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Chapter 3 Biological Foundations, Prenatal Development, and Birth.docx

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Ella Daniel

Chapter 3 Biological Foundations, Prenatal Development, and Birth Directly observable characteristics are called phenotypes, which depend in part on the individual’s genotype—the complex blend of genetic information that determines our species and influences all our unique characteristics Phenotypes are also affected by environmental influences—ones that begin even before conception The question of how nature and nurture work together to shape the course of development GENETIC FOUNDATIONS People are made up of trillions of units called cells. Within every cell is a control center, or nucleus, that contains rodlike structures called chromosomes, which store and transmit genetic information Chromosomes come in 23 matching pairs (an exception is the XY pair in males) One is inherited from the mother and one from the father THE GENETIC CODE Chromosomes are made up of deoxyribonucleic acid, or DNA, which is a long double-stranded molecule that looks like a twisted ladder. Each rung of the ladder consists of a pair of chemical substances called bases The bases always pair up in the same way across the ladder rungs—A with T, C with G—they can occur in any order along its sides Agene is a segment of DNAalong the length of the chromosome An estimated 20,000 to 25,000 genes lie along the human chromosomes Between 98 and 99 percent of chimpanzee and human DNAis identical, so only a small portion of our heredity is responsible for the traits that make us human, from our upright gait to our extraordinary language and cognitive capacities Individuals around the world are about 99.1 percent genetically identical Aunique feature of DNAis that it can duplicate itself through a process called mitosis Asingle cell, formed at conception, to develop into a complex human being composed of a great many cells Each new body cell contains the same number of chromosomes and the identical genetic information 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 and break up and reassemble in staggering variety—about 10 to 20 million altogether Awide range of environmental factors modify gene expression Biological events are the result of both genetic and nongenetic forces THE SEX CELLS New individuals are created when two special cells called gametes, or sex cells—the sperm and ovum—combine Gamete contains only 23 chromosomes 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 resulting cell, called a zygote, will again have 46 chromosomes Crossing over in meiosis creates new hereditary combinations Because it generates offspring that vary in phenotype, it increases the chances that at least some members of a species will cope with ever-changing environments and will survive BOY OR GIRL? The 22 matching pairs of chromosomes, which geneticists number from longest (1) to shortest (22)—these are called autosomes (meaning not sex chromosomes) The twenty-third 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 X-bearing or a Y-bearing sperm fertilizes the ovum. Scientists have isolated a gene on the Y chromosome that initiates the formation of male sex organs during the prenatal period Other genes, some yet to be discovered, are involved in the development of sexual characteristics 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 Frequency is about 1 in every 330 births Avariety of environmental influences that prompt this type of twinning, including temperature changes, variation in oxygen levels, and late fertilization of the ovum, but could be genetic also Fraternal, or dizygotic, twins, the most common type of multiple birth, result from the release and fertilization of two ova Major causes of the dramatic rise in fraternal twinning 1. Ethnicity: Occurs in 4 per 1,000 births amongAsians, 8 per 1,000 births among whites, 12 to 16 per 1,000 births among blacks 2. Family history of twinning: Occurs more often among women whose mothers and sisters gave birth to fraternal twins 3. Age: Rises with maternal age, peaking between 35 and 39 years, and then rapidly falls 4. Nutrition: Occurs less often among women with poor diets; occurs more often among women who are tall and overweight or of normal weight as opposed to slight body build 5. Number of births: Is more likely with each additional birth 6. Fertility drugs and in vitro fertilization: Is more likely with fertility hormones and in vitro fertilization (see page 86), which also increase the chances of bearing triplets, quadruplets, or quintuplets PATTERNS OF GENETIC INFLUENCE Two forms of each gene occur at the same place on the chromosomes, one inherited from the mother and one from the father Each form of a gene is called an allele 7. If the alleles from both parents are alike, the child is homozygous and will display the inherited trait. 