/n Biological Foundations Heredity, Prenatal Development, and Birth 2 2.1 2.2 2.3 2.4 In the Beginning: 23 Pairs of Chromosomes Mechanisms of Heredity Heredity, Environment, and Development REAL PEOPLE: Applying Human Development: Ben and Matt Pick Their Niches From Conception to Birth Period of the Zygote (Weeks 1 and 2) Period of the Embryo (Weeks 3-8) Period of the Fetus (Weeks 9-38) Influences on Prenatal Development General Risk Factors Teratogens: Drugs, Diseases, and Environmental Hazards How Teratogens Influence Prenatal Development Prenatal Diagnosis and Treatment Labor and Delivery Stages of Labor and Delivery Approaches to Childbirth Adjusting to Parenthood SPOTLIGHT on Research: Links Between Maternal Depression and Children's Behavior Problems Birth Complications Infant Mortality LINKING RESEARCH TO LIFE: Conception in the 21st Century SUMMARY KEY TERMS f you ask parents to name the most memorable experiences of their lives, many immediately mention events associated with the birth of their children. For parents, pregnancy and birth evoke awe and wonder. The period before birth is the foundation for all human development and the focus of this chapter. In the first section, you'll see the merger of heredity material from egg and sperm cells that marks the onset of preg- nancy. The second section of the chapter traces the events that transform sperm and egg into a living, breathing human. In the third section, we talk about some problems that can occur during predevelopment. The last section focuses on labor, delivery, and the newborn baby. : 33 2.1 In the Beginning: 23 Pairs of Chromosomes Learning Objectives What are chromosomes and genes? How do they carry hereditary information? ■ How is children's heredity influenced by their environment? chromosomes threadlike structures in the nuclei of cells that contain genetic material Leslie and Glenn are excited at the thought of starting a family. But they're also nervous because Leslie's grandfather had sickle-cell disease and died when he was a young adult. Leslie is terrified that her baby may inherit the disease that killed her grandfather. She and Glenn wish that someone could reassure them that their baby will be okay. How can we reassure Leslie and Glenn? When a person has sickle-cell disease, the red blood cells that carry oxygen are long and curved like a sickle. These stiff, misshapen cells cannot pass through small capillaries, so oxygen cannot reach all parts of the body. The trapped sickle cells also block the way of white blood cells that are the body's natural defense against bacteria. As a result, many people with sickle-cell disease are often tired, may experience acute pain for hours or days, and are prone to infections. About 10% of people with the disease die by age 20 and 50% die by age 50 (Kumar et al., 2010). Sickle- cell disease is inherited, and because Leslie's grandfather had the disorder, it runs in her family. Will Leslie's baby inherit the disease? To answer that question, we need to exam- ine the mechanisms of heredity. Mechanisms of Heredity At conception, egg and sperm create a new organism that incorporates characteristics of each parent. Each egg and sperm cell has 23 chromosomes, threadlike structures that con- tain genetic material. When a sperm penetrates an egg, their chromosomes combine to Red blood cells carry oxygen throughout the body. YAKOBCHUK VASYL/Shutterstock.com 34 | Part 1: PRENATAL DEVELOPMENT, INFANCY, AND EARLY CHILDHOOD Sickle-shaped blood cells associated with sickle-cell disease cannot pass through the body's smallest blood vessels. Science Photo/Custom Medical Stock Photo # # * # * # * 6 08 魚魚 13 10 11 12 88 14 15 16 17 18 歌 19 20 21 22 X Y Humans have 23 pairs of chromosomes, including 22 pairs of autosomes and 1 pair of sex chromosomes. autosomes the first 22 pairs of chromosomes sex chromosomes the 23rd pair of chromosomes, which determines the sex of the child deoxyribonucleic acid (DNA) the molecule that composes one chromosome, making it the biochemical basis of heredity gene a group of compounds that provides a specific set of biochemical instructions genotype the complete set of genes that makes up a person's heredity phenotype physical, behavioral, and psychological features that result from the interaction between an individual's genes and the environment alleles variations of genes produce 23 pairs of chromosomes. The first 22 pairs of chromosomes are called auto- somes. The 23rd pair determines the sex of the child, so these are called sex chromosomes. When the 23rd pair consists of an X and a Y chromosome, the result is a boy; two X chromosomes produce a girl. Each chromosome consists of one molecule of deoxyribonucleic acid-DNA for short- that is the biochemical basis of heredity. To understand the structure of DNA, imagine four colors of beads placed on two strings. The strings complement each other: Wherever a red bead appears on one string, a blue bead appears on the other; wherever a green bead appears on one string, a yellow one appears on the other. DNA is organized this way, except that the four colors of beads are four chemical compounds known as nucleotide bases: adenine, thymine, guanine, and cytosine. The strings, which are made up of phosphates and sugars, wrap around each other to create the double helix shown in Figure 2.1. The order of the chemical "beads" is a code that causes cells to create specific amino acids, proteins, and enzymes-important biological building blocks. For example, three consecutive thymine “beads" is the instruction to create the amino acid phenylalanine. Each group of compounds that provides a set of biochemical instructions is a gene. Thus, genes are the functional units of heredity because they determine the production of chemical substances that are the basis for all human characteristics and abilities. A person's 46 chromosomes include roughly 20,500 genes. Most genes are the same for all people-fewer than 1% of genes cause differences among people (Human Genome Project, 2003). Through biochemical instructions that are coded in DNA, genes regulate the development of all human characteristics and abilities. The complete set of genes that makes up a person's heredity is the person's genotype. Genetic instructions, with environmental influ- ences, produce a phenotype, an