Experience Human Development 15th Edition PDF

type, varies (Smits & Monden, 2011). For example, multiple birth rates in West African women have been reported at 1 in 40, while rates in Japan are T A...

type, varies (Smits & Monden, 2011). For example, multiple birth rates in West African women have been reported at 1 in 40, while rates in Japan are T A far lower at 1 in 200 (Blencowe et al., 2013). Influences on multiple births DNA is the genetic A T are (1) the trend toward delayed childbearing and (2) the increased use of material in all living cells. It consists of four fertility drugs, which spur ovulation, and of assisted reproductive techniques chemical units, called such as in vitro fertilization, which tend to be used by older women (Marti- bases. These bases are G C net al., 2009). the letters of the DNA alphabet. A (adenine) T A pairs with T (thymine) C G Mechanisms of Heredity and C (cytosine) pairs T A with G (guanine). There are 3 billion base pairs in human DNA. The science of genetics is the study of heredity, the genetic transmission of Letters of the DNA C G heritable characteristics from biological parents to offspring. alphabet C G T = Thymine T A A = Adenine T A THE GENETIC CODE G = Guanine C = Cytosine The “stuff” of heredity is a chemical called deoxyribonucleic acid (DNA). The double-helix structure of a DNA molecule resembles a long, spiraling ladder whose C G steps are made of pairs of chemical units called bases (Figure 1). The bases— T A adenine (A), thymine (T), cytosine (C), and guanine (G)—are the “letters” of the genetic code, which cellular machinery “reads.” FIGURE 1 Chromosomes are coils of DNA that consist of smaller segments called DNA: The Genetic Code genes, the functional units of heredity. Each gene is located in a specific posi- Source: Adapted from Ritter (1999). tion on its chromosome and contains thousands of bases. The sequence of bases in a gene tells the cell how to make the proteins that enable it to carry out specific functions. The complete sequence of genes in the human body constitutes the human genome. Of course, every human has a unique genome. The human genome is not meant to be a recipe for making a particular human. Rather, the human genome is a reference point, or representative genome, that shows the location of all human genes. (a) A useful analogy is to consider the DNA of an individual as a series of books in a library. Until those books are “read” by an enzyme called RNA polymerase and transcribed into a readable copy of messenger RNA (m-RNA), the knowledge contained within the books is not actualized. And what books will be pulled down from the shelf and read is in part determined by environ- mental factors turning genes on and off at different points in development (Champagne & Mashoodh, 2009). Every cell in the normal human body except the sex cells (sperm and ova) has 23 pairs of chromosomes—46 in all. Through a type of cell division (b) called meiosis, which the sex cells undergo when they are developing, each sex cell ends up with only 23 chromosomes—one from each pair. When sperm and ovum fuse at conception, they produce a zygote with 46 chromosomes, 23 from the father and 23 from the mother (Figure 2). Ovum Sperm FIGURE 2 Hereditary Composition of the Zygote (c) (a) Body cells of women and men contain 23 pairs of chromosomes, which carry the genes, the basic units of inheritance. (b) Each sex cell (ovum and sperm) has only 23 single chromosomes because of a special kind of cell division (meiosis). (c) At fertilization, the 23 chromosomes from the sperm join the 23 from the ovum Zygote so that the zygote receives 46 chromosomes, or 23 pairs. 50 EXPERIENCE HUMAN DEVELOPMENT CHAPTER 3 Forming a New Life  At conception, then, the single-celled zygote has all the bio- logical information needed to guide its development into a unique individual. Through mitosis, a process by which the non- sex cells divide in half over and over again, the DNA replicates Mother Father itself, so that each newly formed cell has the same DNA struc- ture as all the others. Each cell division creates a genetic dupli- cate of the original cell, with the same hereditary information. Sometimes a mistake in copying is made, and a mutation may Father has an X chromosome and a Y result. Mutations are permanent alterations in genetic material. chromosome. Mother When development is normal, each cell (except the sex cells) has two X chromosomes. continues to have 46 chromosomes identical to those in the Male baby receives an X original zygote. As the cells divide, they differentiate, special- chromosome from the mother and a Y izing in a variety of complex bodily functions that enable the chromosome from X X X Y child to grow and develop. the father. Female Genes spring into action when they are turned on or off, baby receives X either by external environmental factors such as nutrition or chromosomes from both mother and father. stress, or by internal factors such as hormone levels in the mother or fetus. Thus, from the start, heredity and environment are inter- twined. X X X X X Y Baby boy SEX DETERMINATION Baby girl Twenty-two pairs of our 23 pairs of chromosomes are autosomes, FIGURE 3 chromosomes that are not related to sexual expression. The twenty- Genetic Determination of Sex third pair are sex chromosomes—one from the father and one from Because all babies receive an X chromosome from the mother—that govern the baby’s sex. the mother, sex is determined by whether an X or a Sex chromosomes are either X chromosomes or Y chromo- Y chromosome is received from the father. somes. The sex chromosome of every ovum is an X chromosome, but the sperm may contain either an X or a Y chromosome. The deoxyribonucleic acid (DNA) Chemical that carries inherited instruc- Y chromosome contains the gene for maleness, called the SRY gene. When an ovum tions for the development of all cellular (X) is fertilized by an X-carrying sperm, the zygote formed is XX, a genetic female. forms of life. When an ovum (X) is fertilized by a Y-carrying sperm, the resulting zygote is XY, genetic code a genetic male (Figure 3). Sequence of bases within the DNA mol- Sexual differentiation is a more complex process than simple gene determination. ecule; governs the formation of proteins Early in development, the embryo’s rudimentary reproductive system appears almost that determine the structure and func- identical in males and in females. Research with mice has found that once hormones tions of living cells. signal the SRY gene on the Y chromosome to turn on, cell differentiation and for- chromosomes mation of the testes are triggered. At 6 to 8 weeks after conception, the testes start Coils of DNA that consist of genes. to produce the male hormone testosterone. Exposure of a genetically male embryo genes to steady, high levels of testoster- Small segments of DNA located in defi- one ordinarily results in the devel- nite positions on particular chromo- somes; functional units of heredity. opment of a male body with male sexual organs (Arnold, 2017). With- The Neanderthal genome human genome out this hormonal influence, a Complete sequence of genes in the hu- was sequenced in 2013, man body. genetically male mouse will develop and analysis of the commonalities mutations genitals that appear female rather than male. between Neanderthal and human Permanent alterations in genes or chro- genes suggests that we engaged in mosomes that may produce harmful The development of the female characteristics. reproductive system is equally com- limited interbreeding. In other words, some of their genes live on in us. autosomes plex and depends on a number of In humans, the 22 pairs of chromo- genetic variants. These variants pro- Indeed, estimates are that only 1.5 to somes not related to sexual expression. mote ovarian development and 7 percent of the human genome is sex chromosomes inhibit testicular development (Ono uniquely human (Schaefer et al., Pair of chromosomes that determines & Harley, 2013). This includes the 2021). sex: XX in the normal human female, HOX genes (Taylor, 2000) and a XY in the normal human male. Mechanisms of Heredity  EXPERIENCE HUMAN DEVELOPMENT 51  signaling molecule called Wnt-4, a variant form of which can masculinize a geneti- cally female fetus (Bouty et al., 2020). checkpoint can you... Describe the structure of DNA PATTERNS OF GENETIC TRANSMISSION and its role in the inheritance During the 1860s, Gregor Mendel, an Austrian monk, laid the foundation for our of characteristics? understanding of patterns of inheritance. By crossbreeding strains of peas, he discov- ered two fundamental principles of genetics. First, traits could be either dominant or Distinguish between meiosis recessive. Dominant traits are