8. If the alleles differ, then the child is heterozygous, and relationships between the alleles determine the phenotype Dominant-Recessive Relationships Dominate-recessive inheritance occurs: 9. Only one allele affects the child’s characteristics—it is called dominant 10. The second allele, which has no effect, is called recessive Heterozygous individuals with just one recessive allele (Db) can pass that trait to their children 11. Therefore, they are called carriers of the trait Many disabilities and diseases are the product of recessive alleles In dominant-recessive inheritance, if we know the genetic makeup of the parents, we can predict the percentage of children in a family who are likely to display or carry a trait Asingle gene can affect more than one trait Modifier genes enhance or dilute the effects of other genes Children who inherit the dominant allele always develop the disorder They seldom live long enough to reproduce, so The harmful dominant allele is eliminated from the family’s heredity in a single generation Incomplete Dominance 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 Heterozygous individuals are protected from the disease under most circumstances X-Linked Inheritance Males and females have an equal chance of inheriting recessive disorders carried on the autosomes X-linked inheritance applies. Males are more likely to be affected because their sex chromosomes do not match Besides X-linked disorders, many other sex differences reveal the male to be at a disadvantage Rates of miscarriage, infant and childhood deaths, birth defects, learning disabilities, behavior disorders, and mental retardation all are greater for boys The female, with two X chromosomes, benefits from a greater variety of genes In recent decades, the proportion of male births has declined in many industrialized countries, including the United States, Canada, and European nations Some researchers attribute the trend to a rise in stressful living conditions, which heighten spontaneous abortions, especially of male fetuses Genomic Imprinting In 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 The imprint is often temporary; it may be erased in the next generation, and it may not occur in all individuals Imprinting helps, us understand certain puzzling genetic patterns. For example, children are more likely to develop diabetes if they are father, rather than their mother, suffers from it.And people with asthma or hay fever tend to have mothers, not fathers, with the illness Genomic imprinting can also operate on the sex chromosomes, as fragile X syndrome—the most common inherited cause of mental retardation The defective gene at the fragile site is expressed only when it is passed from mother to child Females usually have a normally functioning gene on their other X chromosome (inherited from the father) that partially compensates for the abnormal gene Mutation Although less than 3 percent of pregnancies result in the birth of a baby with a hereditary abnormality, these children account for about 20 percent of infant deaths and contribute substantially to lifelong impaired physical and mental functioning Amutation may affect only one or two genes, or it may involve many genes, as in the chromosomal disorders Some mutations occur spontaneously, simply by chance. Others are caused by hazardous environmental agents Germline mutation, which takes place in the cells that give rise to gametes In somatic mutation, normal body cells mutate, an event that can occur at any time of life Some people harbor a genetic susceptibility that causes certain body cells to mutate easily in the presence of triggering events Polygenic Inheritance Certain characteristics that vary on a continuum among people, such as height, weight, intelligence, and personality, are due to polygenic inheritance, in which many genes determine the characteristic in question Polygenic inheritance is complex, and much about it is still unknown REPRODUCTIVE CHOICES Many couples with genetic disorders in their families chose not to bear a child at all rather than risk the birth of a baby with abnormalities Today, genetic counseling and prenatal diagnosis help people make informed decisions about conceiving, carrying a pregnancy to term, or adopting a child GENETIC COUNSELING Genetic counseling is a communication process designed to help couples assess their chances of giving birth to a baby with a hereditary disorder and choose the best course of action in view of risks and family goals Those who have had difficulties bearing children—for example, repeated miscarriages—or who know that genetic problems exist in their families Women who delay childbearing past age 35 are often candidates for genetic counseling because rate of chromosomal abnormalities rises sharply Maternal needs, not age, should determine referral for genetic counseling If a family history of mental retardation, psychological disorders, physical defects, or inherited diseases exists, the genetic counselor interviews the couple and prepares a pedigree, a picture of the family tree in which affected relatives are identified to estimate the likelihood that parents will have an abnormal child The genetic counselor helps people consider appropriate options. These include taking a chance and conceiving, choosing from among a variety of reproductive technologies PRENATAL DIAGNOSIS AND FETAL MEDICINE Prenatal diagnostic methods—medical procedures that permit detection of developmental problems before birth—are available Women of advanced maternal age are prime candidates for amniocentesis or chorionic villus sampling Prenatal diagnosis should not be used routinely because of injury risks to the developing organism By inserting a needle into the uterus, doctors can administer drugs to the fetus Surgery has been performed to repair such problems as heart, lung, and diaphragm malformations, urinary tract obstructions, and neural defects…but these techniques frequently result in complications, the most common being premature labor and miscarriage Genetic engineering, the Human Genome Project—an ambitious