chpt 2THE BEGINNINGS From Conception to Birth

The Embryonic Period

(a) At 4 weeks past conception, the embryo is only about 1/8 inch (3 millimeters) long, but already the head has taken shape. (b) By 7 weeks, the organism is somewhat less than an inch (2 centimeters) long. Eyes, nose, the digestive system, and even the first stage of toe formation can be seen. (page 56

Less common are nonadditive genes

, which do not contribute equal shares. In one nonadditive form alleles interact in a dominant-recessive pattern. Then for a pair of genes (one from each parent), one gene, called dominant, is far more influential than the other, called recessive. When someone has a recessive gene that is not expressed that person is a carrier of that gene. The recessive gene is carried on the genotype. dominant-recessive pattern The interaction of a heterozygous pair of alleles in such a way that the phenotype reflects one allele (the dominant gene) more than the other (the recessive gene). carrier A person whose genotype includes a gene that is not expressed in the phenotype. The carried gene occurs in half of the carrier's gametes and thus is passed on to half of the carrier's children. If such a gene is inherited from both parents, the characteristic appears in the phenotype.

The most common extra-chromosome condition that results in a surviving child is Down syndrome,

with three chromosomes at the 21st site. In 1868, Dr. Langdon Down and his wife opened a home for such children (then called "Mongolian Idiots"), proving that they could be quite capable. The World Health Organization officially named trisomy-21 Down syndrome in 1965. Some 300 distinct characteristics can result from trisomy-21. No individual with Down syndrome is identical to another, but this trisomy usually produces telltale physical characteristics—a thick tongue, round face, and slanted eyes, as well as distinctive hands, feet, and fingerprints. The brain is somewhat smaller; the hippocampus (important for memory) is especially affected. However, remember plasticity. The impact of that third chromosome varies with every step of development, from conception on

but that is exactly why a genetic counselor, trained to make information clear, is needed—especially for:

women who fear that something they did or experienced affected a future or developing embryo; individuals who have a parent, sibling, or child with a serious genetic condition; couples who have had several spontaneous abortions or stillbirths; couples who are infertile; women over age 35 and men over age 40; and couples from the same ethnic group, particularly if they are relatives.

X-linked A gene carried on the X chromosome. If a male inherits an X-linked recessive trait from his mother, he expresses that trait because the Y from his father has no counteracting gene. Females are more likely to be carriers of X-linked traits but are less likely to express them.

A special case of the dominant-recessive pattern occurs with genes that are X-linked (located on the X chromosome). If an X-linked gene is recessive—as are the genes for most forms of color blindness, many allergies, several diseases (including hemophilia and Duchenne muscular dystrophy), and some learning disabilities—the fact that it is on the X chromosome is critical (see Table 2.1). Since the Y chromosome is much smaller than the X, an X-linked recessive gene almost never has a dominant counterpart on the Y. Therefore, recessive traits carried on the X affect the phenotypes of sons more often than daughters.

THE MOST COMMON GENETIC DISORDERS

About 1 in 12 North American men and women carries an allele for cystic fibrosis, thalassemia, or sickle-cell disease, all devastating in children who inherit the recessive gene from both parents. These conditions are common because carriers have benefited from the gene.

monozygotic (MZ) twins Twins who originate from one zygote that splits apart very early in development. (Also called identical twins.) Other monozygotic multiple births (such as triplets and quadruplets) can occur as well.

About once in every 250 human conceptions, the zygote not only duplicates but splits apart completely, creating two, or four, or even eight separate cells, each genetically identical to that original zygote. If each cell implants and grows, multiple births occur, One separation results in monozygotic (MZ) twins, from one (mono) zygote. Two or three separations create monozygotic quadruplets or octuplets. (An incomplete split creates conjoined twins, once called Siamese twins.) However, because nurture always affects nature, identical twins do not have exactly the same phenotype.

embryo The name for a developing human organism from about the third week through the eighth week after conception.

After implantation, the embryonic period begins. The formless mass of cells becomes a distinct being—not yet recognizably human but with a new name, embryo. (The word embryo is often used loosely, but each stage has a particular name. Here, embryo refers to the developing human from day 14 to day 56.) (See Table 2.2.)

copy number variations The various repeats or deletions of base pairs that genes have.

Attention has focused on copy number variations, which are repeats or deletions (from one to hundreds) of base pairs. Copy number variations are widespread—everyone has them—and they correlate with almost every disease and condition, including heart disease, intellectual disability, mental illness, and many cancers. Most, however, are insignificant. For example, about 30 percent of our skin cells include copy number variations (Macosko & McCarroll, 2013). No matter, our skin still protects us just fine.

