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Chapter 3

Chapter 3 - Introduction to Developmental Psychology

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Psychology 2410A/B
Sandra Hessels

Biological Foundations Prenatal Development and Birth directly observable characteristics are called phenotypes they depend in part on the individuals genotypethe complex blend of genetic information that determines our species and influences all our unique characteristics Genetic Foundations each of us is made up of trillions of units called cells within every cell is a control centre or nucleus containing rodlike structures called chromosomes which store and transmit genetic information the number varies from species to species46 for human beings human chromosomes come in 23 matching pairs each member of a pair corresponds to the other in size shape and genetic functions with one chromosome inherited from the mother and the other form the father The Genetic Code chromosomes are made up of a chemical substance called deoxyribonucleic acid or DNA DNA is a long doublestranded molecule that looks like a twisted ladder each rung of the ladder consists of a pair of chemical substances called bases although the bases always pair up in the same way across the ladder rungsA with T C with Gthey can occur in any order along its sides it is this sequence of base pairs that provides genetic instructions a gene is a segment of DNA along the length of the chromosome genes can be of different lengths between 98 and 99 percent of chimpanzee and human DNA is identical individuals around the world are about 991 percent genetically identical a unique feature of DNA is that it can duplicate itself through a process called mitosis this special ability permits a single cell formed at conception to develop into a complex human being composed of a great many cells during mitosis the chomosomes copy themselves as a result each new body cell contains the same number of chromosomes and the identical genetic information genes accomplish their task by sending instructions for making a rich assortment of proteins to the cytoplasm the area surrounding the cell nucleus proteins which trigger chemical reactions throughout the body are the biological foundation on which our characteristics are built how do humans with far fewer genes than scientists once thought manage to develop into such complex beingsthe answer lies in the proteins our genes make which break up and resemble in staggering varietyabout 10 to 20 million altogether simpler species have far fewer proteins the communication system between the cell nucleus and cytolasm which finetunes genetic activity is more intricate in humans than in simpler organisms within the cell a wide range of environmental factors modify gene expression The Sex Cells new individuals are created when two special cells called gametes or sex cellsthe sperm and the ovumcombine a gamete contains only 23 chromosomes half as many as a regular body cell gametes are formed through a cell division process called meiosis which halves the number of chromosomes normally present in body cells when sperm and ovum unite at fertilization the cell that results called a zygote will again have 46 chromosomes the steps involved in meiosisfirst chromosomes pair up and each one copies itself then a special event called crossing over occurs in which chromosomes next to each other break at one or more points along their length and exchange segments so that genes from one are replaced by genes from another this shuffling of genes creates new hereditary combinations next the chromosome pairs separate into different cells but chance determines which member of each pair will gather others and end up in the same gamete finally each chromosome leaves its partner and becomes part of a gamete containing only 23 chromosomes instead of the usual 46 meiosis helps us understand why siblings differ even though they also have features in common because their genotypes come from the same pool of parental genes the genetic variability produced by meiosis is important in an evolutionary sense because it generates offspring that vary in phenotype it increases the chances that at least some members of a species will cope with everychanging environments and will survive in the male four sperm are produced when meiosis is complete the cells from which sperm arise are produced continuously throughout life for this reason a healthy man can father a child at any age after sexual maturity in the female meiosis results in just one ovum the remaining genetic material degenerates the female is born with all her ova already present in her ovaries and she can bear children for only three to four decades there are plenty of female sex cells about 1 to 2 million are present at birth 40000 remain at adolescence and approximately 350 to 450 will mature during a womans childbearing years Boy or Girl 22 matching pairs of chromosomesgeneticists number from longest 1 to shortest 22 these are called autosomes meaning not sex chromosomes the twentythird pair consists of sex chromosomes in females this pair is called XX in males it is called XY the X is a relatively large chromosome whereas the Y is short and carries little genetic material when gametes form in males the X and Y chromosomes separate into different sperm cells the gametes that form in females all carry an X chromosome therefore the sex of the new organism is determined by whether an Xbearing or a Ybearing sperm fertilizes the ovum Multiple Offspring sometimes a zygote that has started to duplicate separates into two clusters of cells that develop into two individuals these are called identical or monozygotic twins because they have the same genetic makeup the frequency of identical twins is the same around the worldabout 1 in every 330 births fraternal or dizygotic twins the most common type of multiple birth result from the release and fertilization of two ova fraternal twins are genetically no more alike than ordinary siblings older maternal age fertility drugs and in vitro fertilization are major causes of the dramatic rise in fraternal twinning and other multiple births in industrializated nations over the past several decades currently fraternal twins account for 1 in about every 60 births in the United States
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