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Chapter Three 11.33.02 AM.docx

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Western University
Psychology 2040A/B
Jackie Sullivan

Chapter Three: Biological Foundations, Prenatal Development & Birth Genetic Foundations - We are made up of cells and the nucleus in the center is made up of the chromosomes (transmit genetic info) The Genetic Code - Chromosomes are made up of deoxyribonucleic acid (DNA) – double-stranded molecule which consists of chemical substances called bases (A with T & C with G) - A gene is a segment of DNA along the length of the chromosome (genes can be different in length) - DNA can duplicate itself through mitosis - Genes duplicate by sending instructions for making a rich assortment of proteins to the cytoplasm, the area surrounding the cell nucleus The Sex Cells - Gametes, or sex cells – the sperm & ovum combine - Gametes are formed through cell division process called meiosis, which halves the number of chromosomes normally present in body cells - When sperm & ovum unite at fertilization, a zygote is produced - Steps of meiosis (pg. 75); first the chromosomes pair up and each one copies itself, then crossing over occurs in which chromosomes next to each other break at one or more points along their length & exchange segments, so that genes from one are replaced by genes from another (creates new hereditary combinations), next the chromosome pairs separate into different cells, but chance determines which member of each pair will gather with others & end up in the same gamete and then finally each chromosome leaves it partner & becomes part of a gamete containing only 23 chromosomes - Men produce 4 sperm cells when meiosis is complete, whereas females only create 1 ovum o Thus, men can father a child all through life but women only have 3-4 decades as their ova are produced before birth and only a few hundred mature Boy or Girl? - The 22 matching pairs of chromosomes, which geneticists nrdber from longest (1) to shortest (22) – these are called autosomes (meaning not sex chromosomes) and the 23 pair consists of sex chromosomes o XX – female & XY – male o X is a relatively large chromosome, whereas Y is short & carries little genetic material o The sperm determines gender, whether the sperm contains X or Y Multiple Offspring - Sometimes a zygote that has started to duplicate separates into 2 clusters of cells that develop into 2 individuals o These are called identical, or monozygotic, twins because they have the same genetic makeup o Frequency of these twins is equal around the world, a variety of environmental influences & most likely a genetic influence as well (tends to run in families) - Fraternal, or dizygotic, twins, the most common type of multiple birth, result from release & fertilization of 2 ova o There are genetic & environmental influences like, ethnicity, family history, age, nutrition, number of births and fertility drugs & in vitro fertilization (pg. 77) Patterns of Genetic Inheritance - Two forms of each gene occur at the same place on the chromosomes, one inherited from the mother & one from the father. - Each form of a gene is called an allele - If the alleles from both parents are alike, the child is homozygous (DD or dd) & will display the inherited trait - If alleles differ, then the child is heterozygous (Dd) & relationships between the alleles determine the phenotype Dominant-Recessive Relationships - In many heterozygous pairings, dominant-recessive inheritance occurs - Only one allele affects the child’s characteristics (the dominant), the second allele has no effect on characteristics (the recessive) – list of traits (pg. 77) & list of diseases (pg. 78) - Heterozygous individuals with just one recessive allele (Dd) can pass that trait to their children, they are called carriers of the trait - Many diseases are the product of recessive alleles, one of the most frequently occurring recessive disorders is phenylketonuria (PKU) which affects the way the body breaks down proteins contained in many foods; without the enzyme phenylalanine builds up and can cause mental retardation (treated by a modified diet) - Can predict % of chance a child will have certain traits - Modifier genes enhance or dilute the effects of other genes, that is why some have a more mild case of PKU Incomplete Dominance - A pattern of inheritance in which both alleles are expressed in the phenotype, resulting in a combined trait or one that is intermediate between the two - The sickle cell trait, a heterozygous condition present in many black Africans, happens with 2 recessive alleles which changes red blood cells (a mild