CONSEQUENCES OF LOW BIRTHWEIGHT

Children who were extremely SGA or preterm tend to have neurological impairments in middle childhood, including smaller brain volume, lower IQ, and behavioral difficulties Even in adulthood, risks persist: Adults who were VLBW are more likely to develop diabetes and heart disease. They also are more likely to become depressed However, remember plasticity. By age 4, some ELBW infants exhibit typical brain development, especially if they had no medical complications and their mother was well educated. In adulthood, for the fortunate ones, early arrival may no longer be relevant.

complications

Complications vary by hospital. A study of 750,000 births in the United States divided hospitals into three categories—low, average, and high quality—based on obstetric complications for the woman. In low-quality hospitals, cesareans led to five times as many complications (20 percent) and vaginal births twice as many (23 percent) compared to high-quality hospitals (4 and 11 percent) (Glance et al., 2014).

new fathers

Currently, about half of all U.S. women are not married when their baby is born (Martin et al., 2017, p. 2), but fathers are usually listed on the birth certificate. When fathers acknowledge their role, birth is better for mother and child. For example, a study of 151,869 babies and mothers (every single birth in Milwaukee from 1993 to 2006) found that complications correlated with several expected variables (e.g., maternal cigarette smoking) and one unexpected one—no father listed on the birth record. This connection was especially apparent for European Americans: When no father was listed, rates of long labor, cesarean section, and other complications increased ( Fathers may experience pregnancy and birth biologically, not just psychologically. Many fathers experience symptoms, including weight gain and indigestion during pregnancy and pain during labor.

dizygotic (DZ) twins Twins who are formed when two separate ova are fertilized by two separate sperm at roughly the same time. (Also called fraternal twins.)

Dizygotic (DZ) twins, also called fraternal twins, are born three times as often as monozygotic twins. They began life as two zygotes created by two ova fertilized by two sperm. (Usually the ovaries release only one ovum per month, but sometimes two or more ova are released.) A woman's tendency to ovulate more than one ovum is influenced by her genes, and thus it is more common in some families and groups than others. Because genes endure lifelong, if a woman has one set of DZ twins, she is more likely to have another set the genotype doesn't skip, but the phenotype might.

chromosome One of the 46 molecules of DNA (in 23 pairs) that virtually every cell of the human body contains and that, together, contain all of the genes. Other species have more or fewer chromosomes.

Each molecule of DNA is packaged into a chromosome. Almost all humans have 46 chromosomes; other creatures have more than or fewer than 46. Chromosomes contain genes, each located on a particular chromosome. Humans have about 21,000 genes. With one exception, every cell has a copy of that person's chromosomes, arranged in pairs. The exception is the reproductive cell, called a gamete. Each gamete—sperm in a man and ovum in a woman―has only 23 chromosomes, one from each of that person's 23 pairs.

uncertain sex

Every now and then, a baby is born with "ambiguous genitals," meaning that the child's sex is not abundantly clear. When this happens, a quick analysis of the chromosomes is needed to make sure that there are exactly 46 and to see whether the 23rd pair is XY or XX. The karyotypes shown here indicate a typical baby boy (left) and girl (right).

teratogen An agent or condition, including viruses, drugs, and chemicals, that can impair prenatal development and result in birth defects or even death.

Every week, scientists discover another teratogen, which is anything—drugs, viruses, pollutants, malnutrition, stress, and more—that could harm an embryo or fetus.

immigrant paradox The surprising, paradoxical fact that low-SES immigrant women tend to have fewer birth complications than native-born peers with higher incomes.

Evidence for this is in the immigrant paradox. Many immigrants have difficulty getting education and well-paid jobs; their socioeconomic status is low. Low SES correlates with low birthweight, especially in the United States (Martinson & Reichman, 2016). Thus, newborns born to immigrants are predicted to be underweight.

genotype An organism's entire genetic inheritance, or genetic potential.

For each individual, the collection of his or her genes is called the genotype. It was once thought that the genotype led directly to facial characteristics, body formation, intelligence, personality, and so on, but this is much too simplistic. As you just read, not every gene is expressed.

female and male

Forty-five of a human's 46 chromosomes are equally likely to be inherited by a boy or a girl. That includes both halves of the first 22 pairs (called autosomes) and one half of the 23rd pair (the X). Thus, sex and gender are irrelevant for 97.8 percent of who we are, genetically.

Finally, a Baby

Ideally, infants are born full term, weighing more than 5 ½ pounds, with an Apgar of at least 8. Medical assistance speeds contractions, dulls pain, and saves lives, but some interventions may be unnecessary, including about half of the cesareans performed in the United States. Newborns are primed for social interaction, and fathers and mothers are often emotionally connected to their baby and to each other. Paternal support correlates with shorter labor and fewer complications. About one women in seven experiences postpartum depression, feeling unhappy, incompetent, or unwell after giving birth. The most vulnerable time is when the baby is several weeks old, with social support crucial for mother, father, and infant.

However, one chromosome on the 23rd pair is crucial.

In females, the 23rd pair is composed of two large X-shaped chromosomes. Accordingly, it is XX. In males, the 23rd pair has one large X-shaped chromosome and one quite small Y-shaped chromosome. That 23rd pair is XY. XX A 23rd chromosome pair that consists of two X-shaped chromosomes, one each from the mother and the father. XX zygotes become females. XY A 23rd chromosome pair that consists of an X-shaped chromosome from the mother and a Y-shaped chromosome from the father. XY zygotes become males. ecause a female's 23rd pair is XX, when that pair splits, every ovum contains one X or the other—but always an X. Because a male's 23rd pair is XY, when his 46 chromosomes divide to make gametes, half of his sperm carry an X chromosome and half carry a Y. (See Figure 2.2.)

