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Chapter Five

Genetics and the

Development of theHuman Brain

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CHAPTER 5GENETICS AND DEVELOPMENT OF THE

HUMAN BRAIN

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Genetics and Behavior

• Genotype – set of genetic instructions – 23 pairs of chromosomes made up of DNA

• Phenotype – observable traits

• Gene Epression – genetic instructionsconverted into a feature of a living cell

• !itochondrial DNA "mDNA# originates

from mother• Alleles – alternative versions of

particular gene

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$igure %&' !itochondrial DNA Allo(s)esearchers to *race Population

+istory

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$igure %&2 *hree Alleles Give )ise to$our *ypes of Blood

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$igure %&3 *he Process of GeneEpression

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$igure %& ,ell Division by !eiosis

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$igure %&% ,rossing ver ,ontributes toGenetic Diversity

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$igure %&4 Probabilities of +emophilia

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$igure %&5 .NPs and Disease

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Genetics and Behavior

•  *he )oles of +eredity andEnvironment – +eritability al(ays refers to a population not to

individuals – +eritability cannot be assessed (ithout ta0ing

the environment into account

 – *(in and adoption studies

• !innesota .tudy of *(ins )eared Apart

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$igure %&'6 +eritability 7nteracts (ithEnvironment

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$igure %&'' .imilarities in 7dentical *(ins

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Development

• Gro(th and Di8erentiation of theNervous .ystem – Early di8erentiation

• ,ell germ layers – ectoderm- mesoderm- and endoderm• Neural plate- neural groove- neural tube

 – $ormation of neurons and glia• riginate from cells in the ventricular 9one

•Progenitor cells divide by mitosis

 – ,ell migration• Guided by radial glia

• ,ells in cerebral corte arrive in an inside/out fashion

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$igure %&': )adial Glia Guide the!igration of Ne( ,ells

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Development

• Di8erentiation – Di8erentiation of the dorsal and ventral halves

of neural tube

 – Di8erentiation of the neural tube along therostral/caudal ais

• Gro(th of Aons and Dendrites – Developing aons and dendrites end in gro(th

cones

 – $ilapodia and lamellipodia

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$igure %&'4 Gro(th ,ones Guide Aonsto *heir *argets

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$igure %&'; Gro(th ,ones )espond toa

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Development

• $ormation of .ynapses – 7nteraction (ith target cells in=uences the type

of neurotransmitter released by the presynaptic

cell – !ovement of receptors to the synaptic site

guided by chemical release by presynaptic andpostsynaptic structures

• ,ell Death – Apoptosis > programmed cell death

 – Neurotrophins in=uence the survival of a neuron

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$igure %&'5 .teps in the $ormation of a.ynapse at the Neuromuscular

 ?unction

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$igure %&26 Gro(ing Aons ,ompetefor Nerve Gro(th $actor

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Development

• .ynaptic Pruning – Number of functional synapses is reduced

• !yelination – ccurs in rostral direction starting (ith the

spinal cord- then hindbrain- midbrain- andforebrain

 – Burst in myelination around the time of birth

 – Prefrontal corte not completely myelinateduntil early adulthood

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$igure %&2' .ynaptic )earrangementover the @ifespan

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E8ect of Eperience on Development

• Plasticity

• Eperience and the

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$igure %&22 7nput 7n=uences theDevelopment of the ptic *ectum

$i % 23 7 f B h E

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$igure %&23 7nput from Both Eyes,ompetes for the ,ontrol of *arget

,ells in the @GN

$i % 2 E l E i A8 t

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$igure %&2 Early Eperiences A8ectthe rgani9ation of cular Dominance

,olumns

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E8ect of Eperience on Development

• Eperience and .ocial Behavior – @oren9 demonstrated imprinting in several

species of birds

 – )omanian children and social deprivation

• Ending a ,ritical Period – ,onclusion of gro(th spurt in myelin coincides

(ith reduced abilities to learn additional

languages

 – Presence or absence of neurotrophins mayin=uence timing of critical periods

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Disorders of Brain Development

• Neural *ube Defects – Anencephaly

 – .pinal bida

• Genetic Disorders – Do(n syndrome

 – $ragile/1 syndrome

 – PC

• Environmental *oins – $etal alcohol syndrome

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)edevelopment in )esponse toDamage

• Anterograde degeneration- retrogradedegeneration- and transneuronal

degeneration• Genetic therapy and Nogo inhibitors as

possible treatments

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$igure %&24 Neuronal )esponses toDamage

$i % 2; Al h i Di

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$igure %&2; Al9heimers DiseaseProduces .tructural Abnormalities in

Neurons