always expressed, whereas recessive traits are expressed and mitosis? only if both copies of the gene are recessive. Second, traits are passed down indepen- dently of each other. For example, the color of your hair and your height are both Explain why the sperm nor- hereditable traits that are not linked. mally determines a baby’s sex? Although some human traits are inherited via simple dominant transmission, most human traits fall along a continuous spectrum and result from the actions of many genes in concert. Nonetheless, Mendel’s groundbreaking work laid the foundations for our modern understanding of genetics. Dominant and Recessive Inheritance Genes that can produce alternative expres- alleles sions of a characteristic (such as the presence or absence of dimples) are called alleles. Two or more alternative forms of a gene Alleles are alternate versions of the same gene. Every person receives one maternal and that occupy the same position on paired one paternal allele for any given trait. When both alleles are the same, the person is chromosomes and affect the same trait. homozygous for the characteristic; when they are different, the person is heterozygous. homozygous In dominant inheritance, the dominant allele is always expressed, or shows up as a trait Possessing two identical alleles for a trait. in that person. The person will look the same whether or not he or she is heterozygous heterozygous or homozygous because the recessive allele doesn’t show. For the trait to be expressed Possessing differing alleles for a trait. in recessive inheritance, the person must have two recessive alleles, one from each parent. If a recessive trait is expressed, that person cannot have a dominant allele. dominant inheritance Let’s take red hair as an example. Because red hair is a recessive trait, you must Pattern of inheritance in which, when a child receives different alleles, only the receive two recessive copies (r) of the gene—one from each parent—in order to express dominant one is expressed. red hair. Having hair that is not red (R; brown in this example) is a dominant trait, so you will have brown hair if you receive at least one copy (R) from either parent (Rr or recessive inheritance RR) (Figure 4). If you receive one copy of the red hair allele (r) and one copy of an Pattern of inheritance in which a child receives identical recessive alleles, allele for brown hair (R), you are heterozygous (Rr or rR); if you have two copies of the resulting in expression of a nondominant allele for brown hair, you are homozygous dominant (RR). In both cases, you will have trait. brown hair. If you inherited one allele for red hair from each parent, you are homozygous FIGURE 4 Mother Father Dominant and Recessive Inheritance Because of dominant inheritance, the same observable phenotype (in this case, brown hair) can result from two different Rr Rr genotypes (RR and Rr). A phenotype expressing a recessive characteristic (such as red hair) must have a homozygous recessive genotype (rr). (Left to Right) Dougal Waters/Photodisc/Getty Images; Ioannis Pantzi/Shutterstock; Ed-Imaging; Ed-Imaging; BDLM/Cultura/Getty Images; Pete Pahham/Shutterstock RR Rr rR rr 52 EXPERIENCE HUMAN DEVELOPMENT CHAPTER 3 Forming a New Life recessive for this trait (rr) and will have red hair. Thus, the only situation in which you polygenic inheritance would have red hair is if you received two recessive copies (r), one from each parent. Pattern of inheritance in which multiple genes at different sites on chromo- Relatively few traits are determined in this simple fashion. Most traits result from somes affect a complex trait. polygenic inheritance, the interaction of several genes. For example, there is not an “­intelligence” gene that determines whether or not you are smart. Rather, a large number phenotype of genes work in concert to determine your intellectual potential. Like intelligence, most Observable characteristics of a person. individual variations in complex behaviors or traits are governed by the additive influ- genotype ences of many genes with small but identifiable effects. Although single genes often Genetic makeup of a person, containing determine abnormal traits, there is no single gene that by itself significantly accounts both expressed and unexpressed for individual differences in any complex behavior. characteristics. Multifactorial Transmission If you have brown hair, that is part of your phenotype, the observable characteristics through which your genotype, or underlying genetic makeup, is expressed. The phenotype is the product of the genotype and any relevant Your genotype is environmental influences. The difference between genotype and phenotype helps explain the recipe for making you. why a clone (a genetic copy of an individual) or even an identical twin can never be an Your phenotype is how you exact duplicate of another person. actually turn out. As Figure 4 illustrates, people with different genotypes may exhibit the same phe- notype. For example, a child who is homozygous for a dominant brown hair allele will have brown hair, but so will a child who is heterozygous for that same allele. Because it is dominant, the brown hair is expressed, and the recessive red hair allele is hidden. Environmental experience modifies the expression of the genotype for most traits— a phenomenon called multifactorial transmission. Multifactorial transmission illustrates multifactorial transmission the interaction of nature and nurture and how they affect outcomes. Imagine that Rio Combination of genetic and environ- has inherited athletic talent and comes from a family of avid athletes. If his family mental factors to produce certain com- plex traits. nurtures his talent and he practices regularly, he may become a skilled athlete. How- ever, if he is not encouraged or not motivated to engage in athletics, his genotype for epigenesis athletic ability may not be expressed (or may be expressed to a lesser extent) in his Mechanism that turns genes on or off phenotype. Some physical characteristics (including height and weight) and most psy- and determines functions of body cells. chological characteristics (such as intelligence and musical ability) are products of multifactorial transmission. Many disorders (such as attention-deficit/hyperactivity dis- order) arise when an inherited predisposition (an abnormal variant of a normal gene) interacts with an environmental factor, either before or after birth (Yang et al., 2013). Epigenetic Influences on Gene Expression Have you ever wondered why identical twins—who share 100 percent of their genetic code— look and act slightly different? Epi- genetic variation can help explain this. The field of epigenetics includes the study of biochemical modifications of genetic expression “above the genome”—without altering DNA sequence. The differences arise as certain genes are turned off or on as they are needed by the developing body or when triggered by the environment. This phenomenon is called epigenesis, or epigenetics. Epigenesis works via chemical molecules, or “tags,” attached to a gene that affect the way a cell “reads” the gene’s DNA. Because every cell in the body inherits the same DNA sequence, the function of the chemical tags is to differentiate various types of Identical twins can body cells, such as brain cells, skin cells, and liver cells—somewhat like placing sticky usually open each other’s notes in your textbook to tell you where to look for information. These tags work by switching particular genes on or off during embryonic formation. phones with facial Environmental factors, such as nutrition, smoking, sleep habits, stress, and physical recognition. However, activity, can cause epigenetic changes (Wong et al., 2014). In turn these epigenetic changes because every person— can contribute to such common ailments as cancer, diabetes, and heart disease (Biswas even identical twins—has & Rao, 2018; Ling & Rönn, 2019; Prasher et al., 2020). It may explain why one mono- unique fingerprints (Tao et zygotic twin is susceptible to a disease such as schizophrenia whereas the other twin is al., 2012), they cannot open not and why some twins get the same disease but at different ages (Demir & Demir, each other’s phones with a 2018). Environmental influences can also be social in nature. For example, ­childhood fingerprint scanner. adversity can lead to a variety of health vulnerabilities, including cardiovascular disease, Mechanisms of Heredity  EXPERIENCE HUMAN DEVELOPMENT 53  decreased immune responses, and an increased risk of psycho- logical disorders (Pierce et al., 2020; Berens et al., 2017). Cells are particularly susceptible to epigenetic modification during critical periods such as pregnancy (Naffee et al., 