Chapter Five

Genetics and the Development of the

Human Brain

CHAPTER 5GENETICS 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