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Chapter 5

PSYB64 chapter 5

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Pare, Dwayne

GENETICS AND BEHAVIOR  different phenotypic traits result from the interaction between alternative forms of a gene called alleles  there are three different alleles for blood type A, B and O giving rise to four blood types  at a given site if someone has two identical alleles, individual will be considered homozygous for genes  If a person has two different alleles, such as gene for type A and gene for type O blood. individual will be considered heterozygous for that gene  recessive vs dominant allele; a dominant allele produces a phenotypical trait regardless of the homozygous and heterozygous  non-typically you could have genes that are imprinted where only one allele is expressed and its identity depends on the parent that supplied the allele. these genes have been implicated in number of genetic, behavioural and cancer disorders From genes to proteins  4 nucleic acids, ATGC. codons are group of 3 bases  each codon contains instruction for synthesis of 20 proteins  what gives rise to the complexity of human genome?  the fact that humans and other creatures differ greatly in terms of their expression of genes in brain, it is much higher..whereas our rate of expression in liver is same as those of chimpanzees  proteome also contribute to this complexity, which is the set of proteins encoded and expressed by genome Sources of Genetic Diversity  meiosis is the process through which egg and sperm are formed and divided  during this process, parental chromosome pairs are divided in half, leaving only one chromosome from each pair in egg or sperm cell  when the egg and sperm from two parents combine, the resulting zygote, once again, contains the full complement of 23 pair of chromosome  basically, meiosis results in two reproductive cells each containing one set of 23 chromosomes  some genes are physically located close to one another and they could be passed on as a group through process known as linkage  however, chromosomes linking up prior to meiosis physically cross each other and exchange equivalent sections of genetic material, resulting in combination of genes not seen in either parent  mutations  replication errors  inheriting a dominant mutant allele, or two copies of recessive mutant allele will affect an organisms phenotype (expression of mutations affecting phenotype)  mutations with positive affect are spread while negative ones usually disappear from the population  the special case of sex chromosome  most of the active genes on the Y chromosome are involved with male fertility and X chromosome contains the wide variety of genes  sex linked characteristics result from genes on X that are not duplicated on Y  on chromosomes other than X, Y, two copies of recessive genes are required or one copy of dominant gene to produce the trait in organism  genes occurring on X, single recessive gene influences the phenotype without any correspondent gene on the Y chromosome. Why are males more likely to experience sex- linked disorders than females.  this phenomenon is also known as X chromosome inactivation. since many genes on X are not implicated on Y, females are able to produce more proteins than males  example can be seen from calicos cats, only female cats can be calicos because genes for orange or black fur are located in the same area of X chromosome . a male cat could only have orange or black fur but not both because he only has one X chromosome  SNP or single nucleotide polymorphism  alleles whose genetic code differs in only one location The Roles of Heredity and Environment  genetics explains how much variation in trait observed in population is due to genetic difference, so heritability refers to population and not individuals  heritability includes environment as well  for example, IQ, families with environments of proper nutrition, well-educated, affluent, their IQ would be impacted by both IQ and heredity than those lacking those environment. if the environment is held constant than the genetics have more of an impact  identical, MZ, or fraternal, DZ twin studies reveal much about the nature of nurture n nature debate  a study found that identical twins raised apart or together were very similar, whether the correlation for a particular trait was high or low DEVELOPMENT Growth and differentiation of the Nervous System  a week after conception human zygote has already developed three differentiated bands of cells known as Germ Layers  outer layer or ectoderm eventually develops into CNS, skin and hair  middle layer or the mesoderm forms the connective tissue, muscles, blood vessels, bone and the urogenital systems  final layer is the endoderm forms internal organs including stomach + intestines  post 3rd week, after conception, cells of the ectoderm differentiate into a new layer known as the neural plate.  cells differentiate in resposne to the combination of genes and inducing factors or chemical signals from other cells.  skin cells only form when BMP protein is present. inducing factors can block these BMPs and allow for the growth of neural tissue  18-23 days the developing ectoderm folds and forms the neural tube. tube remains to be the ventricles and central canal of spinal cord but the surrounding tissue transforms into brain and spinal cord (how are spinal cord and brain formed)  cell migration  radial glia or specialized progenitor cells guide this process. two thirds of migrating cells wrap around radial glia and move along them. . rest of the cells move on horizontal direction. after migration, radia pull back their branches but can stay in place throughout adhulthood  migrating cells form the cerebral cortex(how is cerebral cortex formed) in an inside-out fashion. they have to pass through many levels upwards. this migration can be disrupted at certain points  disruption of migration during second trimester is implicated in the hippocampal disarray in schizophrenic patients  differentiation  the neural tube goes through further diff. first process differentiates the ventral and dorsal parts of the neural tube. second one further differentiates the neural tube along its rostral- caudal axis)  dorsal-ventral specialization  neurons in the ventral half develop into motor neurons and neurons in the dorsal part develop into sensory neurons.  regulation of ventral process is done by proteins known as sonic hedgehog released by notochord in the mesoderm. notochord develops into vertebrae surrounding spinal cord.  differentiation of dorsal parts happens in response to BMP proteins, the inducing agent, as mentioned above, is also responsible for earlier differentiation of ectoderm  the dorsal BMP and ventral sonic hedgehog differentiation extends up through the hindbrain and midbrain  Rostral-caudal specialization  this specialization results in the division of NS into spinal cord, hindbrain, midbrain and forebrain.  hindbrain differentiation is controlled by inducing proteins encoded by Hox genes. the specific cranial nerve nuceli result because of the different Hox genes expression  not much is clear about the differentiation processes of mid and forebrain but most likely include same process as hindbrain  organization of cortex into association, primary and somatosensory areas seems to be the result of combination of internal genetic factors and the external inducing factors.  from research it is apparent that cortex is capable of being modified  Growth of Axons and Dendrites  their growth is guided at regular intervals  (Sperry, 1945) axons follow some type of chemical pathway to their target  developing axons and dendrites end up in growth cones which have three basic sttructural parts  1. main body containing organelles like mitochondria, microtubules and others  2. filopodia: long fingerlike extensions from the core  3. lamellipodia: flat, sheetlike extensions from the core, located between the filopodia  FIGURE 5.18! Filopodia and Lemellipodia are both capable of movement and pull the growing branches behind them  microtubules from the main body of the cone move forward as the cone extends, forming a new segment of axon or dendrite  Filopodia can signal the growth cone to move in any direction.  respond to the attracting and inhibiting chemicals released by the guidepost cells along the way  Axons that are growing in the same direction stick together in a process of fasiculation. this sticking together is caused by Cell adhesion molecules or CAMS. which are molecules on the surface of growing axon  Formation of new synapses  interaction with the target cell influences the type of NT released by the presynaptic cell.  Steps in the formation of synapse at the Neuromuscular junction  1. Growth cone approaches muscle fiber  Growth cone makes contact  Synaptic Vesicles accumulate in axon terminal and synaptic receptors cluster at point of contact  to complete the process of wiring the NS, the incoming axon must identify appropriate post synaptic cells and appropriate pre and post synaptic structures must develop. this process can be observed at the neuromuscular junction  movement of receptor to synaptic site: a deli
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