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Chapter 2

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Mark Schmuckler

Child Psychology –3 Canadian Edition Chapter 2 – Heredity and the Environment - Transactions between genes and the environment that begin before birth, make each individual unique and continue to shape their characteristics throughout their life - Genotype: particular set of genes that a person inherits from parents - Phenotype: observable, visible, measurable expression of a person’s physical and behavioural characteristics; product of genotype and environment The Process of Genetic Transmission - Egg, or ovum – largest human cell, sperm – smallest human cell - Chromosomes and Genes o Chromosomes, located inside the nucleus, carry genetic information that help direct development o Human body cells each have 46 chromosomes, human sex cells have 23 chromosomes o Meiosis: process where cells are divided to produce new cells with only half of the normal complement of chromosomes (i.e. sex cells, so that they can unite as a zygote and have 46 chromosomes)  Chromosome replication, crossing over between chromosomes, chromosome pairs separate to form two cells, chromosomes separate again forming another 2 cells, 4 cells in total o Crossing over: process where equivalent sections of homologous chromosomes switch places ensuring unique genetic inheritance o The zygote continues to divide producing new cells with the full 46 chromosomes, gradually becoming a multi-cellular organism through mitosis - Genes, DNA, and Protein o DNA – binding element of chromosome, stores genetic information and transmits it during reproduction, made up of nucleotides (compound containing nitrogen base, a simple sugar, and a phosphate group) o 4 nucleotides bond, but only adenine and thymine will bond, as will cytosine and guanine o Genes (portions of the DNA) located on particular sites of the chromosome, code for production of certain proteins each of which give cells different properties (i.e. certain protein gives bone cells their hardess) Genetic Influences on Development - Genes never work in isolation, but always in combination with environmental influences - The Transmission of Traits: A Basic Model o Mendel, laws of inheritance of characteristics  Principle of segregation  Each inherited trait comes from one’s parents as a separate unit  Principle of independent assortment  Inheritance of various traits occurs independently of one another o Allele: another form of any given gene; an alternate; a gene typically has 2 alleles, one from the mother and one from the father o If one inherits the same allele from each parent, they are homozygous for the gene and trait that goes with it, if the alleles are different, they are heterozygous (i.e. allele combination: AA, Aa, aa, aA) o Heterozygous genes can produce an outcome intermediate between the traits, can both express their traits at the same time, or can express only the trait of the dominant, rather than the recessive, allele o Co-dominance: genetic pattern where heterozygous alleles express their traits for which they code simultaneously and with equal force - Genes on the Sex Chromosomes: Exceptions to the Rule o One pair of chromosomes are the sex chromosomes which determine an individual’s gender (Female – XX, male – XY) o X chromosomes are about 5 times bigger than Y chromosomes and carries more genes o Some genes on the X won’t have equivalent genes on the Y, which then results in automatic expression of any recessive genes on the X o Females, with two X chromosomes, have a chance of inheriting a dominant gene to counteract the recessive and thereby have less frequent x-linked recessive gene expressions - Interactions Among Genes o Some characteristics are determined by many pairs acting together o A single pair of alleles may influence more than one trait; modifier genes: genes that exert their influence indirectly, by affecting the expression of other genes - Genetic Disorders o Why Harmful Alleles Survive  Harmful alleles are not harmful in the heterozygous state  When two individuals possess the heterozygous state (ex. Np), there is a 25% chance that their offspring will receive both recessive genes (i.e. NN, Np, Np, pp – pp indicates an offspring with both recessive genes)  Some potentially harmful alleles survive because they are beneficial in combination with a normal allele o Chromosomal Abnormalities  Developmental disorders can be caused by defects on the entire chromosome  Chromosomal defects are not normally present in a child’s parents, but arise during meiosis  Defects often result in spontaneous abortion, however, sometimes with certain chromosomes, the zygote survives and a child is born with a chromosomal defect o Down Syndrome  Heightened susceptibility to leukemia, heart disorders, respiratory infections, and moderate to severe mental retardation, greater risk of developing Alzheimer’s  Trisomy 21 - Caused by extra chromosome at chromosome set 21  Extra chromosome most often comes from the mother’s egg when 21 st chromosome pair fails to separate in meiosis, happens more often in older, pregnant women  Combination of both an older father and older mother (>35) increases risk of having a child with Down Syndrome  Rate of intellectual growth begins to decline after about a year, unless they receive special therapy  Children with Down Syndrome can become competent adults granted caregivers provide the necessary stimulating environment and enlist the appropriate training/therapy o Sec-Chromosomal Anomalies  Turner Syndrome – occurs in females with only one X chromosome, remain short, have unusually shaped mouths and ears, do not develop secondary sex characteristics unless given female hormones, sterile because internal reproductive organs do not develop normally  Triple X females – females inherits 3 X chromosomes, physically normal, cognitive abilities are affected  Klinefelter’s Syndrome – extra X chromosome in males, has female characteristics, is sterile, cognitive impairments  Fragile X chromosome – X is narrowed in some areas, more frequent in males, physical, psychological, and social problems  Severity of the symptoms that arise with hereditary disorders related to degree to which the person has a supportive environment - Box 2.1 – Sickle Cell Anemia: A Double-Edged Sword o Sickle cell anemia – severe and often fatal; red blood cells of an afflicted person become elongated and take on the shape of a sickle when they are low in oxygen (as opposed to remaining disk shaped as normal RBCs do) o Sickle shape causes them to get stuck in small blood vessels resulting in severe pain, tissue damage, and possible death if critical vessels are blocked o The spleen works to remove abnormal cells from the blood which then results in anemia (too few RBCs) o Allele on chromosome 11 causes sickle cell anemia o Sickle cell allele also confers protection against malaria; people with one sickle cell allele have built in resistance to malaria – malarial parasites invade the RBCs, which become sickle shaped when low in oxygen, but when the spleen removes the abnormal cells, it also removes the malarial parasite Genetic Counselling and Genetic Engineering - Genetic counselling – health care service, provides medical information about genetic disorders and risks to couples, can help make personal decisions regarding their health, their pregnancy, and their child’s health - Box 2.2 – The New Reproductive and Genetic Technologies o In vitro fertilization – fertilization “in glass”, used to make child-bearing possible for women who fallopian tubes are blocked  Egg is fertilized with sperm in a glass dish, when it has divided to the 8-cell stage, it is inserted into the woman’s uterus  Eggs can be frozen, or inserted into a surrogate if the mother’s uterus is unfit o Legal, ethical issues are plenty on the matter  I.e. What legal rights does a donor or a surrogate have over the zygote? - Prenatal Diagnostic Techniques o Commonly Used Tests  Risk of disorder can prompt testing of a fetus
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