individual's physical, behavioral, and psychological features. How do genetic instructions produce the misshapen red blood cells of sickle-cell disease? Genes come in different forms that are known as alleles. For example, with Chapter 2: BIOLOGICAL FOUNDATIONS | 35 Biophoto Associates/Getty Images Strands of phosphates and sugars red blood cells, one allele has instructions for normal red blood cells; another has instruc- tions for sickle-shaped red blood cells. The alleles in the pair of chromosomes are some- times the same, which is known as being homozygous. The alleles sometimes differ, which is known as being heterozygous. Leslie's baby would be homozygous if it had two alleles for normal cells or two alleles for sickle-shaped cells. The baby would be heterozygous if it had one allele of each type. How does a genotype produce a phenotype? When a person is homozygous, the alleles are the same and have the same chemical instructions, which typically yield the phenotype. If Leslie's baby had two alleles for normal red blood cells, then the baby would be almost guaranteed to have normal cells. If, instead, the baby had two alleles for sickle- shaped cells, then it would almost certainly suffer from the disease. When a person is heterozygous, the process is more complex. Often one allele is dominant, which means that its chemical instructions are followed while those of the other, recessive allele are ignored. In sickle-cell disease, the allele for normal cells is dominant and the allele for sickle-shaped cells is recessive. This is good news for Leslie: As long as either she or Glenn contributes the allele for normal red blood cells, their baby will not develop sickle-cell disease. ▸ Figure 2.2 summarizes what you've learned about sickle-cell disease: A denotes the allele for normal blood cells, and a denotes the allele for sickle-shaped cells. Only if Leslie's baby inherits two recessive alleles for sickle-shaped cells is it likely to develop sickle-cell disease. But this is unlikely: No one in Glenn's family has had sickle-cell dis- ease, so he probably has two alleles for normal blood cells. The simple genetic mechanism responsible for sickle-cell disease-a single gene pair with one dominant allele and one recessive allele-is also responsible for many other common traits, as shown in Table 2.1. In each instance, individuals with the recessive DAI X X P O C Nucleotide bases (A = Adenine, T = Thymine, G = Guanine, C = Cytosine) ▸ Figure 2.1 DNA is organized in a double helix, with strands of phosphates and sugars linked by chemical compounds (adenine, thymine, guanine, cytosine). homozygous alleles in a pair of chromosomes that are the same heterozygous alleles in a pair of chromosomes that Father Mother A 口 differ from each other Aa A a O a dominant the allele whose chemical instructions are followed Normal child Sickle-cell trait Sickle-cell trait Sickle-cell disease recessive the allele whose instructions are ignored in the presence of a dominant allele ▸ Figure 2.2 In single-gene inheritance, a heterozygous father and a heterozygous mother can have a healthy child, a child with the sickle-cell trait, or a child with sickle-cell disease. 36 Part 1: PRENATAL DEVELOPMENT, INFANCY, AND EARLY CHILDHOOD Human Development in Action Imagine that you are a licensed nurse practitioner. Your patient is a young woman whose father has cystic fibrosis, an inherited disease in which the lungs fill with mucus, making breathing difficult. Cystic fibrosis is caused by a recessive gene, and your patient wonders whether her baby will inherit the disease. What would you tell her? Common Phenotypes Associated with Single Pairs of Genes TABLE 2.1 Dominant Phenotype Curly hair Normal hair Dark hair Thick lips Cheek dimples Normal hearing Normal vision Farsightedness Normal color vision Type A blood Type B blood Rh-positive blood Source: McKusick, 1995. Recessive Phenotype Straight hair Pattern baldness (men) Blond hair Thin lips No dimples Some types of deafness Nearsightedness Normal vision Red-green color blindness Type O blood Type O blood Rh-negative blood Children with Down syndrome typically have upward slanting eyes with a fold over the eyelid, a flattened facial profile, and a smaller-than-average nose and mouth. phenotype have two recessive alleles, one from each parent. Individ- uals with the dominant phenotype have at least one dominant allele. However, sometimes individuals do not receive 46 chromo- somes because a sperm or egg cell has more or fewer than the usual 23 chromosomes. In this case, development is always disturbed. The best example is Down syndrome, which is usually caused by an extra 21st chromosome. People with Down syndrome have almond- shaped eyes and a fold over the eyelid. Their head, neck, and nose are usually smaller than normal. During the first months of life, development of babies with Down syndrome seems normal. There- after, their mental and behavioral development begins to lag behind the average child's. For example, a child with Down syndrome might first sit up without help around 1 year, walk at 2 years, and talk at 3 years, reaching these developmental milestones months or even years behind children without Down syndrome. By childhood, most aspects of cognitive and social development are seriously delayed. iStockphoto.com/Diloute The odds that a woman will bear a child with Down syndrome increase markedly as she gets older, from about 1 in 1,000 during her 20s to 1 in 50 during her 40s. Why? A woman's eggs have been in her ovaries since her own prenatal development. Eggs may deteriorate over time due to aging, and older women may have greater exposure to hazards in the environment that may damage eggs (e.g., X-rays). Abnormal sex chromosomes can also disrupt development. ■ Table 2.2 lists four disorders associated with atypical numbers of X and Y chromosomes. There are no dis- orders consisting solely of Y chromosomes. The presence of an X chromosome appears to be necessary for life. Heredity, Environment, and Development Traits controlled by single genes are usually either-or phenotypes. A person has either normal color vision or red-green color blindness; a person's blood either clots normally or does not. In contrast, most important behavioral and psychological characteristics are Chapter 2: BIOLOGICAL FOUNDATIONS | 37