Child Psychology
A Canadian Perspective
THIRD EDITION
Younger, Adler, Vasta
Chapter 3
Genetics: The Biological Context of
Development
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Learning Objectives
• Learning Objective 3.1:
• Identify and describe the mechanisms and processes by
which physical and behavioural characteristics are inherited.
• Learning Objective 3.2:
• Describe different types of genetic disorders and their impact
on child development.
• Learning Objective 3.3:
• Describe the influence that genes have on the development of
psychological abilities and traits.
• Learning Objective 3.4:
• How do genes and environment interact to influence the
development of behaviour?
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Mechanisms of Inheritance:
Cell Division
• Cells are comprised of three divisions
• Nucleus contains the chromosomes
• Chromosomes: strands of the genetic material DNA
• Each human cell contains 23 chromosome pairs
(yielding 46 total chromosomes per cell)
• Autosomes comprise 22 of the 23 pairs
• Sex chromosomes comprise the 23rd pair
• Males are XY, females are XX
• Cytoplasm fills the cell interior
• Cell membrane encases the cell
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Mechanisms of Inheritance:
Cell Division
• Cells form two groups based on function
• Body cells: form the structures of the body
• Reproduce by mitosis
• Forming two identical cells, each equipped with 23 pairs
of chromosomes
• Germ cells: form the reproductive cells
• Reproduce by meiosis
• Forming four cells with each cell containing only 23
chromosomes
• These cells are the gametes (ova or sperm)
• During conception, a sperm merges with an ovum to form a
new cell containing 23 pairs of chromosomes
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Mitosis refers to a process
by which 2 identical cells
are produced
Meiosis refers to a process
in 4 cells are produced, with
each containing only 23
chromosomes
Figure 3.1: (Mitosis and meiosis). Mitosis results in
two cells identical with the parent and with each
other. Meiosis results in four cells different from the
parent cell and from each other. Adapted from
Biology: Exploring Life (p.152) by G. D. Brum & L. K.
McKane, 1989, New York: John Wiley & Sons.
Adapted by permission of the authors.
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Mechanisms of Inheritance:
Cell Division
Crossing over:
During meiosis,
the x-shaped
chromosomes
line up and
intermix, yielding
a novel genetic
product
Figure 3.2 Crossing over results in the exchange
of genetic material. After the cross-over, all four
strands are different. Adapted from Biology:
Exploring Life (p. 44) by G. D. Brum & L. K.
McKane, 1989, New York: John Wiley & Sons.
Adapted by permission of the authors.
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Inside the Chromosome
• DNA: the basic genetic material,
formed from pairs of base
nucleotides
• The bases form pairs such
as adenosine-thymine or
guanine-cytosine
• The DNA strand is in the
form of a double helix made
up of a series of base pairs
Figure 3.3: Structure and
Replication of DNA
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Mendel’s Studies:
Principles of Heredity
• Mendel argued that certain traits are transmitted from
parents to child
• Each trait is governed by two elements with one
from each parent
• Phenotype: Expressed trait
• Genotype: Underlying genes that govern the trait
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Mendel’s Studies:
Principles of Genetic Transmission
• Principle of dominance:
• Some genes are always expressed (dominant gene),
others are recessive (recessive gene)
• Polygenic inheritance:
• Occurs when traits are determined by a number of genes
• Incomplete dominance:
• Occurs when the dominant gene does not completely
suppress the recessive gene
• Codominance:
• Occurs when both genes are dominant and thus both are
expressed
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Mendel’s Studies:
Common Genetic Traits
Dominant
Recessive
Brown eyes
Blue, gray, or green eyes
Normal hair
Baldness (in men)
Dark hair
Blond hair
Normal colour vision
Colour blindness
Freckles
No freckles
Dimples
Free earlobes
Double-jointed
thumbs
No dimples
Attached earlobes
Tight thumb ligaments
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Genetic Disorders:
Hereditary Disorders
• Mutations (genetic variations) can be
adaptive or maladaptive
• Dominant disorders
• Huntington’s chorea:
• Fatal syndrome in which the nervous
system degenerates in adulthood
(age 30-40)
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Genetic Disorders:
Hereditary Disorders
• Recessive disorders
• Diseases with errors of metabolism
• Tay-Sachs disease:
• Fatal disease in which the nervous system
disintegrates because the body cannot break
down fats in brain cells
• Phenylketonuria (PKU):
• Inherited disease in which the body cannot
process the amino acid phenylalanine
• Treatment: Eat a diet low in this amino acid
during critical periods of brain development
• Diseases without errors of metabolism
• Sickle-cell anemia (SCA)
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Sickle-Cell Anemia (SCA)
Figure 3.4: Scanning electron micrographs of red blood cells from normal
individuals (left) and individuals with sickle-cell anemia (right). (Bill Longcore,
Photo Researchers, Inc.)