2008). Furthermore, epigenetic modifications, especially those that occur early in life, may be heritable. For example, the granddaughters of women and the grandsons of men who experienced famine while in the womb lived shorter lives on average (Pembrey et al., 2014). GENETIC AND CHROMOSOMAL ABNORMALITIES Most birth disorders are fairly rare (see Table 1), affecting only about 3 percent of live births (Centers for Disease Control and Epigenetic changes can explain why identical twins look Prevention, 2020). Nevertheless, they are the leading cause increasingly divergent with age. of infant death in the United States, accounting for 21 percent of LPETTET/E+/Getty Images infant deaths (Ely & Driscoll, 2020). The most prevalent defects are clubfoot, cleft palate, Down syndrome, and heart defects, in that order (Mai et al., 2019). Rates of disorders vary with race and ethnicity. For example, Asian/Pacific Islander infants have the lowest prevalence of anencephaly, a variety of heart and aortal defects, clubfoot, gastrointestinal abnormalities, limb defects, and spina bifida. Hispanic and American Indian/Alaska native infants, by contrast, have among the highest rates of these defects. Non-Hispanic Black infants have higher rates of omphalocele (a birth defect in the abdominal wall) and trisomy-13 (Mai et al., 2019). Survival rates also differ by ethnicity. In particular, African American and Hispanic infants are at higher risk than White, non-Hispanic infants (Wang et al., 2015). Not all genetic or chromosomal abnormalities are apparent at birth. Tay-Sachs, a fatal degenerative disease of the central nervous system, and sickle-cell anemia, a blood disorder, do not generally appear until at least 6 months of age. Likewise, cystic fibrosis, a condition in which excess mucus accumulates in the lungs and digestive tract, may not checkpoint appear until age 4. Some diseases show an even later onset, such as glaucoma, a disease can you... in which fluid pressure builds up in the eyes, and Huntington’s disease, a progressive degeneration of the nervous system, which do not typically appear before middle age. Explain how epigenesis and It is in genetic defects and diseases that we see most clearly the operation of domi- genome imprinting occur, and nant and recessive transmission, as well as of a variation, sex-linked inheritance, dis- give examples? cussed in the the section on Sex-Linked Inheritance of Defects. Dominant or Recessive Inheritance of Defects Most of the time, typical genes are dominant over those carrying abnormal traits, but sometimes the gene for an abnormal trait is dominant. When this is the case, even one copy of the “bad” gene will result in a child expressing the disorder. Among the 1,800 disorders known to be transmitted by dominant inheritance are achondroplasia (a type of dwarfism) and Huntington’s disease. Defects transmitted by dominant inheritance are less likely to be lethal at an early age than those transmitted by recessive inheritance because any affected children would be likely to die before reproducing. Therefore, that gene would not be passed on to the next generation and would soon disappear from the population. Recessive defects are expressed only if the child is homozygous for that gene; in other words, a child must inherit a copy of the recessive gene from each parent. Because recessive genes are not expressed if the parent is heterozygous for that trait, it may not always be apparent that a child is at risk for receiving two alleles of a recessive gene. Defects transmitted by recessive genes tend to be lethal at an earlier age, in contrast to those transmitted by dominant genes, because recessive genes can be transmitted by heterozygous carriers who do not themselves have the disorder. Thus, they are able to reproduce and pass the genes down to the next generation. Normally the presence of a dominant/recessive gene pair results in the full ­expression of the dominant gene and the masking of the recessive gene. However, in 54 EXPERIENCE HUMAN DEVELOPMENT CHAPTER 3 Forming a New Life  TABLE 1 Some Birth Defects and Genetic Disorders Problem Characteristics of Condition Who Is at Risk What Can Be Done Alpha thalassemia Severe anemia that reduces ability of the Primarily families of Frequent blood blood to carry oxygen; nearly all affected Malaysian, African, transfusions infants are stillborn or die soon after birth and Southeast Asian descent Beta thalassemia Severe anemia resulting in weakness, Primarily families of Frequent blood (Cooley’s anemia) fatigue, and frequent illness; usually fatal in Mediterranean transfusions adolescence or young adulthood descent Cystic fibrosis Overproduction of mucus, which collects in 1 in 2,000 White Physical therapy to the lung and digestive tract; children do not births loosen mucus; antibiotics grow normally; short life span; the most for lung infections; common inherited lethal defect among enzymes to improve White people digestion; lung transplant Duchenne Fatal disease usually found in males, marked 1 in 3,000 to 5,000 No treatment muscular by muscle weakness and minor intellectual male births dystrophy disability; respiratory failure and death usually occur in young adulthood Hemophilia Excessive bleeding, usually affecting males; 1 in 10,000 families Frequent transfusions of in its most severe form, can lead to crippling with a history of blood with clotting arthritis in adulthood hemophilia factors Anencephaly Absence of brain tissues; infants are stillborn 1 in 1,000 No treatment or die soon after birth Spina bifida Incompletely closed spinal canal, muscle 1 in 1,000 Surgery prevents further weakness or paralysis, and loss of bladder injury; shunt placed in and bowel control; often accompanied by brain drains excess fluid. hydrocephalus, an accumulation of spinal fluid in the brain, and intellectual disability Phenylketonuria Metabolic disorder resulting in intellectual 1 in 15,000 births Special diet can prevent (PKU) disability intellectual disability Polycystic kidney Infantile form: enlarged kidneys, leading to 1 in 1,000 Kidney transplants disease respiratory problems and congestive heart failure. Adult form: kidney pain, kidney stones, and hypertension resulting in chronic kidney failure Sickle-cell anemia Deformed red blood cells that clog blood 1 in 500 African Painkillers, transfusions vessels, depriving the body of oxygen; Americans for anemia and to symptoms include severe pain, stunted prevent stroke, growth, infections, leg ulcers, gallstones, antibiotics for infections pneumonia, and stroke Tay-Sachs disease Degenerative disease of the brain and Primarily found in No treatment nerve cells, resulting in death before age 5 Eastern European Jewish families Sources: Adapted from Centers for Disease Control and Prevention (2021) and Ross et al. (2013). incomplete dominance, the resulting phenotype is a combination of both genes. For incomplete dominance example, people with only one sickle-cell allele and one “good” allele do not have Pattern of inheritance in which a child receives two different alleles, resulting sickle-cell anemia with its distinctive, abnormally shaped blood cells. Their blood cells in partial expression of a trait. are not the typical round shape either. They are an intermediate shape, which shows that the sickle-cell gene for these people is incompletely dominant. Mechanisms of Heredity  EXPERIENCE HUMAN DEVELOPMENT 55 sex-linked inheritance Sex-Linked Inheritance of Defects In sex-linked inheritance (Figure 5), certain reces- Pattern of inheritance in which certain sive disorders affect male and female children differently. This is due to the fact that characteristics carried on the X chromo- males are XY and females are XX. In humans, the Y chromosome is smaller and carries some inherited from the mother are transmitted differently to her male and far fewer genes than the X chromosome. One outcome of this is that males receive only female offspring. one copy of any gene that happens to be carried on the sex chromosomes, whereas females receive two copies. So, if a woman has a “bad” copy of a particular gene, she has a backup copy. However, if a male has a “bad” copy of a particular gene, that gene will be expressed. Heterozygote females who carry one “bad” copy of a recessive gene and one “good” Children with Turner one are called carriers. If such a woman has children with an unaffected male (a man syndrome have only who has a “good” copy of the gene), she has a 50 percent chance of passing the disor- one X chromosome, are der on to any sons they might have. If they have a son (who is XY by virtue of being missing the second sex male), the father contributed a Y chromosome, and the mother contributed the X chro- chromosome, and