international research program aime1d at deciphering the chemical makeup of human genetic material (genome)—researchers have mapped the sequence of all human DNA base pairs 12. Goal is to understand the estimated 4,000 human disorders, both those due to single genes and those resulting from a complex interplay of multiple genes and environmental factors New treatments are being explored, such as gene therapy—correcting genetic abnormalities by delivering DNAcarrying a functional gene to the cells Another approach, called proteomics, scientists modify gene-specified proteins involved in disease PRENATALDEVELOPMENT The ovum is a tiny sphere, measuring 1/175 inch in diameter But in its microscopic world, it is a giant—the largest cell in the human body It is a perfect target for the much smaller sperm, which measure only 1/500 inch CONCEPTION The ovaries release an ovum every 28 days, which is drawn into one of two fallopian tubes While the ovum is traveling, the spot on the ovary from which it was released, now called the corpus luteum, secretes hormones that prepare the lining of the uterus to receive a fertilized ovum If pregnancy does not occur, the corpus luteum shrinks, and the lining of the uterus is discarded two weeks later with menstruation Males produce about 300 million sperm a day, each with a tail that permits it to swim upstream through the cervix (the opening of the uterus) and into the fallopian tube, where fertilization takes place Most conceptions result from intercourse during a 3-day period—on the day of ovulation or during the 2 days preceding it Three periods: (1) the period of the zygote, (2) the period of the embryo, and (3) the period of the fetes PERIOD OF THE ZYGOTE Lasts about two weeks, from fertilization until the tiny mass of cells drifts down and out of the fallopian tube and attaches itself to the wall of the uterus The zygote’s first cell duplication is long and drawn out; it is not complete until about 30 hours after conception By the fourth day, 60 to 70 cells exist that form a hollow, fluid-filled ball called a blastocyst The cells on the inside of the blastocyst, called the embryonic disk, will become the new organism; the thin outer ring of cells, termed the trophoblast, will become the structures that provide protective covering and nourishment Implantation The blastocyst burrows deep into the uterine lining where, surrounded by the woman’s nourishing blood, it starts to grow It forms a membrane, called the amnion, that encloses the developing organism in amniotic fluid, which helps keep the temperature of the prenatal world constant and provides a cushion against any jolts caused by the woman’s movement Yolk sac emerges that produces blood cells until the developing liver, spleen, and bone marrow are mature enough to take over this function 30 percent of zygotes do not survive this period The Placenta and Umbilical Cord Cells of the trophoblast form another protective membrane—the chorion, which surrounds the amnion. From the chorion, tiny fingerlike villi, or blood vessels, emerge By bringing the mother’s and the embryo’s blood close together, the placenta permits food and oxygen to reach the developing organism and waste products to be carried away The umbilical cord contains one large vein that delivers blood loaded with nutrients and two arteries that remove waste products PERIOD OF THE EMBRYO Lasts from implantation through the eighth week of pregnancy During these brief six weeks, the most rapid prenatal changes take place, as the groundwork is laid for all body structures and internal organs The embryo is especially vulnerable to interference with healthy development The short time span of embryonic growth helps limit opportunities for serious harm Last Half of the First Month The embryonic disk forms three layers of cells: (1) the ectoderm, which will become the nervous system and skin; (2) the mesoderm, from which will develop the muscles, skeleton, circulatory system, and other internal organs; and (3) the endoderm, which will become the digestive system, lungs, urinary tract, and glands The nervous system develops fastest The ectoderm folds over to form the neural tube, or primitive spinal cord The Second Month At 7 weeks, production of neurons begins deep inside the neural tube at the astounding pace of more than 250,000 per minute At the end of this period, the embryo—about 1 inch long and 1⁄7 ounce in weight—can already sense its world PERIOD OF THE FETUS From the ninth week to the end of pregnancy, is the longest prenatal period During this “growth and finishing” phase, the organism increases rapidly in size, especially from the ninth to the twentieth week The Third Month When the brain signals, the fetus kicks, bends its arms, forms a fist, curls its toes, turns its head, opens its mouth, and even sucks its thumb, stretches, and yawns By the twelfth week, the external genitals are well-formed, and the sex of the fetus can be detected with ultrasound Prenatal development is sometimes divided into trimesters, or three equal time periods—at the end of the third month, the first trimester is complete The Second Trimester The mother can feel its movements Awhite, cheese like substance called vernix covers the skin, protecting it from chapping during the long months spent in the amniotic fluid White, downy hair called lanugo also covers the entire body, helping the vernix stick to the skin Many organs are well developed Brain weight increases tenfold from the twentieth week until birth 13. Brain