressed when paired with a dominant
trait eg. Brown eyes are dominant over blue eyes
Additive heredity: A child’s visible traits, his/her
phenotype, is mix of mother and father traits
Regulator genes: Some genes turn other genes on
and off
Environmental influences
ENVIRONMENTAL INFLUENCES ON GENE
EXPRESSION: GENETIC-ENVIRONMENTAL
CORRELATIONS
Passive correlation: parents place their children in
environments that promote certain skills that
match their own genetics
 Evocative Correlation: The child’s genotype is
consistent with certain behaviors; those behaviors
evoke responses in others that are reinforcing
 Active Correlation: taking an active conscious role
in pursuing one’s interests/activities; occupations
that reflect our skills

ENVIRONMENTAL EFFECTS ON
HERITABILITY
Heritability varies from one group to another
 Environment changes the heritability of a trait
 Genetic factors matter less when a characteristic
is largely determined by environment
 Inherited traits lead to different characteristics
in different contexts
 Reaction range – inherited traits as an array of
possibilities rather than fixed points
 Epigenetic Framework: Development reflects
the continual bidirectional exchanges between
genetics and environment

FIGURE 2.7: REACTION RANGE
HOW DO GENES DETERMINE TRAITS?



Traits are determined by pairs of genes and each
member of pair is an allele
 Homozygous: Both alleles for a trait are the same
 Heterozygous: Alleles for a trait are different
Averaging
 Effects of both alleles are shown
 Incomplete dominance or co-dominance
Law of Dominance: When a Dominant allele is paired
with recessive allele, the dominant allele appears in
offspring
USING RESEARCH TO SORT OUT THE EFFECTS
OF GENETICS AND ENVIRONMENTAL
INFLUENCES ON DEVELOPMENT
 Kinship
Studies
 Genetic closeness of relatives
 Twin Studies
 Monozygotic twins share 100% of genes
 Dizygotic twins share 50% of genes
(same as other siblings)
 Reared together versus reared apart
 Adoption Studies