Chapter 3 Summary and Key Terms
What are genes, and how are they transmitted from one generation to the next?
• Each individual’s phenotype, or directly observable characteristics, is a product of both genotype and environment. Chromosomes, rod like structures within the
cell nucleus, contain our hereditary endowment.Along their length are genes, segments of deoxyribonucleic acid (DNA) that send instructions for making a rich
assortment of proteins to the cell’s cytoplasm.
• Gametes, or sex cells, are produced through a cell division process called meiosis. Crossing over and chance assortment of chromosomes into gametes ensure that
each gamete receives a unique set of genes from each parent. Once sperm and ovum unite, the resulting zygote starts to develop into a complex human being
through cell duplication, or mitosis.
If the fertilizing sperm carries an X chromosome, the child will be a girl; if it contains a Y chromosome, a boy. Describe various patterns of genetic inheritance.
• Homozygous individuals have two identical alleles. If the alleles differ, the individual is heterozygous, and relationships between the alleles determine the
phenotype. In dominant–recessive inheritance, only the dominant allele affects the child’s phenotype; a child must inherit two recessive alleles to display the
recessive trait. Heterozygous individuals become carriers of the recessive trait. In incomplete dominance, both alleles are expressed in the phenotype. Modifier
genes enhance or dilute the effects of other genes.
• In X-linked inheritance, a harmful allele is carried on the X chromosome. Males are more likely than females to be affected. In genomic imprinting, one parent’s
allele is activated, regardless of its makeup.
• Harmful genes arise from mutation, which can occur spontaneously or be caused by hazardous environmental agents. Human traits that vary on a continuum,
such as intelligence and personality, result from polygenic inheritance—the effects of many genes. Describe major chromosomal abnormalities, and explain how
• Most chromosomal abnormalities are due to errors during meiosis. The most common is Down syndrome, which results in physical defects and mental
retardation. Sex chromosome abnormalities, such as XYY, triple X, Klinefelter, and Turner syndromes, are milder than defects of the autosomes.
What procedures can assist prospective parents in having healthy children?
• Genetic counseling helps couples at risk for giving birth to children with genetic abnormalities consider appropriate options. Prenatal diagnostic methods allow
early detection of genetic problems.
• Reproductive technologies, such as donor insemination, in vitro fertilization, surrogate motherhood, and postmenopausal-assisted childbirth, enable individuals to
become parents who otherwise would not, but they raise serious legal and ethical concerns.
List the three periods of prenatal development, and describe the major milestones of each.
• The period of the zygote lasts about two weeks, from fertilization until the blastocyst becomes deeply implanted in the uterine lining. During this time, structures
that will support prenatal growth begin to form, including the placenta and the umbilical cord.
• During the period of the embryo, from weeks 2 to 8, the foundations for all body structures are laid down. The nervous system develops fastest, as the neural tube
forms. Other organs follow rapidly. By the end of this period, production of neurons occurs at an astounding pace, and the embryo responds to touch and can
• The period of the fetus, lasting until the end of pregnancy, involves a dramatic increase in body size and completion of physical structures. At the end of the
second trimester, most of the brain’s neurons are in place.
• The fetus reaches the age of viability at the beginning of the final trimester, between 22 and 26 weeks. The brain continues to develop rapidly, and new sensory
and behavioral capacities emerge. Gradually, the lungs mature, the fetus fills the uterus, and birth is near.
Prenatal Environmental Influences
Cite factors that influence the impact of teratogens, noting agents that are known teratogens.
• The impact of teratogens varies with the amount and length of exposure, genetic makeup of mother and fetus, presence or absence of other harmful agents, and
age of the organism at time of exposure. Drugs, tobacco, alcohol, radiation, environmental pollution, and maternal diseases are teratogens that can endanger the
• Babies born to users of heroin, methadone, or cocaine are at risk for prematurity, low birth weight, and physical defects and are born drug-addicted. Evidence on
long-term effects, however, is mixed.
• Infants whose parents use tobacco are often born underweight, may have physical defects, and are at risk for attention, learning, and behavior problems in
• Maternal alcohol consumption can lead to fetal alcohol spectrum disorder (FASD). Fetal alcohol syndrome (FAS) involves slow physical growth, facial
abnormalities, and mental impairments. Milder forms—partial fetal alcohol syndrome (p-FAS) and alcohol-related neurodevelopmental disorder (ARND)—affect
children whose mothers consumed smaller quantities of alcohol.
• Prenatal exposure to high levels of radiation, mercury, lead, dioxins, and PCBs leads to physical malformations and severe brain damage. Low-level exposure has
been linked to cognitive deficits and emotional and behavior disorders.
• Among infectious diseases, rubella causes wide-ranging abnormalities. Babies with prenatally transmitted HIV rapidly develop AIDS, leading to brain damage
and early death. Cytomegalovirus, herpes simplex 2, and toxoplasmosis can also be devastating to the embryo and fetus. Describe the impact of additional
maternal factors on prenatal development.
• Prenatal malnutrition can lead to low birth weight, damage to the brain and other organs, and suppression of immune system development. Maternal vitamin–
mineral supplementation, including folic acid, can prevent prenatal and birth complications.
• Severe emotional stress is associated with many pregnancy complications and may permanently alter fetal neurological functioning. Providing mothers with
social support can reduce its impact.
• Aside from the risk of chromosomal abnormalities in older women, maternal age through the thirties is not a major cause of prenatal problems. Poor health and
environmental risks associated with poverty are the strongest predictors of pregnancy complications in both teenagers and older women.
Describe the three stages of childbirth, the baby’s adaptation to labor and delivery, and the newborn baby’s appearance. In the first stage of childbirth, contractions widen and thin the cervix. In the second stage, the mother feels an urge to push the baby through the birth canal. In the final stage,
the placenta is delivered. During labor, infants produce high levels of stress hormones, which help them withstand oxygen deprivation, clear the lungs for breathing, and
arouse them into alertness at birth.
Newborn babies have large heads and small bodies. TheApgar Scale is used to assess their physical condition at birth.
Approaches to Childbirth
Describe natural childbirth and home delivery, and explain the risks of using pain-relieving drugs during labor and delivery.
In natural, or prepared, childbirth, the expectant mother and a companion attend classes about labor and delivery, master relaxation and breathing techniques to counteract
pain, and prepare for coaching during childbirth. Social support from a partner, relative, or doula reduces the length of labor and the incidence of birth complications.
Home birth is as safe as hospital birth for healthy mothers who are assisted by a well-trained doctor or midwife.
The use of analgesics and anesthetics during childbirth can prolong labor and cause newborns to be withdrawn and irritable and to feed poorly.
What risks are associated with oxygen deprivation and with preterm and low birth weight infants, and what factors can help infants who survive a traumatic birth?
Inadequate oxygen supply during labor and delivery can damage the brain and other organs. Effects of even mild to moderate anoxia often persist, although many children
improve over time.
Low birth weight, a major cause of neonatal and infant mortality and wide-ranging developmental problems, is most common in infants born to poverty-stricken women and
in multiple births. Compared with preterm infants, whose weight is appropriate for time spent in the uterus, small-for-date infants usually have longer-lasting difficulties.
Interventions include providing special stimulation in the intensive care nursery and teaching parents how to care for and interact with their babies. Preterm infants in
stressed, low-income households need long-term, intensive intervention.
When infants experience birth trauma, a supportive family environment or relationships with other caring adults can help restore their growth. Even infants