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PSYB64H3 (93)

Chapter 5

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Janelle Leboutillier

DISCOVERING BIOLOGICAL PSYCHOLOGYCHAPTER 5 GENETICS AND THE DEVELOPMENT OF THE HUMAN BRAIN GENETICS AND BEHAVIOUR p Each of the approximately 100 trillion cells in your body with the exception of red blood cells and sperm or egg cells containing two complete copies of the human genome a set of instructions for constructing a human being y Genome A complete set of chromosomesp Your personal set of genetic instructions is genotype which interacts with environmental influences to produce phenotype or your observable characteristicsy Genotype The genetic composition of an organismy Phenotype The observable appearance of an organismp Your genotype consists of 23 matched pairs of chromosomes y Chromosomes A strand of DNA found within the nucleus of a cell p The chromosomes are made up of molecules of deoxyribonucleic acid DNASmaller segments of DNA form individual genesDeoxyribonucleic acid DNA Molecules that compose chromosomes y y Genes A functional hereditary unit made up of DNA that occupies a fixed location on a chromosome p Gene expression occurs when these genetic instructions are converted into a feature of a living cell o Each cell may contain the instructions for an entire human organism but only a subset of instructions is expressed at any given time and locationy Gene expression The translation of the genotype into the phenotype of an organism p Some DNA is located in mitochondria mitochondrial DNA mDNA originates from the mother o mDNA is especially useful to scientists interested in evolution arguments suggesting that human life first evolved in Africa and migrated to other parts of the world are bolstered by the analysis of mDNAp Different phenotypical traits result from the interactions between alternative versions of a particular gene known as alleleso With two sets of chromosomes an individual can have at most two versions of an alleleo However many more than two versions of an allele can exist within a population Example There are 3 different alleles for blood type A B and Oy Alleles uh LEEL Alternative version of a particular gene y Homozygous Having two identical alleles for a given gene y Heterozygous Having two different alleles for a given gene y Recessive Allele A gene that will produce its characteristic phenotype only when it occurs in a homozygous pair o The gene for Type O blood is recessive which means that the only way a person can have the Type O phenotype is if he or she receives a gene for Type O from each parenty Dominant Allele A gene that produces its phenotype regardless of whether its paired allele is heterozygous or homozygous p Approximately 1 percent of mammals genes are imprinted which means that only one allele is expressedp Imprinted genes have been implicated in a number of genetic disorders behavioral disorders including autism and bipolar disorder and vulnerability to cancery Imprinted gene A gene of which only the mothers of the fathers copy is expressed but not both in the normal Mendelian senseFROM GENES TO PROTEINS p Genes are constructed from combinations of four biochemical known as bases or nucleotides adenine A cytosine C guanine G and thymine Tp A strand of DNA produces a copy of itself on a strand of ribonucleic acid RNA y Ribonucleic acid RNA A molecule that is similar to DNA that participates in the translation of genetic sequences into proteins y Codons KOHdon A sequence of three bases on the DNA molecule that encodes one of 20 amino acids p There are somewhere between 20000 and 25000 genes in the human genome p Humans and other creatures differ substantially in the rate of expression of genes in the brainp We are also uniquely human due to our proteome the set of proteins encoded and expressed by the genomey Proteome The set of proteins encoded by the genomeSOURCES OF GENETIC DIVERSITY p Egg and sperm cells are formed through the process of meiosisp When the egg and sperm cells from the two parents combine the resulting zygote once again contains the full complement of 23 pairs of chromosomesy Meiosis Cell division in sexually reproducing organisms that reduces the number of chromosomes in half in the reproductive cells such as sperm eggs and spores o A single human can produce eggs or sperm with 8388608 combinations of their chromosomesp Genes that are physically located close to one another on the same chromosome are often passed along to offspring as a group in a process known as linkagey Linkage The characteristic of genes located adjacent to one another to be passed along as a group p Linked genes are not automatically inherited together In the process of crossing over chromosomes lining up prior to meiotic division physically cross one another and exchange equivalent sections of genetic material This results in unique combinations of genes not seen in either parenty Crossing over A process occurring during meiosis in which chromosomes exchange equivalentsegments of DNA materialMUTATIONS p In the process of chromosome replication errors or mutations happen p There is some overlap in the genetic encoding of amino acids p Mutations may occur in segments of DNA that do not appear to influence phenotypical traits or a mutation may result in a recessive allele p Inheriting a dominant mutant allele or two copies of recessive mutant allele will affect an organisms phenotype p A mutant allele may have negative even fatal consequences for the organism in the latter case it might disappear from the population y Mutations A heritable alteration of genesTHE SPECIAL CASE OF THE SEX CHROMOSOMES p Most of the active genes on the Y chromosome are involved with male fertility whereas the X chromosome contains a wide variety of genes y Sexlinked characteristics Phenotypical characteristics that result from expression of genes on the X chromosomes that are not duplicated on the Y chromosomep On chromosomes other than the X and Y one would need two copies of a recessive gene or only one copy of a dominant gene to produce the trait in the organism p In the case of genes occurring on the X chromosome however a single recessive gene influences the phenotype when here is no corresponding gene on the Y chromosomeo For this reason males are more likely to experience sexlinked disorders than femalesp The lack of matching pairs for most genes on the sex chromosomes appears to have led to a phenomenon known as X chromosome inactivation y X chromosome inactivation The process by which one X chromosome in each female cell is silenced to equalize the amount of proteins produced by males and femalesy Many genes on the X chromosomes are not duplicated on the Y chromosome females could produce double the amounts of some proteins compared to malesy REFER TO FIGURE 58 FOR EXAMPLE y Sven Bocklandt and his colleagues 2006 have suggested that variations in the degree of xinactivation of mothers may be related to the sexual orientation of sons o In some individuals who have the silenced X from one parent in much greater numbers than the X from the other parent a condition referred to as extreme skewo Extreme skew is more common among mothers who have given birth to gay sons 13 than among mothers of heterosexual sons 4 o Note the majority of mothers of gay sons did not show extreme skewing so this is not a simple solution to the origin of sexual orientation SINGLE NUCLEOTIDE POLYMORPHISMS SNPs
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