What is our basic genetic endowment, and how can human development go off track? How do the environment and genetics work together to determine human characteristics? Which human characteristics are significantly influenced by heredity? What happens during the prenatal stages of development? CHAPTER 3 KEY QUESTIONS
What are the threats to the fetal environment, and what can be done about them? CHAPTER 3 KEY QUESTIONS
This first cell is created when a male reproductive cell, a sperm , pushes through the membrane of the ovum , the female reproductive cell. Gametes The sex cells from the mother and father that form a new cell at conception Zygote The new cell formed by the process of fertilization EARLIEST DEVELOPMENT
Genes The basic unit of genetic information DNA (deoxyribonucleic acid) molecules The substance that genes are composed of that determines the nature of every cell in the body and how it will function Chromosomes Rod-shaped portions of DNA that are organized in 23 pairs GENES AND CHROMOSOMES: THE CODE OF LIFE
Through a process called mitosis , which accounts for the replication of most types of cells, nearly all the cells of the body will contain the same 46 chromosomes as the zygote. When the sperm and ova are formed in the adult human body in a process called meiosis , each gamete receives one of the two chromosomes that make up each of the 23 pairs. GENES AND CHROMOSOMES: THE CODE OF LIFE
Monozygotic twins Twins who are genetically identical Dizygotic twins Twins who are produced when two separate ova are fertilized by two separate sperm at roughly the same time MULTIPLE BIRTHS: TWO—OR MORE— FOR THE GENETIC PRICE OF ONE
If the sperm contributes an X chromosome when it meets an ovum (which will always contribute an X chromosome), the child will have an XX pairing on the 23rd chromosome and will be a female. If the sperm contributes a Y chromosome, the result will be an XY pairing and will be a male BOY OR GIRL? ESTABLISHING THE SEX OF THE CHILD
We need to consider the basic mechanisms involved in the way that the genes we inherit from our parents transmit information. Dominant trait The one trait that is expressed when two competing traits are present Recessive trait A trait within an organism that is present, but is not expressed THE BASICS OF GENETICS: THE MIXING AND MATCHING OF TRAITS
Genotype The underlying combination of genetic material present (but not outwardly visible) in an organism Phenotype An observable trait; the trait that actually is seen Some of the genes form pairs called alleles , genes governing traits that may take alternate forms, such as hair or eye color. THE BASICS OF GENETICS: THE MIXING AND MATCHING OF TRAITS
Homozygous Inheriting from parents similar genes for a given trait Heterozygous Inheriting from parents different forms of a gene for a given trait THE BASICS OF GENETICS: THE MIXING AND MATCHING OF TRAITS
We can see this process at work in humans by considering the transmission of phenylketonuria (PKU) , an inherited disorder in which a child is unable to make use of phenylalanine, an essential amino acid present in proteins found in milk and other foods. TRANSMISSION OF GENETIC INFORMATION
Polygenic inheritance Inheritance in which a combination of multiple gene pairs is responsible for the production of a particular trait Genes also vary in terms of their reaction range , the potential degree of variability in the actual expression of a trait due to environmental conditions. X-linked genes Genes that are considered recessive and located only on the X chromosome POLYGENIC TRAITS
Mapping the specific sequence of genes on each chromosome stands as one of the most important moments in the history of genetics, and, for that matter, all of biology. Already, the mapping of the gene sequence has provided important advances in our understanding of genetics. THE HUMAN GENOME AND BEHAVIORAL GENETICS: CRACKING THE GENETIC CODE
Behavioral genetics The study of the effects of heredity on behavior BEHAVIORAL GENETICS
A recessive gene responsible for a disorder may be passed on unknowingly from one generation to the next, revealing itself only when, by chance, it is paired with another recessive gene. In some cases, genes become physically damaged. INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY
For instance, genes may break down due to wear-and-tear or to chance events occurring during the cell division processes of meiosis and mitosis. And sometimes, for no known reason, genes spontaneously change their form, a process called spontaneous mutation . INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY
Other inherited and genetic disorders include: Down syndrome A disorder produced by the presence of an extra chromosome on the 21st pair; once referred to as mongolism Fragile X syndrome A disorder produced by injury to a gene on the X chromosome, producing mild to moderate mental retardation INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY
Sickle-cell anemia A blood disorder that gets its name from the shape of the red blood cells in those who have it Tay-Sachs disease A disorder that produces blindness and muscle degeneration prior to death; there is no treatment Klinefelter’s syndrome A disorder resulting from the presence of an extra X chromosome that produces underdeveloped genitals, extreme height, and enlarged breasts INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY
Due to advances in behavioral genetics, genetic difficulties increasingly can be forecast, anticipated, and planned for before a child’s birth, enabling parents to take steps before the child is born to reduce the severity of certain genetic conditions. INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY
Genetic counseling The discipline that focuses on helping people deal with issues relating to inherited disorders Possible genetic defects, such as the presence of an extra sex chromosome, can be identified by assembling a karyotype , a chart containing enlarged photos of each of the chromosomes. GENETIC COUNSELING: PREDICTING THE FUTURE FROM THE GENES OF THE PRESENT
Ultrasound sonography A process in which high-frequency sound waves scan the mother’s womb to produce an image of the unborn baby, whose size and shape can then be assessed Chorionic villus sampling (CVS) A test used to find genetic defects that involves taking samples of hairlike material that surrounds the embryo PRENATAL TESTING
Amniocentesis The process of identifying genetic defects by examining a small sample of fetal cells drawn by a needle inserted into the amniotic fluid surrounding the unborn fetus PRENATAL TESTING
The newest role for genetic counselors involves testing people to identify whether they themselves, rather than their children, are susceptible to future disorders because of genetic abnormalities. As our understanding of genetics continues to grow, researchers and medical practitioners have moved beyond testing and counseling to actively working to change flawed genes. SCREENING FOR FUTURE PROBLEMS
As developmental research accumulates, it is becoming increasingly clear that to view behavior as due to either genetic or environmental factors is inappropriate. A given behavior is not caused just by genetic factors; nor is it caused solely by environmental forces. THE ROLE OF THE ENVIRONMENT IN DETERMINING THE EXPRESSION OF GENES: FROM GENOTYPES TO PHENOTYPES
Temperament Patterns of arousal and emotionality that represent consistent and enduring characteristics in an individual TEMPERAMENT
Such findings illustrate that many traits reflect multifactorial transmission, meaning that they are determined by a combination of both genetic and environmental factors. In multifactorial transmission, a genotype provides a particular range within which a phenotype may achieve expression. INTERACTION OF FACTORS
Nonhuman Animal Studies: Controlling Both Genetics and Environment It is relatively simple to develop breeds of animals that are genetically similar to one another in terms of specific traits. By observing animals with similar genetic backgrounds in different environments, scientists can determine, with reasonable precision, the effects of specific kinds of environmental stimulation. STUDYING DEVELOPMENT: HOW MUCH IS NATURE? HOW MUCH IS NURTURE?
Nature conveniently has provided the potential to carry out various kinds of “natural experiments”—in the form of twins. Because the inherited backgrounds of identical, monozygotic twins are precisely the same, any variations in their behavior must be due entirely to environmental factors. CONTRASTING RELATEDNESS AND BEHAVIOR: ADOPTION, TWIN, AND FAMILY STUDIES
Studies of nonidentical, dizygotic twins also present opportunities to learn about the relative contributions of nature and nurture. The general conclusion resulting from decades of research: Virtually all traits, characteristics, and behaviors are the joint result of the combination and interaction of nature and nurture. CONTRASTING RELATEDNESS AND BEHAVIOR: ADOPTION, TWIN, AND FAMILY STUDIES
Monozygotic twins are merely the most extreme example of the fact that the more genetically similar two people are, the more likely they are to share physical characteristics. Other, less obvious physical characteristics also show strong genetic influences. For instance, blood pressure, respiration rates, and even the age at which life ends are more similar in closely related individuals than in those who are less genetically alike. PHYSICAL TRAITS: FAMILY RESEMBLANCES
No other issue involving the relative influence of heredity and environment has generated more research than the topic of intelligence. Genetics plays a significant role in intelligence. INTELLIGENCE: MORE RESEARCH, MORE CONTROVERSY
Studies have shown that the closer the genetic link between two individuals, the greater the correspondence of their overall IQ scores. Although it is clear that heredity plays an important role in intelligence, investigators are much more divided on the question of the degree to which it is inherited. INTELLIGENCE: MORE RESEARCH, MORE CONTROVERSY
Increasing research evidence suggests that some of our most basic personality traits have genetic roots. Neuroticism , as used by personality researchers, is the degree of emotional stability an individual characteristically displays. GENETIC AND ENVIRONMENTAL INFLUENCES ON PERSONALITY: BORN TO BE OUTGOING?
