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Chapter 5

Chapter 5.docx

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University of Toronto Scarborough
Janelle Leboutillier

5 | Genetics and the Development of the Human Brain Genetics and Behaviour - each of approx. 100 trillion cells in your body (exception of red blood cells and sperm or egg cells) contains two complete copies of the human genome: complete set of chromosomes) - genotype: genetic composition of organism; interacts w/ environmental influences = phenotype: observable appearance of an organism - genotype consists of 23 matched pairs of chromosomes: strand of DNA found w/in nucleus of a cell  one chromosome of each pair donated by mother and father - chromosomes made up of molecules of deoxyribonucleic acid (DNA)  smaller segments = individual genes: functional hereditary unit made up of DNA that occupies fixed location on a chromosome  each one has set of instructions for making particular type of protein - gene expression: translation of genotype into phenotype of an organism - vast majority of DNA found in chromosome, some located in mitochondria (mDNA)  all mDNA originates from mother  mutates at fairly regular rate = useful to scientists interested in evolution  figure 5.1, p.125 – mDNA allows researchers to trace population history  foresnsic scientists prefer mDNA over other methods of identifying human remains - alleles: alternative version of a particular gene  w/ two sets of chromosomes, individual can have two versions of an allele at most  more than two versions of allele can exist  figure 5.2, p.126 – three alleles give rise to four types of blood  occur in pairs, one from mom and dad - homozygous: having two identical alleles for a given gene - heterozygous: having two different alleles for given gene - recessive allele: gene that produce its characteristic phenotype only when it occurs in homozygous pair - dominant allele: gene that produces phenotype regardless of whether its paired w/ heterozygous or homozygous - imprinted: gene of which only mother’s or father’s copy is expressed, not both in normal Mendelian (hereditary) sense From Genes to Proteins - genes constructed from combo of four biochemical known as bases or nucleotide: adenine (A), cytosine (C), guanine (G), thymine (T) - figure 5.3, p. 127 – process of gene expression  sequences of bases translated into proteins  strand of DNA produces copy of itself on ribonucleic acid (RNA)  codons: sequence of three bases on DNA molecule that encode one of 20 amino acids  appears on RNA  joined together by ribosomes to form a chain  when complete, chain folds into shape based on amino acid sequence = protein - humans differ substantially in rate of expression of genes in the brain  gene activity in brain is high  also unique due to our proteome: set of proteins encoded by the genome Sources of Genetic Diversity - figure 5.4, p.127 – cell division by meiosis  meiosis: cell division in sexually reproducing organisms that reduce # of chromosomes in half in reproductive cells (such as sperm, eggs, and spores) - linkage: characteristic of genes located adjacent to one another to be passed along as a group - figure 5.5, p.128 – crossing over contributes to genetic diversity  crossing over: process occurring during meiosis in which chromosomes exchange equivalent segments of DNA material Mutations - heritable alterations of genes - segment of DNA that normally encodes particular amino acid somehow switches w/ another that produces same = no effect - some may not influence phenotypical traits, or may be recessive allele - inheriting dominant allele or two recessive alleles = affect phenotype The Special Case of the Sex Chromosomes - figure 5.6, p.128 – X and Y chromosomes are not a matched pair - sex-linked characteristics: phenotypical characteristics that result from expression of genes on X chromosome that are not duplicated on the Y chromosome  figure 5.7, p.129 – probabilities of hemophilia - most of active genes on Y chromosome are involved w/ male fertility; X chromosome contains wide variety of genes  single recessive gene influences phenotype if no corresponding gene on Y chromosome  males more likely to experience sex-linked disorders than females - X chromosome inactivation: process by which one X chromosome in each female cell is silenced to equalize the amount of proteins produced by males and females  because genes on X not duplicated on Y, females produce double amounts of proteins, need to compensate for imbalance  randomly silenced  figure 5.8, p.130 – X chromosome inactivation Single Nucleotide Polymorphisms (SNPs) - variation that occurs in a gene when single base is changed from one version to the next - makes significant difference  e.g., APOE gene  AAGGTTA vs ATGGTTA  figure 5.9, p.131 – SNPs and Disease  Having two E4 gene = 91% chance of getting alzheimers  Having one = 40% chance  Having none = 20% The Roles of Heredity and Environment - heritability always refers to populations, not individuals - figure 5.10, p.131 – heritability interacts w/ environment  if keep environment constant, heritability of trait is likely to be high - studies comparing monozygotic and dizygotic twins raised together or apart are useful in sorting out relative contributions of heredity and environment  monozygotic (MZ) = share same genes  dizygotic (DZ) = share same # of genes (about 50%) as ordinary siblings  twins share similar environment before and after birth; ordinary siblings born at different times exp. greater variations in their environments Disorders of Brain Development Neural Tube Defects - anencephaly: rostral neural tube doesn’t develop normally, resulting in incomplete formation of the brain and skull  majority die in the utero or don’t survive more than few hours after birth - spina bifida: caudal part of tube doesn’t close properly  surgery typically used w/in 24 hrs of life  most will exp. paralysis of lower limbs  produce # of physical and psychological challenges, ppl w/ this live well into adulthood w/ modern treatment  effort to repair prior to birth seems useful - deficiency of folic acid might be responsible for large # of cases  average American diet doesn’t contain enough  42 nations need fortification of products w/ folic acid as preventative measure  fortification of folic acid might lead increased risk for colon cancer Genetic Disorders - trisomy 21, aka down syndrome: abnormal genetic condition resulting from a genotype w/ three copies of chromosome 21, responsible for moderate mental retardation and characteristic physical appearance  major cause is abnormal division 9disjunction) of mother’s 21 chromosome during meiosis  related to maternal age – 1/1000 births for mothers under 33 yrs, 38/1000 births for mothers over 45  physical: small skull, large tongue, almond-shaped eyes, flat nasal bridge, and abnormalities of hands and fingers  subject to heart deformities = shorter life expectancy - fragile-X syndrome: heritable genetic disorder that produces cognitive and physical abnormalities  healthy people have up to 200 codon repeats, over that leads to this condition  large #s of repeats allow chromosome to break physically  physical: low set ears, large forehead and jaw  males often show social withdrawal, fewer than 5% meet formal criteria for autism - other types deal
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