form can occur if experience oxygen deprivation) - Africans get it as carriers of it are more resistant to malaria which is common is Africa X-Linked Inheritance - Males are more likely to get an abnormal recessive allele because their sex chromosomes do not match - In females, any recessive allele on the X chromosome has a good chance of being suppressed by a dominant allele on the other X - Since the Y chromosome is short, it does not suppress it, same with the hemophilia blood clot disorder - Beside X-linked disorders, males have higher risks for many disorders (infant deaths, learning disabilities, retardation) - More males use to be born, but this is declining Genomic Imprinting - Alleles are imprinted, or chemically marked, so that one pair member (either the mother’s or the father’s) is activated, regardless of its makeup - Imprint is often temporary and is erased in the next generation & may not occur in all individuals - Imprinting helps us understand certain puzzling genetic patterns (like asthma, childhood cancers, Huntington disease & Prader-Willis syndrome, a disorder with symptoms of mental retardation & severe obesity) - Genomic imprinting can also operate on the sex chromosomes, as fragile X syndrome, the most common inherited cause of mental retardation, which occurs on the X chromosome from the mother o Fragile X syndrome also have some symptoms of autism, involves impaired social interaction, delayed or absent language & communication and repetitive motor behaviours Mutation - A sudden but permanent change in a segment of DNA - A mutation may affect only 1 or 2 genes, or may involve many - Sometimes occur spontaneously & others are caused by hazardous environmental agents (radiation) - Germline Mutation, takes place in the cells that give rise to gametes, the defective DNA passes through generations - Somatic Mutation, normal body cells mutate, an event that can occur at any time of life; the DNA defect appears in every cell derived from the affected body cell, eventually becoming widespread to cause disease or disability Polygenic Inheritance - In which many genes determine the characteristic in question - Is complex and much is still unknown Chromosomal Abnormalities - Most chromosomal defects result from mistakes during meiosis o Chromosome pair does not separate or part of it breaks off Down Syndrome st - Most common chromosomal disorder, usually from a failure of the 21 pair of chromosomes, so a person receives 3 instead of 2 (sometimes called trisomy 21) - Other less frequent forms are the translocation pattern (21 chromosome attaches to another chromosome) and the mosaic pattern (error occurs during mitosis, symptoms less extreme) - Consequences include mental retardation, memory & speech problems and slow motor development o Brain size reduction in the cerebral cortex area - Also have distinct physical features - Many develop Alzheimer’s, the most common form of dementia - Risk of having a child with Down’s increases with maternal age Abnormalities of Sex Chromosomes - Disorders of the autosomes other than Downs usually cause miscarriages o But sex chromosome disorders are often not recognized until puberty  These children usually have a very specific intellectual deficit - XYY Syndrome o Extra Y chromosome, above average height, large teeth & sometimes severe acne; normal to mildly impaired intelligence; male sexually development & fertility are normal o Incidence: 1 in 1,000 male births o No special treatment necessary - Triple X Syndrome (XXX) o Extra X chromosome, tallness & impaired verbal intelligence; female sexual development & fertility are normal o Incidence: 1 in 500-1,250 female births o Treatment: Special education to treat verbal ability problems - Klinefelter Syndrome (XXY) o Extra X chromosome, tallness, body fat distribution resembling that of females, incomplete development of sex characteristics at puberty, sterility & impaired verbal intelligence o Incidence: 1 in 900 male births o Treatment: Hormone therapy at puberty to stimulate development of sex characteristics; special education to treat verbal ability problems - Tuner Syndrome (XO) o Missing X chromosome, short stature, webbed neck, incomplete development of sex characteristics at puberty, sterility & impaired spatial intelligence o Incidence: 1 in 2,500-8,000 female births o Treatment: Hormone therapy in childhood to stimulate physical growth & at puberty to promote development of sex characteristics; special education to treat spatial ability problems Reproductive Choices Genetic Counseling - Is a communication process designed to help couples assess their chances of giving birth to a baby with a