Extroversion is the degree to which a person seeks to be with others, to behave in an outgoing manner, and generally to be sociable. Clearly, genetic factors play a role in determining personality. At the same time, the environment in which a child is raised also affects personality development. GENETIC AND ENVIRONMENTAL INFLUENCES ON PERSONALITY: BORN TO BE OUTGOING?
If genetics were the sole cause of psychological disorder, the risk for an identical twin would be 100%. However, other factors account for disorders, ranging from structural abnormalities in the brain to a biochemical imbalance. Several other psychological disorders have been shown to be related, at least in part, to genetic factors. PSYCHOLOGICAL DISORDERS: THE ROLE OF GENETICS AND ENVIRONMENT
The role of genetics is often to produce tendency toward a future course of development. When and whether a certain behavioral characteristic will actually be displayed depends on the nature of the environment. PSYCHOLOGICAL DISORDERS: THE ROLE OF GENETICS AND ENVIRONMENT
According to Scarr, the genetic endowment provided to children by their parents not only determines their genetic characteristics, but also actively influences their environment. Children tend to actively focus on those aspects of their environment that are most connected with their genetically determined abilities. CAN GENES INFLUENCE THE ENVIRONMENT?
Children also pay less attention to those aspects of the environment that are less compatible with their genetic endowment. The genetically driven temperament of a child may evoke certain environmental influences. CAN GENES INFLUENCE THE ENVIRONMENT?
Fertilization The process by which a sperm and an ovum—the male and female gametes, respectively—join to form a single new cell When sperm enter the vagina, they begin a winding journey that takes them through the cervix, the opening into the uterus, and into the fallopian tube, where fertilization may take place. FERTILIZATION: THE MOMENT OF CONCEPTION
Only a tiny fraction of the 300 million cells that are typically ejaculated during sexual intercourse ultimately survive the journey. FERTILIZATION: THE MOMENT OF CONCEPTION
The prenatal period consists of three phases: germinal embryonic fetal THE STAGES OF THE PRENATAL PERIOD: THE ONSET OF DEVELOPMENT
Germinal stage The first, and shortest, stage of the prenatal period, which takes place during the first 2 weeks following conception Placenta A conduit between the mother and fetus, providing nourishment and oxygen via the umbilical cord THE GERMINAL STAGE: FERTILIZATION TO 2 WEEKS
Embryonic stage The period from 2 to 8 weeks following fertilization during which significant growth occurs in the major organs and body systems The outer layer of the embryo, the ectoderm , will form skin, hair, teeth, sense organs, and the brain and spinal cord. THE EMBRYONIC STAGE: 2 WEEKS TO 8 WEEKS
The endoderm , the inner layer, produces the digestive system, liver, pancreas, and respiratory system. Sandwiched between the ectoderm and endoderm is the mesoderm , from which the muscles, bones, blood and circulatory system are forged. THE EMBRYONIC STAGE: 2 WEEKS TO 8 WEEKS
Fetal stage The stage that begins at about 8 weeks after conception and continues until birth Fetus A developing child, from 8 weeks after conception until birth The brain becomes increasingly sophisticated during the fetal stage. THE FETAL STAGE: 8 WEEKS TO BIRTH
The two symmetrical left and right halves of the brain, known as hemispheres , grow rapidly, and the interconnections between neurons become more complex. The neurons become coated with an insulating material called myelin which helps speed the transmission of messages from the brain to the rest of the body. THE FETAL STAGE: 8 WEEKS TO BIRTH
Infertility The inability to conceive after 12 to 18 months of trying to become pregnant Artificial insemination A process of fertilization in which a man’s sperm is placed directly into a woman’s vagina by a physician PREGNANCY PROBLEMS
In vitro fertilization (IVF) A procedure in which a woman’s ova are removed from her ovaries, and a man’s sperm are used to fertilize the ova in a laboratory Gamete intrafallopian transfer (GIFT) and zygote intrafallopian transfer (ZIFT) are procedures in which an egg and sperm or fertilized egg are implanted in a woman’s fallopian tubes. PREGNANCY PROBLEMS
In vitro fertilization is increasingly successful, with success rates of as high 33% for younger women (but with lower rates for older women). Furthermore, reproductive technologies are becoming increasingly sophisticated, permitting parents to choose the sex of their baby. PREGNANCY PROBLEMS
