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Chapter 3: Genetic Bases of
Child Development
Module 3.1 Mechanisms of Heredity
Module 3.2 Genetic Disorders
Module 3.3 Heredity is Not Destiny
Children and Their Development, 3/e by Robert Kail
3.1 Mechanisms of Heredity
The Biology of Heredity
Single Gene Inheritance
Behavioral Genetics
3.1 The Biology of Heredity
• The first 22 pairs of chromosomes are
autosomes and the 23rd pair is sex
chromosomes
• Genotype is one’s complete set of genes and
phenotype is one’s physical, behavioral, and
psychological features
3.1 Single Gene Inheritance
• Pairs of alleles can be either homozygous or
heterozygous
• Dominant allele: its chemical instructions are
followed
• Recessive allele: its chemical instructions are
ignored
• Incomplete dominance: one allele doesn’t
dominate the other completely
Sickle Cell Trait: An Example of
Incomplete Dominance
3.1: Single Gene Inheritance
3.1 Behavioral Genetics
• Many behavioral genotypes reflect polygenic
inheritance, which involves many genes
• Behavioral geneticists rely upon twin studies
and adoption studies
• Cognitive abilities, psychological disorders,
substance abuse, and personality are all
affected by heredity
An Example of Polygenic
Inheritance
3.1: Behavioral Genetics
3.2 Genetic Disorders
Inherited Disorders
Abnormal Chromosomes
3.2 Inherited Disorders
• Many disorders are triggered when a child
inherits two recessive alleles
• Examples include cystic fibrosis, PKU,
albinism, and Tay-sachs disease
• Most inherited disorders are very rare
3.2 Abnormal Chromosomes
• Some people are born with too many, too few,
or damaged chromosomes
• People with Down Syndrome usually have an
extra 21st chromosome
• A number of disorders (e.g., Turner’s
Syndrome, Klinefelter’s Syndrome, XYY
complement, XXX Syndrome) are caused by
missing or extra sex chromosomes
3.3 Heredity is Not Destiny
Paths from Genes to Behavior
Reaction Range
Changing Relations Between Nature and
Nurture
The Nature of Nurture
3.3 Paths From Genes to
Behavior
• Genes never cause behavior directly
• The behavioral consequences of genetic
instructions depends on environment
3.3 Reaction Range
• A genotype can lead to a range of phenotypes
depending on the environment
• People with PKU genotype: those who eat
normal diet will be mentally retarded, but those
who eat special diet will have normal
intelligence
3.3 Changing Relations Between
Nature and Nurture
• Passive G-E relation: parents pass on genotype
and environment
• Evocative G-E relation: different genotypes
evoke different responses from the environment
• Active G-E relation: actively seek environments
related to their genes
• Niche-picking: deliberately seeking
environments that fit one’s heredity
3.3 The Nature of Nurture
• Parents don’t provide exactly the same
environments for all of their children (nonshared environmental influences)
• Parents provide the child’s genes and
environment, but the child also influences her
own environment
The Relation Between Genes
and Environment
3.3 The Nature of Nurture