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Chapter 3

Chapter 3 - Textbook Notes

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Western University
Psychology 2040A/B
Ian Mac Donald

CHAPTER 3PRINCIPLES OF HEREDITARY TRANSMISSIONGenotype genetic endowment that an individual inherits Phenotype the ways in which a persons genotype is expressed in observablemeasurable characteristicsConception the moment of fertilization when a sperm penetrates an ovum forming a zygote The Genetic MaterialWhen a sperm cell penetrates the lining of the ovum a biochemical reaction repels other sperm Within a few hours the sperm cell begins to disintegrate releasing its genetic materialsOvum also releases its genetic materialo New cell nucleus forms around the hereditary informationcalled a zygote single cell formed at conception from the union of a sperm and an ovum New cell nucleus contains 46 chromosomes threadlike structures made up of geneseach of which consists of thousands of genes hereditary blueprints for development that are transmitted unchanged from generation to generationo Chromosomes come in matching pairsone member of each chromosome pair comes from the mothers ovum and the other from the fathers spermo Each parent contributes 23 chromosomes to each of their children Genes on each chromosome also function as pairstwo members of each gene pair being located at the same sites on their corresponding chromosomes Genes are actually stretches of deoxyribonucleic acid DNA long doublestranded molecules that make up chromosomeso Resemble a twisted ladder o Provides the chemical basis for developmento Can duplicate itselfthis is what makes it possible for a onecelled zygote to develop into a complex human beingGrowth of the ZygoteProduction of Body Cells As the zygote moves from the fallopian tube to the uterus its begins to reproduce itself through mitosis the process in which a cell duplicates its chromosomes and then divides into two genetically identical daughter cellso Before each division the cell duplicates its 46 chromosomeso Division of the cell then proceeds By the time a child is born they have billions of cells Mitosis continues through life generating new cells that enable growth and replacing old ones that are damaged Every new cell contains an exact copy of the 46 chromosomes we inherited at conception The Germ or Sex CellsGerm cells serve one special hereditary functionto produce gametes sperm in male ova in femalesDifferent type of cell reproduction than mitosisProduction of Gametes Through Meiosis Meiosis the process in which a germ cell divides producing gametes sperm or ova that each contain half of the parent cells original complement of chromosomes in humans the products of meiosis contain 23 chromosomes Germ cell first duplicates its 46 chromosomes Then crossingover takes place a process in which genetic material is exchanged between pairs of chromosomes during meiosiso Transfer of genes during crossingover creates new and unique hereditary combinations Next pairs of duplicated chromosomes segregate into two new cells that each contain 46 chromosomes Finally the new cells divide so that each of their resulting gametes contains 23 single or unpaired chromosomesAt conception then a sperm with 23 chromosomes unites with an ovum with 23 chromosomesproduces a zygote with 46 chromosomes Hereditary UniquenessWhen a pair of chromosomes segregates during meiosis it is a matter of chance which of the two chromosomes will end up in a particular parent cell Independent Assortment each pair of chromosomes segregates independently of all other chromosome pairs during meiosis Each parent can produce over 8 million different genetic combinations in their sperm or ovaany couple could theoretically have 64 trillion babies without producing two children who inherited precisely the same set of geneso Odds of exact genetic replication in two siblings are even smaller than 1 in 64 trillioncrossingover process alters the genetic composition of chromosomes thereby increasing the number of possible variations in an individuals gametes Siblings resemble one another to some extent because their genes are drawn from a gene pool provided by the same two parentsMultiple Births One circumstance under which two people will share a genotypeMonozygotic Identical Twins twins who develop from a single zygote that later divides to form two genetically identical individualso Occur about every 1 in 250 birthso Should show very similar developmental progress if genes have much effect on human development More common in approx 1 in 125 birthsDizygotic Fraternal Twins twins that result when a mother releases two ova at the same time and each is fertilized by a different sperm producing two zygotes that are genetically differento No more genes in common than any other pair of siblingso Often differ considerably in appearance and need not be the same sex
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