The use of surrogate mothers, in vitro fertilization, and sex selection techniques present a web of ethical and legal issues, as well as many emotional concerns. Even more troubling are concerns raised by sex selection techniques. For the moment, many of these ethical issues remain unresolved. ETHICAL ISSUES
A miscarriage —known as a spontaneous abortion—occurs when pregnancy ends before the developing child is able to survive outside the mother’s womb. The embryo detaches from the wall of the uterus and is expelled. MISCARRIAGE AND ABORTION
In abortion , a mother voluntarily chooses to terminate pregnancy. Regarding risk of future psychological problems, the findings are mixed, and there are significant individual differences in how women respond to the experience of abortion. MISCARRIAGE AND ABORTION
There are certain aspects of a mothers’ and fathers’ behavior, both before and after conception, that can produce lifelong consequences for the child. Some consequences show up immediately, but half the possible problems aren’t apparent before birth. Other problems, more insidious, may not appear until years after birth. THE PRENATAL ENVIRONMENT: THREATS TO DEVELOPMENT
Teratogen A factor that produces a birth defect The timing and quantity of exposure to a teratogen are crucial. At some phases of prenatal development, a certain teratogen may have only a minimal impact. At other periods, however, the same teratogen may have profound consequences. THE PRENATAL ENVIRONMENT: THREATS TO DEVELOPMENT
Most of our knowledge of the environmental factors that affect the developing fetus comes from the study of the mother. There are ways to counteract the types of maternal malnourishment that affect prenatal development. MOTHER’S DIET
Dietary supplements given to mothers can reverse some of the problems produced by a poor diet. Research shows that babies who were malnourished as fetuses can overcome some of the effects of their early malnourishment. MOTHER’S DIET
More women are giving birth later in life than was true just 2 or 3 decades ago. Women who give birth when over the age of 30 are at greater risk for a variety of pregnancy and birth complications than are younger mothers. The risks involved in pregnancy are greater not only for older mothers but also for atypically young women. MOTHER’S AGE
The higher mortality rate for babies of adolescent mothers reflects more than just physiological problems related to the mothers’ young age. Young mothers often face adverse social and economic factors which can affect infant health. MOTHER’S PRENATAL SUPPORT
Mothers who eat the right foods, maintain an acceptable weight, and who exercise appropriately maximize the chances of having a healthy baby. A mother’s diseases may affect a developing fetus, depending on when the illness is contracted. MOTHER’S HEALTH
Mothers’ use of many kinds of drugs—both legal and illegal—poses serious risks to the unborn child. Even drugs prescribed by medical professionals have sometimes had disastrous consequences. (Thalidomide) Illicit drugs may pose equally great, and sometimes even greater, risks for the environments of prenatal children. MOTHER’S DRUG USE
Increasing evidence suggests that even small amounts of alcohol and nicotine can disrupt the development of the fetus. Fetal alcohol syndrome (FAS) A disorder caused by the pregnant mother consuming substantial quantities of alcohol during pregnancy, potentially resulting in mental retardation and delayed growth in the child MOTHERS’ USE OF ALCOHOL AND TOBACCO
Fetal alcohol effects (FAE) A condition in which children display some, although not all, of the problems of fetal alcohol syndrome due to the mother’s consumption of alcohol during pregnancy Smokers are two times as likely as nonsmokers to have babies with an abnormally low birthweight, and smokers’ babies are shorter, on average, than those of nonsmokers. MOTHERS’ USE OF ALCOHOL AND TOBACCO
Furthermore, women who smoke during pregnancy are 50% more likely to have mentally retarded children. MOTHERS’ USE OF ALCOHOL AND TOBACCO
It is becoming increasingly clear that fathers’ behavior may indeed influence the prenatal environment. Fathers-to-be should avoid smoking. A father’s use of alcohol and illegal drugs can have significant effects on the fetus. Fathers who are physically or emotionally abusive to their pregnant wives can damage their unborn children. DO FATHERS AFFECT THE PRENATAL ENVIRONMENT?
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