are mosome. Because she has one “good” copy and one “bad” copy, either outcome is equally likely. Daughters (who are XX by virtue of being female) may be protected always girls. Because so because the father will pass on his “good” copy to daughters, so the girls have a 50 little information is carried percent chance either of being completely unaffected or of carrying a hidden recessive on the Y chromosome, an copy of the gene. embryo with only a Y Sex-linked recessive disorders are more common in males than in females. For exam- chromosome and no X ple, red-green color blindness, hemophilia (a disorder in which blood does not clot when chromosome is not viable. it should), and Duchenne muscular dystrophy (a disorder that results in muscle degen- Alternatively, an embryo eration and eventually death) are all more common in males, and all result from genes with only an X chromosome located on the X chromosome. Occasionally, a female does inherit a sex-linked condition. but no Y often is. For this to happen, the father must have a “bad” copy, and the mother must also be a carrier or herself have the condition. Chromosomal Abnormalities Chromosomal abnormalities typically occur because of errors in cell division, resulting in an extra or missing chromosome. For example, Klinefelter syndrome is caused by an extra female sex chromosome (shown by the pattern XXY). Turner syndrome results FIGURE 5 Sex-Linked Inheritance Carrier Unaffected In the most common form, mother father the female sex chromosome of an unaffected mother carries one recessive abnormal gene and one dominant “good” one (X). The father has one “good” male X and Y chromosome complement. The odds for each male child are 50-50: 1. 50% risk of inheriting the abnormal X and the disorder 2. 50% chance of inheriting “good” X X X Y and Y chromosomes The odds for each female child are 50-50: 1. 50% chance of inheriting one abnormal X, to be a carrier like mother 2. 50% chance of inheriting no X Y X X Y X abnormal genesa Unaffected Unaffected Affected Carrier male female male female Possible hereditary results 56 EXPERIENCE HUMAN DEVELOPMENT CHAPTER 3 Forming a New Life  TABLE 2 Sex Chromosome Abnormalities Pattern/Name Typical Characteristics* Incidence Treatment XYY Male; tall stature; tendency toward low IQ, 1 in 1,000 male births No special treatment especially verbal XXX (triple X) Female; normal appearance, menstrual irregu- 1 in 1,000 female births Special education larities, learning disorders, intellectual disability XXY (Klinefelter) Male; sterility, underdeveloped secondary sex 1 in 1,000 male births Hormone therapy, special characteristics, small testes, learning disorders education XO (Turner) Female; short stature, webbed neck, impaired 1 in 1,500 to 2,500 Hormone therapy, special spatial abilities, no menstruation, infertility, female births education underdeveloped sex organs Fragile X Minor-to-severe intellectual disability more 1 in 1,200 male births; Educational and behavioral severe in males; delayed speech and motor 1 in 2,000 female births therapies when needed development, hyperactivity; the most common inherited form of intellectual disability *Not every affected person has every characteristic. from a missing sex chromosome (XO). The likelihood of errors increase in offspring of Down syndrome women age 35 or older. Characteristics of the most common sex chromosome disorders Chromosomal disorder characterized by moderate-to-severe intellectual disability are shown in Table 2. and by such physical signs as a down- Down syndrome, the most common chromosomal abnormality, accounts for about ward-sloping skin fold at the inner cor- 40 percent of all cases of moderate-to-severe intellectual disability (Pennington et al., ners of the eyes. Also called trisomy-21. 2003). The condition is also called trisomy-21 because it is characterized in more than 90 percent of cases by an extra 21st chromosome. Slightly over one of every 700 live births is a child with Down syn- drome (Mai et al., 2019). Although the risk of having a child with Down syndrome rises with age, because of the higher birthrates of younger women, more young mothers have children with Down syndrome (Cen- ters for Disease Control and Prevention, 2021). Research also shows having a father less than 20 or over 40 years old increases the risk (Fang et al., 2020). Additionally, parents who have had one child with Down syndrome are at increased risk for having another child with Down syn- drome (Davidson, 2008). Between 1979 and 2003, an increased tendency to delay child rear- ing and thus a greater number of older mothers resulted in a corre- sponding increase in the number of children born with Down syndrome (Shin et al., 2009). However, this trend was offset by the development of noninvasive prenatal screening tests in 2011, which allowed pregnant women to test for genetic disorders without risk of miscarriage. Esti- mates are that approximately 30 percent of pregnancies in which Down syndrome was diagnosed were electively terminated (de Graaf et al., 2015). Children with Down syndrome, like other children with disabilities, tend to benefit cognitively, socially, and emotionally when provided with regular, intensive therapies designed to help them achieve important skills (Smith et al., 2020; Ruiz-González et al., 2019; Lukowski et al., 2019). As adults, many live in small group homes and support them- selves; they tend to do well in structured job situations. Because of Although Down syndrome is a major cause of increases in the average life span, there now exists a much wider range intellectual disability, people with this chromosomal of ages in the US population of people with Down syndrome than used abnormality can live happy, productive lives. to be the case (de Graaf et al., 2015). Still, a recent international meta- Disability Images/Blend Images LLC Mechanisms of Heredity  EXPERIENCE HUMAN DEVELOPMENT 57  analysis of 34 studies indicated that, across a variety of countries, people with Down syndrome live about 28 fewer years than the general population (O’Leary et al., 2018). Another common sign of Down syndrome involves the lines that palm GENETIC COUNSELING AND TESTING readers use to tell your Genetic counseling can help prospective parents assess their risk of bearing children with fortune. In children with genetic or chromosomal defects. People who have already had a child with a genetic Down syndrome, there is a defect, who have a family history of hereditary illness, who suffer from conditions known single horizontal line across or suspected to be inherited, or who come from ethnic groups at higher risk of passing the palm. on genes for certain diseases can get information about the likelihood their children being affected. genetic counseling A genetic counselor takes a family history and gives the prospective parents and any Clinical service that advises prospective biological children physical examinations. Laboratory investigations of blood, skin, urine, parents of their probable risk of having or fingerprints may be performed. Chromosomes from body tissues may be analyzed children with hereditary defects. and photographed, and the photographs enlarged and arranged according to size and structure on a chart called a karyotype. This chart can show chromosomal abnormalities and can indicate whether a person might transmit genetic defects to a child (Figure 6). The counselor tries to help clients understand the mathematical risk of a particular condition, explains its implications, and presents information about alternative courses of action. Geneticists have made great contributions to avoidance of birth defects. For example, since so many Jewish couples have been tested for genes that carry Tay-Sachs, a fatal disease involving degeneration of mental and physical abilities, far fewer Jewish babies have been born with the disease (Zhang et al., 2019). Similarly, screening and counsel- ing of women of childbearing age from Mediterranean countries, where beta thalassemia (refer to Table 1) is common, has brought a decline in births of affected babies and greater knowledge of the risks of being a carrier (Cao & Kan, 2013). A B 1 2 3 4 5 C 6 7 8 9 10 11 12 D E 13 14 15 16 17 18 checkpoint can you... F G 19 20 21 22 Sex chromosomes Explain the operation of dominant inheritance, recessive FIGURE 6 inheritance, incomplete dominance, sex-linked Karyotype of a Female with Down Syndrome A karyotype is a photograph that shows the chromosomes when they are separated and inheritance, and mutations in aligned for cell division. We know that this is a karyotype of a person with Down transmission of birth defects? syndrome because there are three chromosomes instead of the usual two on pair 21. Explain the purposes of Because pair 23 consists of two Xs, we know that this is the karyotype of a female. genetic counseling? Sources: Babu & Hirschhorn (1992); March of Dimes Birth Defects Foundation (1987). 