hereditary disorder & choose the best course of action in view of risks & family goals - People who have had difficulties bearing children or older women tend to do this - Family history of disorders prepares a pedigree (family tree) to estimate the risk of having an abnormal child o Carrier detection is possible for all the recessive diseases - After all this is done, it is the parents decision to conceive or adopt or use reproductive technology Prenatal Diagnosis & Fetal Medicine - Prenatal Diagnostic Methods: medical procedures that permit detection of development problems before birth o These are available for those who might bear a child with abnormalities - These techniques frequently result in complications - Advances in genetic engineering also offer a new hope for correcting hereditary defects, thousands of genes have been identified already and as a result, new treatments are being explored (gene therapy: correcting genetic abnormalities by delivering DNA carrying a functional gene to the cell) - Another approach is called proteomics, scientists modify gene-specific proteins involved in disease Prenatal Methods - Amniocentesis: o The most widely used technique, a hollow needle is inserted through the abdominal wall to obtain a sample th of fluid in the uterus; cells are examined for genetic defects; can be performed by the 14 week after conception; 1-2 more weeks are required for test results; small risk of miscarriage - Chorionic Villus Sampling o A procedure that can be used if results are desired or needed very early in pregnancy; a thin tube is inserted into the uterus through the vagina or a hollow needle is inserted through the abdominal wall; a small plug of tissue is removed from the end of one or more chronic villi, the hair-like projection on the membrane surrounding the developing organism. Cells are examined for genetic defects; can be performed 9 weeks after conception; results are available within 24 hours; entails a slightly greater risk of miscarriage than amniocentesis; also associated with a small risk of limb deformities, which increases the earlier the procedure is performed - Fetoscopy o A small tube with a light source at one end is inserted into the uterus to inspect the fetus for defects for the limbs & face; also allows a sample of fetal blood to be obtained, permitting diagnosis of such disorders as hemophilia & sickle cell anemia, as well as neural defects; usually performed between 15-18 weeks after conception but can be done as early as 5 weeks; some risk of miscarriage - Ultrasound o High-frequency sound waves are beamed at the uterus; their reflection is translated into a picture on a video screen that reveals the size, shape & placement of the fetus; permits assessment of fetal age, detection of multiple pregnancies and identification of gross physical defects; also used to guide amniocentesis, chorionic villus sampling & fetoscopy; when used more than 5 times, may increase chance of low birth weight - Ultrafast MRI o Sometimes used as a supplement to ultrasound, where brain or other abnormalities are detected & MRI can provide greater diagnostic accuracy; the ultrafast technique overcome image blurring due to fetal movements; no evidence of adverse effects - Maternal Blood Analysis o By the second month of pregnancy, some of the developing organism’s cells enter the maternal bloodstream; an elevated level of alpha-fetoprotein may indicate kidney disease, abnormal closure of the esophagus or neural tube defects, such as anencephaly (absence of most of the brain) & spina bifida (bulging of the spinal cord from the spinal column); isolated cells can be examined for genetic defects - Preimplantation Genetic Diagnosis o After in vitro fertilization & duplication of the zygote into a cluster of about 8-10 cells, 1 or 2 cells are removed & examined for hereditary defects; only if that sample is free of detectable genetic disorders is the fertilized ovum implanted in the woman’s uterus Social Issues: The Pros & Cons of Reproductive Technologies (story) Prenatal Development Conception - About once every 28 days, an ovum bursts from an ovary into a fallopian tube, the ovum then turns into the corpus luteum and secretes hormones to prepare the uterus for implantation - Sperm goes through the cervix to meet the egg in the fallopian tube o Sperm live up to 6 days o Ovum survives for 1 day Period of Zygote - Lasts for 2 weeks - Forms a blastocyst with an embryonic disk inside which will become the organism and the trophoblasts on the outside will become the nourishing structures Implantation - Between the 7 & 9 dayth - Blastocyst burrows in the uterine lining, the trophoblasts