From Zygote to Newborn

In the germinal period (the first two weeks after conception), cells duplicate and differentiate, and the developing organism implants itself in the lining of the uterus. In the embryonic period (third through the eighth week), organs and body structures are formed, except the sex organs. In the fetal period (ninth week until birth) the fetus grows and all the organs begin to function. Crucial for viability is brain development at 22 weeks, when a fetus born that early might survive. Every week after that increases weight and odds of survival.

postpartum depression A new mother's feelings of inadequacy and sadness in the days and weeks after giving birth.

Many women experience significant physical problems soon after birth, such as healing from a c-section, painfully sore nipples, or problems with urination. However, worse than physical problems are psychological ones (O'Hara & McCabe, 2013). When the level of birth hormones drops, about one new mother in seven experiences postpartum depression, a sense of inadequacy and sadness (called baby blues in the mild version and postpartum psychosis in the most severe form). Postpartum depression is affected by anesthesia, hormones, pain, financial stress, marital problems, a birth that did not go as planned, surgery, and a baby with feeding or other problems. Successful breast-feeding reduces maternal depression (Figueiredo et al., 2014), but success is elusive for many new mothers. A lactation consultant may be an important part of the new mother's support team. Some researchers believe that postpartum depression is a consequence of modern life, because contemporary women consume less omega-3 fatty acids (especially found in fish), exercise less (especially in the sun), and are far from their own mothers and other relatives (Hahn-Holbrook & Haselton, 2014). In any case, depressed mothers need help not only for their sake but for the sake of their babies, who begin learning how to respond to people based on how people respond to them.

additive genes Genes that each contribute to the characteristic—they "add up" rather than one being hidden (recessive). For example, skin color is additive: It shows the combined genes of both parents, rather than taking after one or the other.

Most genes are additive genes. Their effects add up to make the phenotype. When genes interact additively, the phenotype may reflect all the genes that are involved. Height, hair curliness, and skin color, for instance, are influenced by additive genes. Indeed, height is probably influenced by 180 genes, each contributing a very small amount (Enserink, 2011).

Brazelton Neonatal Behavioral Assessment Scale (NBAS) A test that is often administered to newborns which measures responsiveness and records 46 behaviors, including 20 reflexes.

Newborns are responsive social creatures (Zeifman, 2013). They listen, stare, cry, stop crying, and cuddle. In the first day or two, a professional might administer the Brazelton Neonatal Behavioral Assessment Scale (NBAS), which records 46 behaviors, including 20 reflexes. A similar but simpler set of responsive behaviors can be assessed at birth (Nugent et al., 2017).

couvade Symptoms of pregnancy and birth experienced by fathers.

Paternal experiences of pregnancy and birth are called couvade —expected in some cultures such as India, normal in many, and considered pathological in others (M. Sloan, 2009; Ganapathy, 2014). In the United States, couvade is unnoticed and unstudied, but many fathers are intensely involved with their future child (Brennan et al., 2007; Raeburn, 2014). Like new mothers, fathers are vulnerable to depression; other people need to help. Indeed, sometimes the father experiences more depression in the first few weeks than the mother (Bradley & Slade, 2011).

genetic counseling Consultation and testing by trained experts that enable individuals to learn about their genetic heritage, including harmful conditions that they might pass along to any children they may conceive.

Professionals who provide genetic counseling help prospective parents understand their genetic risk so that they can make informed decisions, not impulsive, irrational ones. They advise about special hazards, precautions, and treatments, before conception, during pregnancy, and after birth.

siblings

Siblings differ not only in their chromosomes and microbiome but also in the genes themselves. When the genes on the father's chromosome pair up with their counterparts from the mother, the interaction between the two determines the inherited traits of the future person. Since some alleles from the father differ from the alleles from the mother, their combination produces a zygote unlike either parent. Even more than that, each zygote carries genes that are not exact duplicates of those inherited from the parents (Macosko & McCarroll, 2013). Small variations, mutations, or repetitions in the 3 billion base pairs could make a notable difference in the proteins and thus, eventually, in the person.

allele A variation that makes a gene different in some way from other genes for the same characteristics. Many genes never vary; others have several possible alleles. heterozygous Referring to two genes of one pair that differ in some way. Typically one allele has only a few base pairs that differ from the other member of the pair.

Some genes come in slightly different versions, as in eye-color genes. Each version is called an allele. Genes that have various alleles are called polymorphic (many shapes). If the gene from one parent differs from the same gene from the other parent, the zygote is said to be heterozygous for that trait.

behavioral teratogens Agents and conditions that can harm the prenatal brain, impairing the future child's intellectual and emotional functioning.

Some teratogens cause no physical defects but affect the brain, making a child hyperactive, antisocial, or intellectually disabled. These are behavioral teratogens. Behavioral teratogens can be subtle, yet their effects last a lifetime.

reflex An unlearned, involuntary action or movement in response to a stimulus. A reflex occurs without conscious thought.