growth means new behavioral capacities The Third Trimester The point at which the baby can first survive, called the age of viability, occurs sometime between 22 and 26 weeks The cerebral cortex, the seat of human intelligence, enlarges Convolutions and grooves in its surface appear, permitting a dramatic increase in surface area that allows for maximum prenatal brain growth without the full-term baby’s head becoming too large to pass through the birth canal 14. As neural connectivity and organization improve, the fetus spends more time awake By the end of pregnancy, the fetus takes on the beginnings of a personality Sensitive caregiving can modify the temperaments of children who have difficulty adapting to new experiences Third trimester also brings greater responsiveness to external stimulation 15. Suggests that fetuses can remember for at least a brief period PRENATALENVIRONMENTALINFLUENCES Many factors can affect the embryo and fetus There is much that parents—and society as a whole—can do to create a safe environment for development before birth TERATOGENS The term teratogen refers to any environmental agent that causes damage during the prenatal period From the Greek word tears, meaning “malformation” or “monstrosity” They first learned about harmful prenatal influences from cases in which babies had been profoundly damaged Depends on the following: 16. Dose: larger doses over longer time periods usually have more negative effects 17. Heredity: the genetic makeup of the other has an impact on the developing organism, but some individuals are better able than others to withstand harmful environments 18. Other negative influences: several negative factors at once, such as additional teratogens, poor nutrition, and lack of medical care can worsen the impact of a single harmful agent 19. Age: the effects vary with the age of the organism at the time of exposure—think of the sensitive period concept, which is a limited time span in which a part of the body or a behavior is biologically prepared to develop rapidly, and during this time, it is especially sensitive to its surroundings…if the environment is harmful, then damage occurs, and recovery is difficult and sometimes impossible Also, timing can have an impact 20. In the period of the zygote, before implantation, teratogens rarely have any impact 21. The embryonic period is the time when serious defects are most likely to occur because the foundations for all body parts are being laid down 22. During the fetal period, teratogenic damage is usually minor Some health outcomes are delayed and may not show up for decades Psychological consequences may occur indirectly, as a result of physical damage Defect resulting from drugs the mother took during pregnancy can affect others’reactions to the child as well as the child’s ability to explore the environment Parent-child interaction, peer relations, and opportunities to explore may suffer Prenatally exposed children may be less resilient in the face of environmental risks, such as single parenthood, parental emotional disturbance, or maladaptive parenting Bidirectional influences between child and environment Prescription and Non-prescription Drugs Asedative called thalidomide was widely available in Canada, Europe, and SouthAmerica 23. When taken by mothers 4 to 6 weeks after conception, thalidomide produced gross deformities of the embryo’s developing arms and legs 24. As children exposed to thalidomide grew older, many scored below average in intelligence Synthetic hormone called diethylstilbestrol (DES) to prevent miscarriages 25. As daughters of these mothers reached adolescence and young adulthood, they showed unusually high rates of cancer of the vagina, malformations of the uterus, and infertility 26. Young men showed an increased risk of genital abnormalities and cancer of the testes Currently, the most widely used potent teratogen is a VitaminA derivative called isotretinoin to treat severe acne and taken by hundreds of thousands of women of childbearing age Exposure during the first trimester of pregnancy results in eye, ear, skull, brain, heart, and immune system abnormalities Any drug with a molecule small enough to penetrate the placental barrier can enter the embryonic or fetal bloodstream 27. Caffeine contributes to low birth weight and miscarriage rates 28. Antidepressant medications are linked to increased risk of premature delivery and birth complications, including respiratory distress and persistent high blood pressure in infancy The safest course for pregnant women is to avoid such drugs entirely Illegal Drugs Highly addictive mood-altering drugs, such as cocaine and heroin, has become more widespread, especially in poverty-stricken inner-city areas, where these drugs provide a temporary escape from a daily life of hopelessness 29. About 4 percent of U.S. pregnant women take these substances Babies born to users of cocaine, heroin, or methadone (a less addictive drug used to get people off heroin) are at risk for a wide variety of problems, including pre maturity, low birth weight, physical defects, breathing difficulties, and death at or around the time of birth 30. In addition, these infants are born drug-addicted 31. They are often feverish and irritable and have trouble sleeping, and their cries are abnormally shrill and piercing—a common symptom among stressed newborns Throughout the first year, heroin- and methadone-exposed infants are less attentive to the environment than nonexposed babies, and their motor