58 EXPERIENCE HUMAN DEVELOPMENT CHAPTER 3 Forming a New Life Studying the Influences of Heredity and Environment Today it has become clear that, although certain rare physical disorders are virtually 100 percent inherited, phenotypes for most traits, such as those having to do with intelligence and personality, are subject to a complex array of hereditary and environmental forces. MEASURING HERITABILITY One approach to the study of heredity and environment is quantitative: It seeks to mea- sure how much heredity and environment influence particular traits. This is the tradi- tional goal of the science of behavioral genetics. behavioral genetics Behavioral geneticists have developed a means of estimating how much of a trait is Quantitative study of relative hereditary due to genetics and how much is the result of environmental influences by using a con- and environmental influences on behavior. cept known as heritability. Every trait is a consequence of genes and environment. By looking at groups of people with known genetic relationships and assessing whether or heritability not they are concordant, or the same, on a given trait, behavioral geneticists can estimate Statistical estimate of contribution of he- the relative influence of genes and environment. redity to individual differences in a spe- cific trait within a given population. Heritability cannot be measured directly. Thus, researchers in behavioral genetics have developed indirect methods for assessing the relationship between the expression concordant of traits and the genetic and environmental factors influencing them. Although there are Term describing tendency of twins to variations in the details, the underlying logic of the approaches in these types of studies share the same trait or disorder. is the same. If two people are unrelated, we know they are not likely to share any genes. If two people are identical twins, we know they share all their genes. If two people are fraternal twins, siblings, or parent and child, they share roughly 50 percent of their genes with Keep in mind that a each other. And, if we know, on average, how many genes people share, then we can high heritability estimate measure how similar they are on traits (that is, their concordance rate) and work back- does not mean that a trait ward to determine the relative environmental influences. Therefore, if heredity has a cannot be influenced by large influence on a particular trait, identical twins should be more alike on that trait environment. If the than fraternal twins, and adopted children should be more like their biological parents environment changes, the than their adoptive parents. Note that this can be carried out to more distant genetic heritability estimate may relatives as well. For traits with strong genetic influences, for example, siblings should change as well. be more similar than cousins on that trait. We can also use the environment to estimate influences. If the environment exerts a large influence on a trait, people who live together should be more similar than those that live apart, and shared genes should matter less. For example, in this situation, we might compare adopted children to their biological and adoptive parents. If adopted children are more similar to their adoptive parents than their biological parents on a trait, then that trait is likely influenced strongly by the environment. Essentially, this approach boils down to comparisons of shared genes, the same or different environments, and concordance rates. Using these three variables allows researchers to make an estimate of the relative influence of genes and environment on a trait. Different variations of this basic approach are used. In family studies, researchers measure the degree to which biological relatives share certain traits and determine whether or not the closeness of the familial relationship is associated with the degree of similarity. Adoption studies look at similarities between adopted children and their adoptive families and also between adopted children and their biological families. Twin studies compare pairs of monozygotic, or identical, twins with same-sex dizygotic, or fraternal, twins. Heritability is expressed as a percentage ranging from 0.0 to 1.0: the higher the number, the greater the heritability of a trait. A heritability estimate of 1.0 indicates that genes are 100 percent responsible for variances in the trait within the population. A heritability estimate of 0.0 percent would indicate the environment shaped a trait Studying the Influences of Heredity and Environment  EXPERIENCE HUMAN DEVELOPMENT 59  e­ xclusively. Note that heritability does not refer to the influences that shaped any one particular person because those influences are virtually impossible to separate. Nor does checkpoint can you... heritability tell us how traits develop. It merely indicates the statistical extent to which genes contribute to a trait at a certain time within a given population. State the basic assumption underlying studies of behav- ioral genetics and how it HOW HEREDITY AND THE ENVIRONMENT WORK TOGETHER applies to family, twin, and Today many developmental scientists see heredity and environment as fundamentally adoption studies? intertwined. From conception on, throughout life, a combination of constitutional factors (related to biological and psychological makeup) and social, economic, and cultural factors help shape development. Reaction Range Many characteristics vary, within limits, under varying hereditary or environmental conditions. The concept of reaction range can help us visualize how this happens. reaction range Reaction range refers to a range of potential expressions of a hereditary trait. Body Potential variability, depending on envi- size, for example, depends largely on biological processes, which are genetically regu- ronmental conditions, in the expression lated. Tall people have tall children, and short people have short children. Even so, a of a hereditary trait. range of sizes is possible. In societies in which nutrition has dramatically improved, an canalization entire generation has grown up to tower over the generation before. The better-fed chil- Limitation on variance of expression of dren share their parents’ genes but have responded to a healthier world. Ultimately, certain inherited characteristics. height has genetic limits. We don’t see typically developing people who are only 1 foot tall or 10 feet tall. Heredity can influence whether a reaction range is wide or narrow. In other words, the genotype places limits on the range of possible phenotypes. For example, a child born with a defect producing mild cognitive limitations is more able to respond to a favorable environment than a child born with more severe limitations. Likewise, a child IQ (phenotype) with greater native intelligence is likely to benefit more from an 160 enriched home and school environment than a child with a more typical level of intelligence (Figure 7). 140 reaction range 155 Canalization Some traits have an extremely narrow range of reac- 120 tion. The metaphor of canalization illustrates how heredity restricts reaction 125 the range of development for some traits. After a heavy storm, the range rainwater that has fallen on a pavement has to go somewhere. If the 100 reaction 100 street has potholes, the water will fill them. If deep canals have been range 80 dug along the edges of the street, the water will flow into the canals. Highly canalized traits, such as eye color, are analogous to the deep canals. They are strongly programmed by genes, and there is 60 115 100 little opportunity for variance in their expression. Because of the 90 deep, genetically dug channel, it would take an extreme change in 40 environment to alter their course. The canal is too deep for the water to easily slosh over. 20 Many highly canalized traits tend to be those necessary for sur- vival. In the case of very important traits such as these, natural Marcie Juan Andrea selection has designed them to develop in a predictable and reliable (genotype A) (genotype B) (genotype C) way within a variety of environments and a multitude of influences. Enriched environment Restricted environment They are too important to be left to chance. Thus, traits such as these tend to be highly canalized. With respect to motor develop- ment, typical babies follow a predictable sequence: crawling or scoot- FIGURE 7 ing, walking, and then running, in that order, at certain approximate Intelligence and Reaction Range ages. This sequence is said to be canalized, in that children will Children with different genotypes for intelligence will follow this same blueprint irrespective of many variations in the envi- show varying reaction ranges when exposed to a ronment. A similar process occurs for language. Despite differences restricted (blue portion of bar) or enriched (entire bar) in linguistic environments, babies the world over reach language mile- environment. stones at approximately the same time and in the same order. 