Technically, a reflex is an involuntary response to a particular stimulus. Humans of every age reflexively protect themselves (the eye blink is an example). Reflexes seem automatic. Not quite. The strength and reliability of newborn reflexes varies depending on genes, drugs at birth, and overall health.

implantation The process, beginning about 10 days after conception, in which the developing organism burrows into the uterus, where it can be nourished and protected as it continues to develop.

The first task of those outer cells is implantation —that is, to embed themselves in the lining of the uterus. This is far from automatic; half of all conceptions do not implant. Most new life ends before an embryo begins (Sadler, 2015). Successful implantation allows the cell mass to tap into nourishment from the mother's uterine wall, beginning the interdependence of mother and child.

germinal period The first two weeks of prenatal development after conception, characterized by rapid cell division and the beginning of cell differentiation.

The first two weeks are called the germinal period, when the single cell, smaller than the period at the end of this sentence, germinates into an embryo with thousands of cells. Within hours after conception, the zygote begins duplication and division.

how Proteins Are Made

The genes on the chromosomes in the nucleus of each cell instruct the cell to manufacture the proteins needed to sustain life and development. The code for a protein is the particular combination of four bases, T-A-G-C (thymine, adenine, guanine, and cytosine).

Remember that nurture starts at conception.

The newborn sex ratio is higher in developed nations (105:100 in Northern Europe and North America) but lower in poor nations (e.g., Zimbabwe, 101:100) (United Nations, Department of Economic and Social Affairs, 2017). The probable reason: Male embryos are more vulnerable if their pregnant mother is hungry and lacks prenatal care. That is nurture interacting with nature. During infancy and childhood, boys die at slightly higher rates; by early adulthood, the usual sex ratio is finally even. Adult males die more often: In the United States, people over age 85 are twice as likely to be women than men. Is this nature? Perhaps hormonal? Maybe not. The environment profoundly affects survival of males and females lifelong, as explained in Opposing Perspectives.

Twelve of 3 Billion Pairs

This is a computer illustration of a small segment of one gene. Even a small difference in one gene can cause major changes in a person's phenotype.

The Moment of Conception

This ovum is about to become a zygote. It has been penetrated by a single sperm, whose nucleus now lies next to the nucleus of the ovum. Soon, the two nuclei will fuse, bringing together about 21,000 genes to guide development.

Most of the 7,000 known single-gene disorders are dominant

always expressed) (Milunsky & Milunsky, 2016). Most dominant disorders are relatively mild; severe ones are infrequent because children with a severe disorder usually die before puberty, and thus they never pass on that lethal gene. However, a few dominant disorders are latent until adulthood. One is Huntington's disease, a fatal central nervous system disorder caused by a copy number variation—more than 35 repetitions of a particular set of three base pairs.

Every human has at least 44 autosomes and one X chromosome;

an embryo cannot develop without those 45. However, about 1 in every 300 infants is born with only one sex chromosome (no Y) or with three or more (not just two) (Benn, 2016). Each particular combination of sex chromosomes results in a particular syndrome (see Table 2.3). page 60

Less studied is the epidural,

an injection in the spine that alleviates pain. Epidurals are often used in hospital births, but they may increase the rate of cesarean sections, decrease newborn sucking, and lead to other complications—at least according to a large study in Pennsylvania (Kjerulff, 2014).

The genomes of brewer's yeast and a tiny worm (the nematode) are the only ones that have been

completely sequenced, down to every letter of code. Virtually complete are the genomes of Homo sapiens and many other species, including the sweat bee, the olive fruit fly, the komodo dragon, the kakapo bird, and the monk seal (a list provided to help readers realize how many species and genomes there are) (Pennisi, 2017). Plant genomes are more complex, but several have been sequenced, including several kinds of rice.

Additional DNA and RNA (another molecule) surround each gene. In a process called methylation, this material

enhances, transcribes, connects, empowers, silences, regulates, and alters genes. This material used to be called junk―but now "there is no such thing as junk DNA" (Larson, 2018, p. 1). As one team explains: One of the most important discoveries in genetics in the last 10 years is that the vast majority of trait-associated DNA variations occur in regions of the genome that were once labeled as 'junk DNA' because they do not code for proteins. We now know that these regions harbor genetic elements that control where, when, and to what extent specific genes are expressed.

Researchers who sought the gene for, say, schizophrenia, or homosexuality, or even for a tiny detail such as memory for chemistry formulas,

have been disappointed. No such single genes exist. Instead, almost every trait arises from a combination of genes, each with a small potential impact, each dependent on epigenetic factors that determine if that gene is expressed or silenced

After about the eight-cell stage, a third process, differentiation,

joins duplication and division. In differentiation, cells specialize, taking different forms and reproducing at various rates, depending on where they are located. They are no longer omnipotent stem cells that could develop into a new person. About a week after conception, the multiplying cells (now numbering more than 100) separate into two distinct masses.