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Genetic Disorders
• Differences between the two sex chromosomes
have implications for the transmission of certain
disorders called X-linked disorders
• X-linked disorders: disorders that result from
recessive genes located on the X chromosome,
leaving males more vulnerable to them
• Example: hemophelia—bleeding disorder
caused by low or no blood protein essential for
clotting; results from a recessive allele on the X
chromosome
• Other examples of X-linked disorders:
Duchenne muscular dystrophy, red-green
colour blindness, and fragile X syndrome
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Structural Defects in the Chromosome
• Autosomal disorders:
• Down syndrome
• 21st pair of chromosomes has a third member
• Results in mental retardation, poor muscle
tone, and distinctive facial features
• Greatest risk occurs in mothers between the
ages of 45-49 years
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Structural Defects in the Chromosome
• Disorders of the sex chromosomes:
• Fragile X syndrome:
• Caused by an abnormal gene on the X chromosome
• Results in a variety of physical and behavioural
symptoms, including mental retardation
• Turner’s syndrome:
• Occurs when a female has only one X chromosome
(XO)
• Klinefelter’s syndrome:
• Occurs when a male inherits an extra X
chromosome (XXY)
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Studying the Effects of Genes on
Behaviour
• Three principal areas of behaviour
• Intellectual abilities
• Psychiatric disorders (including children’s
behavioural problems)
• Personality
• Four major approaches are used to study the
impact of genes on behaviour
• Family studies
• Adoption studies
• Twin studies
• Combined Twin Study and Adoption Study
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Family and Adoption Studies
• Family studies:
• Compare different family members and their similarity
in certain characteristics
• Explore whether the phenotypic similarity on a trait
follows the genotypic similarity among the people being
compared
• Adoption studies:
• Compare similarities in characteristics between
adopted children and their biological and adoptive
parents
• Explores the contribution of shared genes versus
shared environment to a trait
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Twin Studies
• Two types of twins:
• Identical twins – Monozygotic (MZ)
• From the same fertilized egg
• Fraternal twins – Dizygotic (DZ)
• From two different eggs
•Twin studies:
• Compare the similarity between the twins in regard to a
behaviour
• Concordance: the degree of similarity of twins on a
behavior
• Most studies indicate that MZ twins show greater similarity
than do DZ twins and this effect is larger as they get older
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Twin Studies
Age-Related Changes in Concordance for MZ and DZ Twins
Figure 3.7: Concordance in IQ changes in (a) identical, or monozygotic (MZ), twins and (b)
fraternal, or dizygotic (DZ), twins from 3 months to 6 years of age.The scales are different to
accommodate different ranges of scores.The important point is that changes in performance are
more similar for monozygotic twins. Adapted from “The Louisville Twin Study: Developmental
Synchronies in Behavior” by R. S.Wilson, 1983, Child Development, 54, p. 301. Copyright © 1983
by The Society for Research in Child Development, Inc. Adapted by permission.
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Models of Gene-Environment
Interaction
• Gottesman’s limit-setting model:
• Range of ability is determined by genes—
actual value of that ability is determined by the
environment (range of reaction)
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Range of Reaction Model
Figure 3.8: The reaction range concept, showing the simultaneous influences of genes and
environment. Adapted from “Developmental Genetics and Ontogenetic Psychology: Overdue
Détente and Propositions from a Matchmaker” by I. I. Gottesman, 1974. In A. D. Pick (Ed.),
Minnesota Symposia on Child Psychology, vol. 8, p. 60, University of Minnesota Press.
Copyright © 1974 by the University of Minnesota. Adapted by permission.
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Models of Gene-Environment
Interaction
• Scarr’s niche-picking model:
• Passive gene-environment correlation
• Evocative gene-environment correlation
• Active gene-environment correlation
• Plomin’s environmental genetics model:
• Different children experience the same environment
differently (nonshared environment)
• Bronfenbrenner and Ceci’s biological model:
• Child’s genes and immediate environment interact
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