development is slow Prenatally exposed babies develop lasting problems Cocaine can alter the production and functioning of neurons and the chemical balance in the fetus’s brain Compared to nonexposed infants, cocaine-exposed babies show greater stress reactivity By school age, children with a history of prenatal cocaine exposure display a reduced cortisol response during stressful mental activity Cortisol levels that are either too high or too low signal a disrupted stress response system—a risk factor for illness and learning and behavior problems Marijuana is used more widely than heroin or cocaine Researchers have linked prenatal marijuana exposure to smaller head size (a measure of brain growth); to attention, memory, and academic achievement difficulties; to impulsivity and overactivity; and to depression as well as anger and aggression in childhood and adolescence Tobacco Estimated 14 percent of U.S. women smoke during their pregnancies Low birth weight The likelihood of other serious consequences, such as miscarriage, prematurity, cleft lip and palate, impaired heart rate and breathing during sleep, infant death, and asthma and cancer later in childhood, also increases The more cigarettes a mother smokes, the greater the chances that her baby will be affected If a pregnant woman stops smoking at any time, even during the third trimester, she reduces the likelihood that her infant will be born underweight and suffer from future problems Newborns of smoking mothers are less attentive to sounds, display more muscle tension, are more excitable when touched and visually stimulated, and more often have colic (persistent crying)—findings that suggest subtle negative effects on brain development Tend to have shorter attention spans, difficulties with impulsivity and overactivity, poorer memories, lower mental test scores, and higher levels of disruptive, aggressive behavior Nicotine, the addictive substance in tobacco, constricts blood vessels, lessens blood flow to the uterus, and causes the placenta to grow abnormally reducing the transfer of nutrients, so the fetus gains weight poorly Also raises carbon monoxide levels in bloodstreams of both mother and child, which displaces oxygen from red blood cells, damaging the central nervous system and slowing body growth in the fetuses of laboratory animals “Passive smokers”: husbands, relatives, or co-workers use cigarettes—also linked to low birth weight, infant death, childhood respiratory illnesses, and possible long-term attention, learning, and behavior problems Alcohol Fetal alcohol spectrum disorder (FASD), a term that encompasses a range of physical, mental, and behavioral outcomes caused by prenatal alcohol exposure Types of FASD: 1. Fetal alcohol syndrome (FAS), distinguished by (a) slow physical growth, (b) a pattern of three facial abnormalities (short eyelid openings; a thin upper lip; a smooth or flattened philtrum, or indentation running from the bottom of the nose to the centre of the upper lip), and (c) brain injury, evident in a small head and impairment in at least three areas of functioning—for example, memory, language and communication, attention span and activity level (overactivity), planning and reasoning, motor coordination, or social skills. Other defects—of the eyes, ears, nose, throat, heart, genitals, urinary tract, or immune system—may also be present. Infants at risk when their mothers drink heavily throughout the whole pregnancy. 2. Partial fetal alcohol syndrome (p-FAS), characterized by (a) two of the three facial abnormalities just mentioned and (b) brain injury, again evident in at least three areas of impaired functioning. Mothers of children with p-FAS generally drank alcohol in smaller quantities, and children’s defects vary with the timing and length of alcohol exposure. Furthermore, recent evidence suggests that paternal alcohol use around the time of conception can induce genetic alterations, thereby contributing to symptoms. 3. Alcohol-related neurodevelopmental disorder (ARND), in which at least three areas of mental functioning are impaired, despite typical physical growth and absence of facial abnormalities.Again, prenatal alcohol exposure, though confirmed, is less pervasive than in FAS. The more alcohol a woman consumes during pregnancy, the poorer the child’s motor coordination, speed of information processing, attention, memory, reasoning, and intelligence and achievement test scores during the preschool and school years In adolescence and early adulthood, FASD is associated with persisting attention and motor-coordination deficits, poor school performance, trouble with the law, inappropriate social and sexual behaviours, alcohol and drug abuse, and lasting mental health problems, including high stress reactivity and depression Alcohol interferes with production and migration of neurons in the primitive neural tube The body uses large quantities of oxygen to metabolize alcohol—a pregnant woman’s heavy drinking draws away oxygen that the developing organism needs for cell growth About 25 percent of U.S. mothers report drinking at some time during their pregnancies.As with heroin and cocaine, alcohol abuse is higher in poverty-stricken women Even mild drinking, less than one drink per day, is associated with reduced head size and body growth among children followed into adolescence Expectant mothers should avoid alcohol entirely Radiation Ionizing radiation can cause mutation, damaging DNAin ova an
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