A hundred years ago, it was believed that humans had 48 chromosomes,

not 46; 20 years ago, it was thought that humans had 100,000 genes, not 20,000 or so. Why? (see response, page 79)

epigenetic,

not solely genetic. As noted earlier, genes are affected from the moment of conception by other material. Epi- is a prefix that means "above, on, over, nearby, upon; outer; besides, in addition to; among; attached to; or toward." All important human characteristics are epigenetic including diseases known to be inherited, such as cancer, schizophrenia, and autism spectrum disorder (Kundu, 2013; Plomin et al., 2013). [Life-Span Link: Epigenetics is introduced in Chapter 1.]

If an entire chromosome is missing or added,

that leads to a recognizable syndrome, a cluster of distinct characteristics that tend to occur together. Usually the cause is three chromosomes at a particular location instead of the typical two (a condition called a trisomy).

Another medical intervention is induced labor,

when labor is started, speeded, or strengthened with a drug. The rate of induced labor in developed nations has more than doubled since 1990, up to 20 or 25 percent. Sometimes induction is necessary for the health of the mother or the fetus. However, induced labor itself increases complications, including higher rates of cesareans (Grivell et al., 2012; Mikolajczyk et al., 2016).

The DNA code on those chromosomes contains about

3 billion base pairs of chemicals organized in triplets (sets of three pairs), each of which specifies production of one of 20 possible amino acids. Those amino acids combine to produce proteins, and those proteins combine to produce a person. Small variations, mutations, or repetitions (called copy number variations) in the base pairs or triplets make a notable difference in the proteins and thus, eventually, in the person. Some genes have triplet transpositions, deletions, or repetitions not found in other versions of the same gene. Thus, genes "are themselves transmitted to individual cells with large apparent mistakes—somatically acquired deletions, duplications, and other mutations" (

finally a baby- page 71

About 38 weeks (266 days) after conception, the fetal brain signals the release of hormones (especially oxytocin) to start labor. The average baby is born after about 12 hours of active labor for first births and 7 hours for subsequent births, with wide variations. The definition of "active" labor is usually decided by the woman herself, especially since women are encouraged to stay home until contractions are difficult to manage. Some women believe they are in active labor for days, and others say 10 minutes. Women's birthing positions also vary—sitting, squatting, lying down. Some women give birth while immersed in warm water, which helps the woman relax (the fetus continues to get oxygen via the umbilical cord). Preferences and opinions on birthing positions are partly cultural and partly personal. In general, physicians find it easier to see the head emerge if the woman lies on her back. However, it is easier for women to push the fetus out if they sit up. (Figure 2.6 shows the stages of birth.)

deoxyribonucleic acid (DNA) The chemical composition of the molecules that contain the genes, which are the chemical instructions for cells to manufacture various proteins.

All living things are composed of cells that promote growth and sustain life according to instructions in their molecules of deoxyribonucleic acid (DNA)

age of viability The age (about 22 weeks after conception) at which a fetus might survive outside the mother's uterus if specialized medical care is available.

At about 22 weeks past conception, the brain is sufficiently mature to reach the age of viability , when a fetus born early might become a baby who survives. Note that brain maturation, not body size, is crucial: Twins born at 22 weeks sometimes survive, although they weigh less than a single fetus that young. In the last trimester (months 7, 8, and 9) organs mature, weight is gained (an ounce a day!), and the fetus prepares for life outside the uterus with no medical help needed. T

primitive streak

At about day 14, a thin line called the primitive streak appears down the middle of the cell mass; it forms the neural tube 22 days after conception. The neural tube develops into the central nervous system (i.e., the brain and spinal column) (Sadler, 2015). Soon the head appears, as eyes, ears, nose, and mouth start to form and a minuscule blood vessel that will become the heart begins to pulsate.

zygote The single cell formed from the union of two gametes, a sperm and an ovum.

At conception, the genes on each of the 23 chromosomes from the sperm pair up with the genes on the same 23 chromosomes from the ovum, creating a new cell called a zygote. or instance, an eye-color gene from the father on chromosome 15 connects with an eye-color gene from the mother on the zygote's other chromosome 15. If the match between the two genes is exact (as it usually is since most genes are identical for every human), the person is said to be homozygous (literally, "same zygote") for that trait.

surgery/ midwives

Fifty years ago, in developed nations, hospital births required a medical doctor (M.D.), but now many hospitals allow midwives, who are trained specifically in pregnancy and birth but not in surgery. The data show that midwives are as skilled at delivering babies as physicians are, and the rates of various complications and interventions are lower in midwife births (Bodner-Adler et al., 2017; Renfrew et al., 2014). If the birth needs surgical intervention, an M.D. is called. Many midwives try to avoid such a call, believing that doctors are too quick to intervene. On the other hand, most U.S. births are attended by physicians, who deliver via cesarean section (c-section, or simply section) in about one birth in three. The fetus is removed through incisions in the mother's abdomen, avoiding a vaginal birth. C-sections were once very rare: a way to save the baby when it seemed that both mother and fetus were dying. Now c-sections save lives of both mother and child, and some women and doctors prefer them.