60 EXPERIENCE HUMAN DEVELOPMENT CHAPTER 3 Forming a New Life  Cognition and personality, however, are not highly canalized. They are more subject to variations in experience. Consider reading. We are not wired to read: Natural selection has not designed us to naturally develop this skill. The environment plays a large part. In humans, walking Parents who play letter and word games and who read to their children are likely to and talking are canalized have children who learn to read earlier than if these skills are not encouraged or rein- traits. Can you think of forced. Children who are not taught to read do not learn to do so spontaneously. other human physical or behavioral characteristics Genotype-Environment Interaction Genotype-environment interaction usually refers that might be highly to the effects of similar environmental conditions on genetically different individuals, canalized? and a discussion of these interactions is a way to conceptualize and talk about the different ways nature and nurture interact. To take a familiar example, many children are exposed to pollen and dust, but those with a genetic predisposition are more likely genotype-environment interaction The portion of phenotypic variation that to develop allergic reactions (Sordillo et al., 2015). Interactions can work the other results from the reactions of genetically way as well: Genetically similar children often develop differently depending on their different individuals to similar environ- home environments. A child born with a difficult temperament may develop adjust- mental conditions. ment problems in one family and thrive in another, depending largely on parental handling. Genotype-Environment Correlation The environment often reflects or reinforces One environmental genetic differences. This tendency is called genotype-environment correlation, and it works factor that has been in three ways to strengthen the phenotypic expression of a genotypic tendency (Berge- identified as protective man & Plomin, 1989). The first two ways are common among younger children, the against severe allergies third among older children, adolescents, and adults. in children is early Passive correlations: You not only inherit genes from your biological parents, you exposure to animals also inherit environments. For example, a musical parent is likely to create a home (Wegienka et al., 2011). environment in which music is heard regularly, to give a child music lessons, and to take the child to musical events. If the child inherited the parent’s musical talent, genotype-environment correlation the child’s musicality will reflect a combination of genetic and environmental Tendency of certain genetic and envi- influences. This type of correlation is called passive because the child does not ronmental influences to reinforce each control it. Passive correlations are most applicable to young children, whose parents other; may be passive, reactive (evoca- have a great deal of control over their early experiences. In addition, passive tive), or active. Also called genotype- correlations function only when a child is living with a biologically related parent. environment covariance. Reactive, or evocative, correlations: Children with differing genetic makeups evoke different reactions from others. For example, parents who are not musically inclined may make a special effort to provide musical experiences for a child who Behavioral genetics shows interest and ability in music. This response, in turn, strengthens the child’s research with twins and genetic inclination toward music. This type of correlation is called reactive because the other people react to the child’s genetic makeup. siblings shows whether we enjoy or hate exercise is Active correlations: As children get older and have more freedom to choose their genetically influenced and own activities and environments, they actively select or create experiences shows moderate heritability consistent with their genetic tendencies. An adolescent with a talent for music will probably seek out musical friends, take music classes, and go to concerts if such (Schutte et al., 2017). opportunities are available. This tendency to seek out environments compatible with one’s genotype is called niche-picking; it helps explain why identical twins niche-picking reared apart tend to have similar characteristics. Tendency of a person, especially after early childhood, to seek out environ- ments compatible with his or her Nonshared Environmental Influences Although two children in the same family may genotype. bear a striking physical resemblance, siblings can differ greatly in intellect and especially in personality (Plomin & Daniels, 2011). One reason may be genetic differences, which lead children to need different kinds of stimulation or to respond differently to a similar home environment. For example, one child may be more affected by family discord than another (Horowitz et al., 2010). In addition, studies in behavioral genetics suggest that many of the experiences that strongly affect development vary for different children in a family (Dunn & Plomin, 1991). Children may live in the same family, but that does not mean that their experiences are identical. Studying the Influences of Heredity and Environment  EXPERIENCE HUMAN DEVELOPMENT 61  These nonshared environmental effects result from the unique environ- ment in which each child in a family grows up. Children in a family have a shared environment—the home they live in, the people in it, and the activities family members jointly engage in—but they also, even if they are twins, have experiences that are not shared by their brothers and sisters. Parents and siblings may treat each child differently. Certain events, such as illnesses and accidents, and experiences outside the home affect one child and not another. Despite being in the same family, the influences are not identical. Indeed, some behavioral geneticists have concluded that although heredity accounts for most of the similarity between siblings, the nonshared environment accounts for most of the difference (Hetherington et al., 2013). We can also extend the conversation about genotype-environment correlations to explain some of the effects of the nonshared environment on siblings’ experiences. Children’s genetic differences may lead parents to react to them differently and treat them differently. One child may be An adolescent with artistic talent may seek out shy and elicit more gentle behavior from parents; another may be bold opportunities to engage in creative pursuits. and be given greater freedom and encouragement to explore. Children This is an example of niche-picking. also mold their environments by the choices they make—what they do Hill Street Studios/Getty Images and with whom—and their genetic makeup influences these choices. A child who loves to read may spend hours in solitude; an athletic and sociable child may nonshared environmental effects prefer to be outside playing with others. Thus, not only will the child’s talents (such as The unique environment in which each reading or athleticism) develop differently, but their social life will be different as well. child grows up, consisting of distinctive These differences tend to be accentuated as children grow older and have more experi- influences or influences that affect one child differently than another. ences outside the family (Plomin, 1996; Scarr, 1992). CHARACTERISTICS INFLUENCED BY HEREDITY AND ENVIRONMENT checkpoint can you... Keeping in mind the complexity of unraveling the influences of heredity and environ- ment, let’s look at what is known about their roles in producing certain characteristics. Explain and give at least one example of reaction-range Physical Health The risk of developing a wide variety of medical disorders, including canalization and of each of the high blood pressure, heart disease, stroke, rheumatoid arthritis, and epilepsy, has been found to be influenced by genetics (Olczak et al., 2021; Assimes & Roberts, 2016; Dichgans three genotype-environment et al., 2019; Okada et al., 2019; Myers et al., 2019). Life span, too, seems to be influenced correlations? by genes (Melzer et al., 2020). Differentiate the three types of Obesity is