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Chapter Five
Genetics and the
Development of the
Human Brain
CHAPTER 5
GENETICS AND DEVELOPMENT OF THE
HUMAN BRAIN
Genetics and Behavior
• Genotype – set of genetic instructions
– 23 pairs of chromosomes made up of DNA
• Phenotype – observable traits
• Gene Expression – genetic instructions
converted into a feature of a living cell
• Mitochondrial DNA (mDNA) originates from
mother
• Alleles – alternative versions of particular
gene
Figure 5.1 Mitochondrial DNA Allows
Researchers to Trace Population History
Figure 5.2 Three Alleles Give Rise to
Four Types of Blood
Genetics and Behavior
• From Genes to Proteins
– Constructed from adenine, cytosine, guanine, thymine
– Human gene activity in the brain very high
– Proteome – proteins encoded and expressed by genome
• Sources of Genetic Diversity
– Meiosis and crossing over
– Mutations – chromosome replication errors
– The Special Case of the Sex Chromosomes
• Sex-linked characteristics
• X chromosome inactivation
– Single Nucleotide Polymorphisms (SNPs)
Figure 5.3 The Process of Gene Expression
Figure 5.4 Cell Division by Meiosis
Figure 5.5 Crossing Over Contributes to Genetic
Diversity
Figure 5.7 Probabilities of Hemophilia
Figure 5.9 SNPs and Disease
Genetics and Behavior
• The Roles of Heredity and Environment
– Heritability always refers to a population not to individuals
– Heritability cannot be assessed without taking the
environment into account
– Twin and adoption studies
• Minnesota Study of Twins Reared Apart
Figure 5.10 Heritability Interacts with
Environment
Figure 5.11 Similarities in Identical Twins
Development
• Growth and Differentiation of the Nervous
System
– Early differentiation
• Cell germ layers – ectoderm, mesoderm, and endoderm
• Neural plate, neural groove, neural tube
– Formation of neurons and glia
• Originate from cells in the ventricular zone
• Progenitor cells divide by mitosis
– Cell migration
• Guided by radial glia
• Cells in cerebral cortex arrive in an inside-out fashion
Figure 5.14 The Closing of the Neural Tube
Figure 5.15 Neurogenesis
Figure 5.16 Radial Glia Guide the Migration of
New Cells
Development
• Differentiation
– Differentiation of the dorsal and ventral halves of neural
tube
– Differentiation of the neural tube along the rostral-caudal
axis
• Growth of Axons and Dendrites
– Developing axons and dendrites end in growth cones
– Filapodia and lamellipodia
Figure 5.17 Growth Cones Guide Axons to Their
Targets
Figure 5.18 Growth Cones Respond to a Variety
of Cues
Development
• Formation of Synapses
– Interaction with target cells influences the type of
neurotransmitter released by the presynaptic cell
– Movement of receptors to the synaptic site guided by
chemical release by presynaptic and postsynaptic
structures
• Cell Death
– Apoptosis = programmed cell death
– Neurotrophins influence the survival of a neuron
Figure 5.19 Steps in the Formation of a Synapse
at the Neuromuscular Junction
Figure 5.20 Growing Axons Compete for Nerve
Growth Factor
Development
• Synaptic Pruning
– Number of functional synapses is reduced
• Myelination
– Occurs in rostral direction starting with the spinal cord,
then hindbrain, midbrain, and forebrain
– Burst in myelination around the time of birth
– Prefrontal cortex not completely myelinated until early
adulthood
Figure 5.21 Synaptic Rearrangement over the
Lifespan
Effect of Experience on Development
• Plasticity
• Experience and the Visual System
– Early in development cells of LGN and primary visual
cortex receive input from both eyes
– Experience with sensory information influences
segregation of ocular dominance
Figure 5.22 Input Influences the Development of
the Optic Tectum
Figure 5.23 Input from Both Eyes Competes for
the Control of Target Cells in the LGN
Figure 5.24 Early Experiences Affect the
Organization of Ocular Dominance Columns
Effect of Experience on Development
• Experience and Social Behavior
– Lorenz demonstrated imprinting in several species of birds
– Romanian children and social deprivation
• Ending a Critical Period
– Conclusion of growth spurt in myelin coincides with
reduced abilities to learn additional languages
– Presence or absence of neurotrophins may influence
timing of critical periods
Disorders of Brain Development
• Neural Tube Defects
– Anencephaly
– Spinal bifida
• Genetic Disorders
– Down syndrome
– Fragile-X syndrome
– PKU
• Environmental Toxins
– Fetal alcohol syndrome
Redevelopment in Response to
Damage
• Anterograde degeneration, retrograde degeneration,
and transneuronal degeneration
• Genetic therapy and Nogo inhibitors as possible
treatments
The Adult Nervous System
• Fully mature at 25; weight of brain starts to decrease
at 45
• Neurogenesis in Adulthood
• Alzheimer’s Disease
– neurofibrillary tangles
– amyloid
Figure 5.27 Neuronal Responses to Damage
Figure 5.28 Alzheimer’s Disease Produces
Structural Abnormalities in Neurons