Genes and Chromosomes

Genes are the foundation for development. Human conception occurs when two gametes (an ovum and a sperm, each with 23 chromosomes) combine to form a zygote. Those 46 chromosomes contain the genes, about 20,000 in all. Biological sex is determined by the 23rd pair of chromosomes, with a Y sperm creating an XY (male) zygote or an X sperm creating an XX (female) zygote. If one zygote splits, that creates monozygotic twins; if two ova are fertilized at the same time, that creates dizygotic twins. Genes may interact additively, or they can follow a dominant-recessive pattern. The genotype may not be expressed in the phenotype, which is the actual characteristics of the person.

Problems and Solutions

If a zygote has more or fewer than 46 chromosomes, it usually does not implant. However, if an extra chromosome is at the 21st site (Down syndrome) or at the 23rd site, that person has lifelong disabilities but may have a good life. Everyone is a carrier for genetic abnormalities. Usually these conditions are recessive, and no fetus is affected unless both parents carry the same disorder. Genetic testing and counseling can help many couples avoid having a baby with serious chromosomal or genetic problems. Thousands of teratogens, especially drugs and alcohol, have the potential to harm the embryo or fetus. Actual harm occurs because of a cascade: Genes, critical periods, dose, and frequency all have an impact. Low birthweight (less than 5 ½ pounds, or 2,500 grams) may result from multiple fetuses, maternal illness, genes, malnutrition, smoking, drinking, or drug use. Underweight babies may experience physical and intellectual problems lifelong. Newborns that are small for gestational age (SGA) are especially vulnerable. Maternal behavior increases the risk of every problem, including low birthweight. Fathers, other relatives, and the society also can affect the incidence of disabilities.

gene disorders

If all anomalies and disorders are included, 92 percent of people do not develop a serious genetic condition by early adulthood—but that means 8 percent have a notable problem in their phenotype as well as their genotype (Chong et al., 2015). Everyone carries about 40 alleles that could cause serious disease. The phenotype is affected only when: the inherited gene is dominant, or a zygote received the same recessive gene from both parents, or multiple additive genes combine to cause a problem.

Recessive diseases are more numerous than dominant ones because they are passed down by carriers who are not affected.

Most recessive disorders are on the autosomes and thus are not X-linked, which means that either parent could be a carrier (Milunsky & Milunsky, 2016). Only in the rare case when two carriers have a child who inherits the double recessive (true for one child in four when both parents are carriers) is the gene expressed. There are thousands of recessive diseases; advance carrier detection is currently possible for only several hundred. A few recessive conditions are X-linked, which means they are carried on the X chromosome. One is fragile X syndrome, which is caused by more than 200 repetitions of one gene (Plomin et al., 2013). (Some repetitions are normal, but not this many.) The cognitive deficits caused by fragile X syndrome are the most common form of inherited intellectual disability. (Many forms are not usually inherited.) Boys are more often impaired by fragile X than are girls, because they have only one X.

Apgar scale A quick assessment of a newborn's health, from 0 to 10. Below 6 is an emergency—a neonatal pediatrician is summoned immediately. Most babies are at 7, 8, or 9—almost never a perfect 10.

Newborn health is often measured by the Apgar scale, first developed by Dr. Virginia Apgar. When she earned her M.D. in 1933, Apgar wanted to work in a hospital but was told that only men did surgery. She became an anesthesiologist, present at many births but never the doctor in charge. Apgar saw that "delivery room doctors focused on mothers and paid little attention to babies. Those who were small and struggling were often left to die" (Beck, 2009, p. D1). To save those young lives, Apgar developed a simple rating scale of five vital signs—color, heart rate, cry, muscle tone, and breathing. Nurses could use the scale and raise the alarm immediately if a newborn was in crisis. Since 1950, birth attendants worldwide have used the Apgar (often using the name as an acronym: Appearance, Pulse, Grimace, Activity, and Respiration) at one minute and again at five minutes after birth, assigning each vital sign a score of 0, 1, or 2. Most babies are 8 or higher; below 6 requires immediate attention from a neonatal pediatrician. (See Visualizing Development on page 73.) A study comparing Apgar rates in 23 nations found that birth attendants in some nations tended to score newborns as almost perfect (97 percent of newborns scored 9 or 10 in some nations, only 73 percent in others). Culture and custom, not objective data, seemed to be the reason, since high scores did not correlate with excellent obstetric practice. However, everywhere, babies with Apgars below 7 were at risk of early death (Siddiqui et al., 2017). Thus, worldwide, a low Apgar signals "baby emergency."

microbiome All of the microbes (bacteria, viruses, and so on) with all of their genes in a community; here, the millions of microbes of the human body.

One epigenetic influence that profoundly affects each person is the microbiome, which refers to all of the microbes (bacteria, viruses, fungi, archaea, yeasts) that live within the body. The microbiome includes what people call "germs," which they try to kill with disinfectant and antibiotics. However, most microbes are helpful, not harmful. Microbes have their own DNA, reproducing throughout life. There are thousands of varieties of these microbes. Together they have an estimated 3 million different genes—influencing immunity, weight, diseases, moods, and much else that affects us every day (Dugas et al., 2016; Koch, 2015). Particularly intriguing is the relationship between the microbiome and nutrition, since bacteria in the gut break down food for nourishment (Devaraj et al., 2013; Pennisi, 2016). A fetus gains weight because of the mother's microbiome. The mother's diet affects the fetal microbiome, and thus it affects the child (Prince et al., 2017). The microbiome, like other epigenetic aspects, changes over each person's life, from birth to death. affects genetic expression

false positive The result of a laboratory test that reports something as true when in fact it is not true. This can occur for pregnancy tests, when a woman might not be pregnant even though the test says she is, or during pregnancy, when a problem is reported that actually does not exist.

