Download Chapter 2: The Human Heritage: Genes and the Environment

The Human Heritage:
Genes and the
Environment
The Development of Children (5th ed.)
Cole, Cole & Lightfoot
Chapter 2
A World of Possibilities…
“If we go to all the trouble it takes
to mix our genes with those of
somebody else, it is in order to make
sure that our child will be different
from ourselves and from all our
other children.”
—François Jacob, The Possible and the Actual
Overview of the Journey

Sexual
Reproduction
and Genetic
Transmission

Genotype and
Phenotype

Mutations and
Genetic
Abnormalities

Biology and
Culture
Sexual Reproduction
and Genetic
Transmission
Mitosis: A Process of
Cell Replication
Meiosis: A Source of
Genetic Variation
Sexual Determination:
Another Source of Variation
Genetic Code



Chromosome: A threadlike
structure made up of genes
 46 in all human cells, except
sperm and ova which have 23
Deoxyribonucleic acid (DNA):
A long double-stranded molecule
that makes up chromosomes
Genes: The segments on a DNA
molecule that act as hereditary
blueprints for the organism’s
development
Model of DNA
DNA Replication


A strand of DNA replicates
by splitting down the middle
of the rungs of its ladderlike
structure
Each free base picks up a
new complementary partner:


Cytosine (C) pairs with
guanine (G)
Adenine (A) pairs with
thymine (T)
Cell Replication
Mitosis
The process of
somatic (body)
cell duplication
and division that
generates all the
individual’s cells
except sperm
and ova
Genetic
Variation
Meiosis
The twice-over
process of germ cell
(i.e., sperm and ova)
division that results in
a zygote (with 46
chromosomes) at
conception
Variation enhanced
by “crossing over”
during first phase...
The Process of Crossing Over
The Case of Twins

Monozygotic twins: Come from a single fertilized
egg and have exactly the same genes (i.e., identical
genotypes)


Sometimes called “identical
twins”, but may not actually
look identical due to the fact
that phenotype interacts with
the environment
Dizygotic twins: Come
from two ova that have
been fertilized at the same time, and consequently
are no more alike than any two siblings
Sexual Determination


In 22 of the 23 pairs of chromosomes found human
somatic cells, the two chromosomes are of the same
size and shape, and carry corresponding genes
Chromosomes of pair 23 can differ, however, and
this determines a person’s sex
 Females: Both members of chromosome pair 23
are of the same type and are called X
chromosomes (i.e., XX)
 Males: In chromosome pair 23, one X chromosome is paired with a different, much smaller
chromosome called a Y chromosome (i.e., XY)
Sexual Determination
Human
X and Y
chromosomes
Sexual Determination


Since a female is always XX, each of her eggs
contains an X chromosome
In contrast, half of a man’s sperm carry an X
chromosome and half carry a Y chromosome


If a sperm containing an
X chromosome fertilizes
the egg, the resulting
child will be XX, a female
If the sperm contains a
Y chromosome, the child
will be XY, a male
Genotype and
Phenotype
The Laws of Genetic Inheritance
Genes, the Person, and
the Environment
The Study of Genetic Influences
on Human Behavior
An Interaction
Genotype + Environment  Phenotype

Genotype: An individual’s genetic endowment

Environment: The totality of conditions and
circumstances that surround an individual

Phenotype: The observable characteristics of
an individual

Physical and psychological traits, health, behavior
Laws of Genetic Inheritance
Gregor Mendel (1822-1884): Garden peas



Allele: The specific form of a gene coded for a
particular trait
Homozygous: Having inherited two genes of
the same allelic form for a trait
Heterozygous:
Having inherited two
genes of different allelic
forms for a trait
Heterozygous Possibilities
1.
Dominant gene: The allele that
is expressed when an individual
possesses two different alleles
for the same trait
2.
Recessive genes: The allele that
is not expressed when an individual possesses
two different alleles for the same trait
3.
Codominance: An outcome in which a trait that is
determined by two alleles is different from the trait
produced by either of the contributing alleles alone
The Case of Blood Type
Sex-Linked Traits


Given that the X chromosome is much larger than the
Y chromosome, most inherited sex-linked
characteristics are carried on the X chromosome
Males are more susceptible to genetic defects than
are females