form a membrane (amnion), that encloses the developing organism in amniotic fluid, which helps keep the temperature of the prenatal world constant & provides a cushion - A yolk sac emerges that produces blood cells until the liver, spleen & bone marrow are mature - 30% of zygotes don’t survive this period The Placenta & Umbilical Cord - By the end of the 2 ndweek the trophoblasts form another protective membrane, the chorion, which surrounds the amnion, from it, tiny finger-like villi or blood vessels emerge and the placenta starts to develop - Placenta permits food & oxygen to reach the developing organism & waste to be carried away (bloods don’t mix) - Placenta is connected to the umbilical cord, which first appears as a primitive body stalk and grows 1-3 feet, it contains 1 large vein that delivers blood with nutrients & 2 arteries that remove waste Period of the Embryo - Lasts from implantation through to the 8 week of pregnancy - Most rapid prenatal changes occur (thus, most vulnerable) Last Half of the First Month - Embryonic disk forms 3 layers o Ectoderm (NS & skin), Mesoderm (muscles, skeleton, circulatory system & internal organs) & Endoderm (digestive system, lungs, urinary tract & glands) o NS develops the fastest, ectoderm folds to become a neural tube, forms the spine and the brain The Second Month - Growth continues rapidly - Eyes, ears, nose, jaw & neck form & tiny buds for limbs, internal organs are more distinct (yolk sac not needed) - At 7 weeks, production of neurons begin inside the neural tube and travel to form the major parts of the brain - Fetus can now move and respond to some stimuli (touch) Period of tth Fetus - 9 week to the end of pregnancy - Longest prenatal period (growth & finishing phase), increases rapidly in size The Third Month - In 3 month, muscles & NS become connected & organized - Body position changing, kicking, lungs expand & contract th - External genitals arrdwell-formed & sex can be detected by the 12 week - By the end of the 3 month, the first trimester is complete The Second Trimester - Between 17-20 weeks - A white cheese-like substance (vernix) covers the skin, protecting it from chapping during the long months spent in the amniotic fluid - White, downy hair (lanugo) also covers the body to help the vernix stick to the skin - Many of the organs are well-developed by the end and billions of neurons are in place, the glial cell, which support & feed the neurons continue to grow during pregnancy & after birth - At the same time, neurons are forming synapses, or connections at rapid pace - 20 week old fetus is also sensitive to noises, eye movement appears (slow then fast) The Third Trimester - The point at which the baby can survive (age of viability), occurs between 22-26 weeks o Still need oxygen assistance if born too early - Cerebral cortex enlarges and becomes more defined - Neural connectivity improves, HR reveals periods of alertness later on (28 weeks) - By the end of pregnancy, fetus takes on the beginning of a personality o Some studies show that a more active baby sometimes has a infant temperament - Greater responsiveness to external stimulation (pain sensitivity) - Habituation also occurs (adaptation to sound), can distinguish different voices & melodies - Grows more than 5 pounds & decreases movement, also a layer of fat is added for temperature regulation Prenatal Environmental Influences Teratogens - Refers to any environmental agent that causes brain damage during the prenatal period - Harm by teratogens are dependent on: o Dose o Heredity o Other negative influences o Age - Before implantation (period of zygote), teratogens barely have an impact, if so, they die - The embryonic period is the time when serious defects are most likely to occur because foundations are being made - In the fetal period, teratogens can cause a minor affect - Cause immediate & delayed damage - Bidirectional Influences between child & environment Prescriptions & Nonprescription Drugs - A sedative, thalidomide, produced gross deformities & intellectual damage - A synthetic hormone, diethylstilbestrol (DES), prescribed to prevent miscarriages, caused daughters to have high rates of vaginal cancer, malformations of the uterus & infertility & men increase rate of genital abnormalities & testes cancer - The most potent teratogen is a vitamin A derivative, Accutane, prescribe to treat severe acne, which caused eye, ear, skull, brain, heart & immune system abnormalities - Any molecule that can breech the placental barrier can cause problems, but women tend to not heed the advice o Aspirin has been linked to low bi
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