One specific example comes from a test in place for decades: alpha-fetoprotein (AFP). If it is too high or too low, it may indicate multiple fetuses, abnormal growth, or Down syndrome. Many such warnings are false positives; that is, they falsely suggest a problem that does not exist. Any warning, whether false or true, requires further testing, worry, and soul-searching.

home births

Only about 1 percent of U.S. births occur at home, about half of them planned and half unexpected because labor happened too quickly. The latter situation is hazardous if no one is nearby to rescue a newborn in distress. Higher rates of newborn death occur in poor nations, where most births occur at home without trained midwives or doctors. Compared with the United States, planned home births are more common in many other developed nations (2 percent in England, 30 percent in the Netherlands) where professional birth attendants are supported by the government. In the Netherlands, special ambulances called flying storks speed mother and newborn to a hospital if needed. In nations where low-risk mothers can choose home births, and good medical care is available, mothers have fewer complications and newborn survival rates in home births are as good as or better than hospital births (de Jonge et al., 2015). One reason women choose a home birth is that they want family members, friends, and nonmedical helpers nearby. Many U.S. hospitals now allow such people, although some still forbid anyone except nurses and doctors, and others limit the number of nonmedical people allowed at birth. One helper often chosen is a doula, a person trained to support the laboring woman. Doulas time contractions, use massage, provide encouragement, and do whatever else is helpful. Often they come to the woman's home during early labor, and they provide breast-feeding advice for days after birth. Every comparison study finds that the rate of medical intervention is lower when doulas are part of the birth team. Doulas have proven to be particularly helpful for immigrant, low-income, or unpartnered women who may be intimidated by doctors. Fathers also may be crucial supports, depending on their training and temperament (Kang, 2014; Saxbe, 2017). The midwife who delivered my grandson praised Elissa's husband, saying he was "as good as any doula."

SPONTANEOUS MUTATIONS

Spontaneous mutations are more likely if the parents have been exposed to various pollutants or radiation, which then affects the sperm, ova, or zygote. Age matters, too: The frequency of chromosomal miscounts rises when the mother is over age 35; genetic mutations increase in the sperm when the father is over age 40. This does not mean that older parents should not have children: Serious problems are unusual no matter how old the parents are.

low birth weight

The World Health Organization defines low birthweight (LBW) as under 2,500 grams. LBW babies are further grouped into very low birthweight (VLBW), under 1,500 grams (3 pounds, 5 ounces), and extremely low birthweight (ELBW), under 1,000 grams (2 pounds, 3 ounces). Some viable newborns weigh as little as 500 grams, but even with excellent care, about half of them die and most survivors suffer physical and intellectual disabilities (Lau et al., 2013) (see Figure 2.4). low birthweight (LBW) A body weight at birth of less than 2,500 grams (5½ pounds). very low birthweight (VLBW) A body weight at birth of less than 1,500 grams (3 pounds, 5 ounces). extremely low birthweight (ELBW) A body weight at birth of less than 1,000 grams (2 pounds, 3 ounces). Some public health experts consider the rate of low birthweight to be indicative of national health, since both are affected by the same causes. If that is true, the world is getting healthier, since the LBW world average was 28 percent in 2009 but 16 percent in 2012. When all nations are included, 47 report LBW at 6 per 100 or lower. (The United States and the United Kingdom are not among them.) Babies born preterm (two or more weeks early; no longer called premature) are often LBW, because fetal weight normally doubles in the last trimester of pregnancy, with 900 grams (about 2 pounds) of that gain occurring in the final three weeks. Some LBW babies are not preterm. They gain weight slowly throughout pregnancy and are small-for-dates, or small for gestational age (SGA). A full-term baby weighing only 2,600 grams and a 30-week-old fetus weighing only 1,000 grams are both SGA, even though the first is not technically a low-birthweight baby. SGA is a sign of something amiss in the pregnancy, often a teratogen, with drug use particularly harmful. Another common reason for slow fetal growth is malnutrition.