usually measured by body mass index, or BMI (comparison of weight to genotype-environment height), and is a risk factor for many negative health outcomes. Children between the 85th correlation? and 95th percentiles are classified as overweight, and those above the 95th percentile as obese. The risk of obesity is 2 to 3 times higher for a child with a family history of obesity List three kinds of influences (Bahreynian et al., 2017). Therefore, we might reasonably conclude that obesity involves that contribute to nonshared genetic contributions. environmental effects? Research shows that obesity is indeed affected by genetics. There is not “a” gene for obesity; rather it is a multifactorial condition. Twin studies, adoption studies, and other research suggest the heritability of obesity is 40 to 50 percent across the general popula- tion. However, heritability varies across weight status. In people who are obese, heritability estimates are 60 to 65 percent. In people who are severely obese, heritability estimates are over 80 percent (Bouchard, 2021). However, this increased risk is not solely genetic. Environmental experiences also con- tribute to obesity (Willyard, 2014). The kind and amount of food eaten in a particular home and the amount of exercise that is encouraged can increase or decrease the likelihood that a child will become obese. Moreover, the wider social context is at play as well. Obe- obesity sity rates rise in countries with rapid socioeconomic growth and increases in gross domes- Extreme overweight in relation to age, sex, height, and body type as defined tic product (Min et al., 2013). In Western countries, obesity likely stems from the by having a body mass index at or interaction of a genetic predisposition with overeating, supersized portions, and inadequate above the 95th percentile. exercise (Lakerveld et al., 2017). 62 EXPERIENCE HUMAN DEVELOPMENT CHAPTER 3 Forming a New Life Intelligence Heredity exerts a strong influence on general intelligence, as measured by intelligence tests, and a moderate effect on specific abilities such as memory, verbal ability, and spatial ability (Plomin & Von Stumm, 2018). Note that although specific genes might contribute to intelligence, intelligence is best described as shaped by large numbers of genes working together. Indirect evidence of the role of heredity in intelligence comes from adoption and twin studies. Adopted children’s scores on standardized intelligence tests are consistently closer to the scores of their biological mothers than to those of their adoptive parents and siblings; monozygotic twins are more alike in intelligence than dizygotic twins (Bri- ley & Tucker-Drob, 2013). Intelligence also depends in part on brain size and structure, which are under strong genetic control (Goriounova & Mansvelder, 2019). Experience counts, too. An enriched or impoverished environment can substantially affect the development and expression of innate ability (Kempermann, 2019). Environmental influence is greater, and heritability lower, among poor families than among more economically privileged families (Nisbett et al., 2012). The influence of genes increases sharply with age (Plomin & Dreary, 2015). This increase is probably a result of niche-picking. The shared family environment has a strong temperament influence on young children but little influence on adolescents, who are more apt to find Characteristic disposition, or style of ap- their own niche by actively selecting environments compatible with their hereditary proaching and reacting to situations. abilities and related interests (Bouchard, 2013). schizophrenia Mental disorder marked by loss of Temperament and Personality When babies are exposed to a new experience, say contact with reality; symptoms include riding on a train or playing with a new noisy toy, some infants respond with interest and hallucinations and delusions. excitement, and others with apprehension and withdrawal. Some babies are active, others less so. Some babies sleep and eat at the same time every day, others have difficulty set- tling into a consistent schedule. Right from the beginning, infants are utterly unique. Another trait Psychologists call babies’ unique and characteristic ways of approaching and react- ing to environmental stimuli temperament. Temperament is largely inborn and is rela- influenced by genetics tively consistent over the years, although it may respond to special experiences or is religiosity. Behavioral parental handling (Goldsmith et al., 1987). In support of the role of genes, siblings— genetics research suggests both twins and singletons—tend to be similar in temperament on such traits as positive that the tendency to believe affect, activity level (Saudino & Micalizzi, 2015), and behavioral regulation (Gagne & strongly in a religion is Saudino, 2010). moderately heritable; Temperament underlies adult personality. Given the genetic contributions found for that is, at about the same temperament, one would predict personality research should also illustrate hereditary level as intelligence (Waller influences. This is indeed the case. Scientists have identified genes directly linked with et al., 1990). specific aspects of personality such as neuroticism and extraversion (Vinkhuyzen et al., 2012). Overall, the heritability of personality traits appears to be around 40 percent (Vukasović & Bratko, 2015), and there is little evidence of shared environmental influence (Plomin, 2011). As with intelligence, genetic influences on personality appear to become more important with age (Briley & Tucker-Drob, 2014) and are shaped in part by active niche- picking (Kandler & Zapko-Willmes, 2017). Psychopathology There is evidence for a hereditary influence on such mental disorders as schizophrenia, autism, alcoholism, and depression. All tend to run in families and to show greater concordance between monozygotic twins than between dizygotic twins. However, heredity alone does not produce such disorders; an inherited tendency can be triggered by environmental factors (Smoller et al., 2019). Schizophrenia illustrates the interaction of heredity and genetics. Although temperamental characteristics—such as Schizophrenia is a neurological disorder that affects about 1 percent of shyness—are genetically influenced, the environment the US population each year (Society for Neuroscience, 2008). It is modulates the expression of these tendencies and characterized by loss of contact with reality; hallucinations and delusions; shapes the resulting adult personality. loss of coherent, logical thought; and inappropriate emotionality. Patricia Marks/Shutterstock Studying the Influences of Heredity and Environment  EXPERIENCE HUMAN DEVELOPMENT 63  ­ stimates of heritability range from 60 to 80 percent (Schwab & Wildenauer, 2013). A E wide array of rare gene mutations, some of which involve missing or duplicated segments of DNA, may increase susceptibility to schizophrenia (Giegling et al., 2017). However, monozygotic twins are not always concordant for schizophrenia, perhaps due to epigen- etic processes (Smigielski et al., 2020). checkpoint Researchers also have looked at possible nongenetic influences, such as a series of can you... neurological insults in fetal life (Debnath et al., 2015), exposure to influenza or rubella (Brown, 2012), or high stress experienced by a mother during her pregnancy (Lipner et Discuss the evidence for al., 2019). Infants born in urban areas or those whose mothers experienced obstetric genetic and environmental complications or who were poor or severely deprived as a result of war or famine are influences on physical and at higher risk (Rapoport et al., 2012), as are infants born during the winter months physiological traits such as (Martinez-Ortega et al., 2011). Advanced paternal age is also a risk factor for schizo- obesity, intelligence, tempera- phrenia (Lan et al., 2020), and there are indications that, at least for boys, having very ment, and schizophrenia? young fathers may put children at elevated risk as well (Miller et al., 2010). Prenatal Development For many women, the first clear (though not necessarily reliable) sign of pregnancy is gestation a missed menstrual period. But even before that first missed period, a pregnant woman’s Period of development between con- body undergoes subtle but noticeable changes. Table 3 lists early signs and symptoms ception and birth. of pregnancy. During gestation, the period between conception and birth, an unborn child under- gestational age Age of an unborn baby, usually dated goes dramatic processes of development. The normal range of gestation is between 37 from the first day of an expectant moth- and 41 weeks. Gestational age is usually dated from the first day