Infants and Children in Context. Tara L. Kuther

share similarities, as they are often depicted in the media as identical in appearance, personality, and interests, yet they tend to differ in many unpredictable ways despite sharing parents and a home environment. Twins illustrate the complexity of how characteristics and tendencies are inherited. In this chapter, we discuss the process of genetic inheritance and principles that can help us to understand how members of a family—even twins—can share a great many similarities and also many differences.

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      Learning Objectives

       2.1 Discuss the genetic foundations of development.Video Activity 2.1: Twins

       2.2 Identify examples of genetic disorders and chromosomal abnormalities.

       2.3 Examine the choices available to prospective parents in having healthy children.Video Activity 2.1: Genetics and Pregnancy

       2.4 Summarize the interaction of heredity and environment, including behavioral genetics and the epigenetic framework.

      Chapter Contents

       Genetic Foundations of DevelopmentGeneticsCell ReproductionSex DeterminationGenes Shared by TwinsPatterns of Genetic InheritanceDominant–Recessive InheritanceIncomplete DominancePolygenic InheritanceGenomic Imprinting

       Chromosomal and Genetic ProblemsGenetic DisordersDominant–Recessive DisordersX-Linked DisordersChromosomal AbnormalitiesMutation

       Reproductive ChoicesGenetic CounselingAssisted Reproductive TechnologyReproductive TechnologyAdoptionPrenatal DiagnosisPrenatal Treatment of Genetic Disorders

       Heredity and EnvironmentBehavioral GeneticsMethods of Behavioral GeneticsGenetic Influences on Personal CharacteristicsGene–Environment InteractionsRange of ReactionCanalizationGene–Environment CorrelationsEpigenetic Influences on Development

      Genetic Foundations of Development

      What determines our traits, such as appearance, physical characteristics, health, and personality? We are born with a hereditary “blueprint” that influences our development. The following sections examine the role of heredity in our development.

      Genetics

      The human body is composed of trillions of units called cells, each with a nucleus containing 23 matching pairs of rod-shaped structures called chromosomes (Plomin, DeFries, Knopik, & Neiderhiser, 2013). Each chromosome holds the basic units of heredity, known as genes, composed of stretches of deoxyribonucleic acid (DNA), a complex molecule shaped like a twisted ladder or staircase. Genes carry the plan for creating all of the traits that organisms carry. It is estimated that 20,000 to 25,000 genes reside within the chromosomes, comprising the human genome and influencing all genetic characteristics (Finegold, 2017).

      Much of our genetic material is not unique to humans. Every species has a different genome, yet we share genes with all organisms, from bacteria to primates. We share 99% of our DNA with our closest genetic relative, the chimpanzee. There is even less genetic variation among humans. People around the world share 99.7% of their genes (Lewis, 2017). Although all humans share the same basic genome, every person has a slightly different code, making him or her genetically distinct from other humans.

      Cell Reproduction

      Most cells in the human body reproduce through a process known as mitosis in which DNA replicates itself, duplicating chromosomes, which ultimately form new cells with identical genetic material (Sadler, 2018). The process of mitosis accounts for the replication of all body cells. However, sex cells reproduce in a different way, through meiosis. First, the 46 chromosomes begin to replicate as in mitosis, duplicating themselves. But before the cell completes dividing, a critical process called crossing over takes place. The chromosome pairs align and DNA segments cross over, moving from one member of the pair to the other, essentially “mixing up” the DNA. Crossing over thereby creates unique combinations of genes (Sadler, 2018). The resulting cell consists of only 23 single, unpaired chromosomes. Known as gametes, these cells are specialized for sexual reproduction: sperm in males and ova in females. Ova and sperm join at fertilization to produce a fertilized egg, or zygote, with 46 chromosomes, forming 23 pairs with half from the biological mother and half from the biological father. Each gamete has a unique genetic profile, and it is estimated that individuals can produce millions of genetically different gametes (National Library of Medicine, 2019).

      Sex Determination

      Whether a zygote will develop into a male or female is controlled by the sex chromosomes. As shown in Figure 2.1, 22 of the 23 pairs of chromosomes are matched; they contain similar genes in almost identical positions and sequence, reflecting the distinct genetic blueprint of the biological mother and father. The 23rd pair of chromosomes are sex chromosomes that specify the biological sex of the individual. In females, sex chromosomes consist of two large X-shaped chromosomes (XX). Males’ sex chromosomes consist of one large X-shaped chromosome and one much smaller Y-shaped chromosome (XY).

      Figure 2.1 Chromosomes

      Because females have two X sex chromosomes, all ova contain one X sex chromosome. A male’s sex chromosome pair includes both X and Y chromosomes; therefore, one-half of the sperm males produce contains an X chromosome and one-half contains a Y. The Y chromosome contains genetic instructions that will cause the fetus to develop male reproductive organs. Thus, whether the fetus develops into a boy or girl is determined by which sperm fertilizes the ovum. If the ovum is fertilized by a Y sperm, a male fetus will develop, and if the ovum is fertilized by an X sperm, a female fetus will form, as shown in Figure 2.2. (The introduction of sex selection methods has become more widely available, and some parents may seek to choose the sex of their child. For more on this topic, see the Applying Developmental Science feature.)

      Description

      Figure 2.2 Sex Determination

      Genes Shared by Twins

      All biological siblings share the same parents, inheriting chromosomes from each. Despite this genetic similarity, siblings are often quite different from one another. Twins are siblings who share the same womb. Twins occur in about 1 out of every 33 births in the United States (Martin, Hamilton, Osterman, Driscoll, & Drake, 2018).

      The majority of naturally conceived twins are dizygotic (DZ) twins, or fraternal twins, conceived when a woman releases more than one ovum and each is fertilized by a different sperm. DZ twins share about one-half of their genes, and like other siblings, most fraternal twins differ in appearance, such as hair color, eye color, and height. In about half of fraternal twin pairs, one twin is a boy and the other a girl. DZ twins tend to run in families, suggesting a genetic component that controls the tendency for a woman to release more than one ovum each month. However, rates of DZ twins also increase with in vitro fertilization, maternal age, and each subsequent birth (Pison, Monden, & Smits, 2015; Umstad, Calais-Ferreira, Scurrah, Hall, & Craig, 2019).

      Monozygotic (MZ) twins, or identical twins, originate from the same zygote, sharing the same genotype, or set of genetic instructions for all