c sections

The World Health Organization suggests that cesareans are medically indicated in 10-15 percent of births, but many nations have too few or too many c-sections (World Health Organization, April, 2015). Fifty-four nations are below 10 percent; 69 are above 15 percent (Gibbons et al., 2012b). Nations with low cesarean rates have high rates of childbirth deaths, but nations with high cesarean rates are not necessarily healthier. The world region with the lowest rate is East Africa (4 percent) and with the highest, Latin America (40 percent) (Betrán et al., 2016). In the United States, the cesarean rate rose between 1996 and 2008 (from 21 percent to 34 percent) and since has stabilized or is slightly reduced. Medical reasons for c-section include multiple births, breach (fetus is not positioned head down), prior c-section, long active labor (more than 24 hours), and advanced maternal age. None of those reasons requires a c-section. For instance, a large study of all births (78,880) in the state of Washington focused on the relationship between age and various complications. Of those new mothers aged 50 or older, 60 percent delivered by c-section and 40 percent vaginally (Richards et al., 2016). Cesareans have immediate advantages for hospitals (easier to schedule, quicker, and more expensive than vaginal deliveries) and for women (they can plan ahead, and birth is quick). Convenience, rather than medical necessity, it the likely reason that c-sections are more than twice as common on weekdays than weekends (Martin et al., 2017). Disadvantages appear later. Mothers giving birth by c-section are less likely to breast-feed and more likely to develop medical complications after birth. Children born by cesarean are more likely to develop asthma or become obese (Chu et al., 2017; Mueller et al., 2017). The reason may be that babies delivered vaginally have beneficial bacteria (the microbiome) in their gut, but those delivered surgically do not (Wallis, 2014).

genome The full set of genes that are the instructions to make an individual member of a certain species.

The entire packet of instructions that make a living organism is called the genome. There is a genome for every animal species, from Homo sapiens to the smallest insect, and for every kind of plant. Even yeast has a genome, detailed in 1996. The Human Genome Project found only about 20,000 to 23,000 genes, almost all of which are present in every human being. (Mapping all the possible alleles takes much longer, and is ongoing.) Genomes have since been sequenced for many other creatures, again with surprises. Any two people, of whatever ethnicity, share 99.5 percent of their genetic codes, and humans are much more similar to other mammals than most people imagined. The genetic codes for humans and chimpanzees are 98 percent the same (although chimp genes are on 48, not 46, chromosomes), and the genomes for every other mammal are at least 90 percent the same as for people.

fetus The name for a developing human organism from the start of the ninth week after conception until birth.

The organism is called a fetus from the ninth week after conception until birth. The fetal period encompasses dramatic change, from a tiny creature smaller than the final joint of your thumb to a newborn about 20 inches (51 centimeters) long. At 3 months, the fetus weighs about 3 ounces (87 grams) and is about 3 inches (7.5 centimeters) long. Those numbers—3 months, 3 ounces, 3 inches—are rounded off for easy recollection, but growth rates vary—some 3-month-old fetuses are not quite 3 ounces and others already weigh 4. Mid-pregnancy (months 4, 5, and 6) is the period of the greatest brain growth of the entire life span. The brain increases about six times in size and develops many new neurons (neurogenesis) and synapses (synaptogenesis), and it divides into hemispheres (O'Rahilly & Müller, 2012). Before this, the cortex had been smooth, but now the brain folds and wrinkles to fit inside the head.

First Stages of the Germinal Period

The original zygote as it divides into (a) two cells, (b) four cells, and (c) eight cells. Occasionally at this early stage, the cells separate completely, forming the beginning of monozygotic twins, quadruplets, or octuplets.

phenotype The observable characteristics of a person, including appearance, personality, intelligence, and all other traits.

The phenotype, which is a person's actual appearance and behavior, reflects much more than the genotype. The genotype is the beginning of diversity; the phenotype is the actual manifestation of it, the result of "multiple interactions among numerous genetic and environmental factors" (Nadeau & Dudley, 2011, p. 1015). If a gene is expressed, the influence of many environmental factors determines the particulars of that expression.

Chromosomal Anomalies

The sperm and ova do not always carry exactly 23 chromosomes; about half of all zygotes have more than or fewer than 46 chromosomes (Milunsky & Milunsky, 2016). Almost always they fail to duplicate, divide, differentiate, and implant, and they are spontaneously aborted before anyone knows that conception occurred. If implantation does occur, many embryos with chromosomal miscounts are aborted, either by nature (miscarried) or by choice. Ninety-nine percent of fetuses that survive until birth have the usual 46 chromosomes. For the remaining 1 percent, birth is hazardous (Benn, 2016). Survival is more common if only some cells have 47 chromosomes and the others have 46 (a condition called mosaicism), or if only a piece of a chromosome is missing or extra. Advanced analysis suggests that mosaicism of some sort "may represent the rule rather than the exception" (Lupski, 2013, p. 358). Usually mosaicism has no effect, although cancer in adulthood is more likely.

pain meds

et, one study found that 23 percent of pregnant women on Medicaid were given a prescription for a narcotic (Desai et al., 2014). Hopefully, the prescribing doctor didn't realize the patient was pregnant, and the women didn't take the drug. However, a study of 25 Web sites that, together, approved 235 medications found that TERIS (expert teratologists who analyze drug safety) had declared only 60 (25 percent) safe. The rest were not proven harmful, but TERIS found insufficient evidence to confirm safety (Peters et al., 2013). Those 25 Internet sites sometimes used unreliable data: Some drugs on the safe list of one site were on the danger list of another.

too many boys

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almost every trait is

polygenic (affected by many genes) and multifactorial (influenced by many factors). Almost daily, researchers discover new complexities in multifactorial interaction. Here we describe a few of them.

To understand humans,

polymorphisms in diverse environments are key.