of an expectant moth- er’s last menstrual cycle. er’s last menstrual cycle. TABLE 3 Early Signs and Symptoms of Pregnancy Physical Change Causes and Timing Missed period May indicate pregnancy, although can be misleading Tender, swollen Increased production of the female hormones estrogen and progesterone stimulates breast growth breasts to prepare for producing milk. Slight bleeding or Implantation bleeding may occur about 10 to 14 days after fertilization when ovum attaches to lining cramping of uterus. Women may also cramp as the uterus enlarges. Nausea with or Pregnancy hormones likely play a role. Morning sickness may begin as early as 2 weeks after con- without vomiting ception but usually around 4 to 8 weeks and may occur at any time of day. Frequent urination Enlarging uterus during first trimester exerts pressure on the bladder. Fatigue Heart is pumping harder and faster to carry nutrients to the fetus. Stepped-up production of hormones takes extra effort. Progesterone depresses central nervous system and may cause sleepiness. Mood swings Flood of hormones early in pregnancy can produce emotional highs and lows. Constipation Increase in progesterone may slow digestion, so food passes more slowly through intestinal tract. Food aversions Hormonal changes may alter food preferences, especially during first trimester. Heightened sense of smell may trigger nausea in response to certain foods. Faintness and Lightheaded feeling may be triggered by blood vessel dilation and low blood pressure or by low dizziness blood sugar. Raised basal body Basal body temperature (taken first thing in the morning) normally rises soon after ovulation each temperature month and then drops during menstruation. When menstruation ceases, temperature remains elevated. Source: Mayo Clinic (2021). 64 EXPERIENCE HUMAN DEVELOPMENT CHAPTER 3 Forming a New Life CULTURAL BELIEFS ABOUT PRENATAL DEVELOPMENT Although our modern understanding of pregnancy differs from traditional beliefs found in much of the world, people from all cultures share the understanding that the prenatal environment can profoundly shape the developing human. Much of the research on cultural beliefs about prenatal development has been con- ducted in Asian countries, where common practices during pregnancy include massage, the use of traditional healers, medicines and herbs, taboos against the consumption of hot or cold foods, behavioral taboos, and superstitions (Withers et al., 2018). For exam- ple, in Chiang Mai, Thailand, women are sometimes cautioned against eating papaya, pickled foods, or more than half a banana during pregnancy. Spicy food, too, is advised against as it is thought to be associated with being born hairless, and coffee or tea is believed to negatively affect a child’s intelligence (Liamputtong et al., 2005). Similar taboos are found worldwide. In some areas of India, “cold” foods such as milk, yogurt, coconut, wheat, vegetables, and rice are recommended for pregnant women and believed to guard against miscarriage (Choudry, 1997). Alternatively, Guatemalan mothers are warned to avoid “hot” foods such as meat and beans (Cosminsky, 1982). The Walpiri aboriginal people of Australia warn pregnant mothers to avoid eating food made from spiked animals such as anteaters, monitor lizards, or possums and are told Pregnancy tests to be careful not to harm any animal associated with their developing baby’s spirit, which identify the presence of is shaped by the geographical area in which the child is conceived (Pierroutsakos, 2000). human chorionic Traditional beliefs for the Konya of Turkey specify mothers should eat quince if a dim- gonadotropin, which, under pled baby is desired or apples if they want their child to have ruddy cheeks (Okka et normal circumstances, is al., 2016). And, Canadian First Nations people believe it is important to eat foods such as wild meat, fish, white carrots, potatoes, rice, and berries for the baby’s health, and produced only by embryos also stress the importance of moderate exercise lest the baby stick to the womb and and fetuses. So there are experience a difficult labor (Sokoloski, 1995). no false positives. These cultural beliefs are interesting because they highlight important cultural mes- A pregnancy might not be sages and beliefs about pregnancy. However, they are also important because traditional viable, but a positive beliefs may clash with modern medical treatment, leading some women to forego pre- pregnancy test tells a natal care. For example, pregnant Zambian women in one study avoided medical care woman a conception has during pregnancy in part because they believed if they divulged their use of traditional occurred. healers to health care providers, they would be denied service (Maimbolwa et al., 2003). STAGES OF PRENATAL DEVELOPMENT Prenatal development takes place in three stages: germinal, embryonic, and fetal. (Table 4 gives a month-by-month description.) Both before and after birth, development proceeds according to two fundamental principles: Growth and motor development occur from the top down and from the center of the body outward. The cephalocaudal principle, from Latin, meaning “head to cephalocaudal principle tail,” dictates that development proceeds from the head to the lower part of the trunk. Principle that development proceeds in a head-to-tail direction; that is, that up- An embryo’s head, brain, and eyes develop earliest and are disproportionately large until per parts of the body develop before the other parts catch up. According to the proximodistal principle, from Latin, meaning lower parts of the trunk. “near to far,” development proceeds from parts near the center of the body to outer ones. The embryo’s head and trunk develop before the limbs, and the arms and legs proximodistal principle Principle that development proceeds before the fingers and toes. from within to without; that is, that parts of the body near the center develop Germinal Stage (Fertilization to 2 Weeks) During the germinal stage, from fertiliza- before the extremities. tion to about 2 weeks of gestational age, the zygote divides, becomes more complex, and germinal stage is implanted in the wall of the uterus. First 2 weeks of prenatal development, Within 36 hours after fertilization, the zygote enters a period of rapid cell division characterized by rapid cell division, and duplication (mitosis). Seventy-two hours after fertilization, it has divided first into blastocyst formation, and implantation 16 and then into 32 cells; a day later, it has 64 cells. While the fertilized ovum is divid- in the wall of the uterus. ing, it is also making its way through the fallopian tube to the uterus, a journey of 3 or 4 days. Its form changes into a blastocyst, a fluid-filled sphere, which floats freely in the uterus until the sixth day after fertilization, when it begins to implant itself in the uter- Prenatal Development  EXPERIENCE HUMAN DEVELOPMENT 65  TABLE 4 Milestones in Prenatal Development Age Accomplishments 3 weeks Nervous system begins to form. 4 weeks Heart begins to beat. 5 weeks Head continues rapid growth. 8 weeks Almost all body parts are differentiated. 12 weeks Growth of head slows. Formation of red blood cells by liver slows. Biophoto Associates/Science Source 14 weeks Begins to coordinate limb movement Possible to visually determine baby’s sex 16 weeks Ultrasound shows clearly defined bone structure. 20 weeks Possible to hear heartbeat with stethoscope Baby covered by fine downy hair called lanugo. Fetal movements called quickening are felt by mother. Clouds Hill Imaging Ltd/Corbis Documentary/Getty Images 21 weeks Rapid eye movements commence. Substantial weight gain 24 weeks Fingernails can be seen. 28 weeks Eyes open and close. Lungs capable of breathing 32 weeks Skin pink and smooth Chubby appearance 38 weeks Nervous system can carry out some integrative functions. James Stevenson/Science Source Reacts to light Usually assumes upside-down position as birth approaches Sources: Leifer (2003); Moore & Persaud (2003); Olds et al. (1996). implantation ine wall. Only about 10 to 20 percent of fertilized ova complete the task of implantation The attachment of the blastocyst to the and continue to develop. Where the egg implants will determine the placement of the uterine wall, occurring at about day 6. placenta. Before implantation, as cell differentiation begins, some cells around the edge of the blastocyst cluster on one side to form the embryonic disk, a thickened cell mass from which the embryo begins to develop. This mass will differentiate into three layers. The ectoderm, the upper layer, will become the outer layer of skin, the nails, hair, teeth, sensory organs, and the nervous system, including the brain and spinal cord.