If a daughter has a harmful recessive gene on one X
chromosome, she will usually have a normal dominant gene
on the other X chromosome to override it
A son who inherits a harmful recessive gene on his X
chromosome has no such complementary allele to override
the recessive gene’s harmful effects
Examples: Red-green color blindness, hemophilia,
muscular dystrophy
Principles of
Gene-Environment Interactions
1.
2.
3.
Gene-environment interaction is a two-way
process.
Interactions between organisms and their
environments need to be studied in a broad,
ecological framework because variations in
the environment can have profound effects
on the development of the phenotype.
Genetic factors often play a role in
determining what environments individuals
inhabit and how individuals shape and select
their own experiences.
Range of Reaction
The effect of environment on the
expression of a gene for fur color
in the Himalayan rabbit.
Under normal conditions only the rabbit’s
feet, tail, ears, and nose are black. If fur is
removed from a patch on the rabbit’s
back and an ice pack is placed there,
creating a cold local environment, the
new fur that grows in is black.
Canalization
The process
that makes
some traits
relatively
invulnerable to
environmental
events
Genes and Behavior

Only in cases where it has
been possible to identify a
specific gene that controls
a specific pattern of
phenotypes across all known
environments where humans
live has it been possible to
make clear causal inferences
from genes to behavior.

As a substitute in the vast majority of cases where multiple,
unknown, genes are likely to be involved, behavioral
geneticists rely on the study of various kinship relations
to estimate the relative influences of the genotype and the
environment on the phenotype.
Types of Kinship Methods



Family studies:
Relatives who live
together in a household
are compared
Twin studies:
Monozygotic and
dizygotic twins are compared
Adoption studies: Children living apart from
their biological parents are studied
Family and Adoption
Results for Extroversion
Type of Relative
Correlation % Shared Genes
MZ twins raised together
.51
100%
DZ twins raised together
.18
50%
MZ twins raised apart
.38
100%
DZ twins raised apart
.05
50%
Parents/child. living together
.16
50%
Adoptive parents & children
.01
0%
Siblings raised together
.20
50%
Siblings raised apart
-.07
50%
Loehlin, 1992
Mutations and Genetic
Abnormalities
Down Syndrome
Phenylketonuria
Sickle-Cell Anemia
Klinefelter Syndrome
Causes of
Genetic
Abnormalities
Down Syndrome:
A Chromosomal Error




Cause: More than 95% of children born with Down
syndrome have 3 (vs. 2) copies of chromosome 21
Traits: Mentally/physically retarded; distinctive physical
characteristics (e.g., slanting eyes, rather flat facial
profile, ears lower than normal, short neck, protruding
tongue, dental irregularities, small curved fingers, widespaced toes); more likely to die young
Incidence: 1:1000 births (increases with mother’s age:
1:100 by age 40, 1:32 by age 45, 1:12 by age 49)
Outlook: Supportive intervention that includes special
education by concerned adults can markedly improve
the intellectual functioning of some of these children
Phenylketonuria (PKU):
A Treatable Genetic Disease




Cause: Defective recessive gene that reduces the
body’s ability to convert one amino acid
(phenylalanine) into another (tyrosine), resulting in
retarded development of prefrontal cortex
Traits: Severe retardation
Incidence: 1:10,000 (more prevalent among whites
than blanks; 1:100 people of European descent is a
carrier of the recessive mutant gene)
Outlook: Feeding PKU infants a diet low in
phenylalanine (e.g., avoiding milk, eggs, bread, fish),
reduces the severity of mental retardation, although
this does not eradicate its effects entirely
Sickle-Cell Anemia:
Gene–Environment Interaction




Cause: Recessive gene
(victims are homozygous, but
heterozygous subjects are also
mildly affected)
Traits: Abnormal blood cells
cause circulatory problems
(e.g., heart enlargement) and
severe anemia
Incidence: 8-9% of U.S. blacks
Outlook: Crippling, but
treatable with medication
Klinefelter Syndrome:
A Sex- Linked Abnormality

Cause: Extra X chromosome
(i.e., XXY)

Traits: Males fail to develop
secondary sex characteristics
(e.g., facial hair, voice change),
are sterile, and most have
speech and language problems

Incidence: 1:900 males

Outlook: May be treatable with
testosterone replacement therapy
Biology and Culture
Survival Strategies
Coevolution
Survival Strategies: Culture


Cultural artifacts, such as
tools and clothing, greatly
extend the range of
environments in which
human beings can reproduce
and thrive
Cultural knowledge, such
as the creation of “hazard
prevention strategies,”
serves to protect and support
children to enable them to
mature and reproduce
Coevolution of
Tool-Making Abilities