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Genetic Foundations of Development Genetic influences on behavior evolved over time and across many species. Our many traits and characteristics that are genetically influenced have a long evolutionary history that is retained in our DNA. In other Page 39 words, our DNA is not just inherited from

our parents; it’s also what we’ve inherited as a species from the species that came before our own. Let’s take a closer look at DNA and its role in human development. How are characteristics that suit a species for survival transmitted from one generation to the next? Darwin did not know the answer to this question because genes and the principles of genetics had not yet been discovered. Each of us carries a human “genetic code” that we inherited from our parents. Because a fertilized egg carries this human code, a fertilized human egg cannot grow into an egret, eagle, or elephant.

Each of us began life as a single cell weighing about one twenty-millionth of an ounce. This tiny piece of matter housed our entire genetic code—instructions that orchestrated growth from that single cell to a person made of trillions of cells, each containing a replica of the original code. That code is carried by our genes. What are genes and what do they do? For the answer, we need to look into our cells.

The nucleus of each human cell contains chromosomes, which are threadlike structures made up of deoxyribonucleic acid, or DNA. DNA is a complex molecule that has a double helix shape, like a spiral staircase, and contains genetic information. Genes, the units of hereditary information, are short segments of DNA, as you can see in Figure 2. They help cells to reproduce themselves and to assemble proteins. Proteins, in turn, are the building blocks of cells as well as the regulators that direct the body’s processes (Cowan, 2015; Goodenough & McGuire, 2017).

Figure 2 Cells, Chromosomes, DNA,

and Genes

(Top) The body contains trillions of cells. Each cell contains a central structure, the

nucleus. (Middle) Chromosomes are threadlike structures located in the nucleus of the cell.

Chromosomes are composed of DNA. (Bottom) DNA has the structure of a spiral staircase. A gene is a segment of DNA.

Each gene has its own designated place on a particular chromosome. Today, there is a great deal of enthusiasm about efforts to discover the specific locations of genes that are linked to certain functions and developmental outcomes (Johnson, 2017; Sutphin & Korstanje, 2016). An important step in this direction was taken when the Human Genome Project and the Celera Corporation completed a preliminary map of the human genome—the complete set of developmental instructions for creating proteins that initiate the making of a human organism (Brooker, 2015). Completion of the Human Genome Project has led to use of the genome-wide association method to identify genetic variations linked to a particular disease, such as cancer, cardiovascular disease, or Alzheimer disease (Cho & Suh, 2016; Hou & others, 2016). To conduct a genome-wide association study, researchers obtain DNA from individuals who have the disease and those who don’t have it. Then, each participant’s complete set of DNA, or genome, is purified from the blood or other cells and scanned on machines to determine markers of genetic variation. If the genetic variations occur more frequently in people who have the disease than in those who don’t have it, the variations point to the region in the human genome where the disease-causing problem exists. Genome- wide association studies have recently been conducted for childhood obesity (Zandona & others, 2016); cancer (Johnson & others, 2016); cardiovascular disease (Schick & others, 2016); depression (Knowles & others, 2016; Nho & others, 2015); suicide (Sokolowski, Wasserman, & Wasserman, 2016); glaucoma (Bailey & others, 2016); and Alzheimer disease (Chauhan & others, 2015; Ramos Dos Santos & others, 2016). One of the big surprises of the Human Genome Project was a report indicating that humans have only about 30,000 genes (U.S. Department of Energy, 2001). More recently, the number of human genes has been revised further downward, to approximately 20,700 (Flicek & others, 2013). Further recent analysis proposes that humans may actually have Page 40fewer than 20,000 protein-producing genes (Ezkurida & others, 2014). Scientists had thought that humans had as many as 100,000 or more genes. They had also believed that each gene programmed just one protein. In fact, humans appear to have far more proteins than they have genes, so there cannot be a one-to-one correspondence between genes and proteins (Commoner, 2002). Each gene is not translated, in automaton-like fashion, into one and only one protein. A gene does not act independently, as developmental psychologist David Moore (2001) emphasized by titling his book The Dependent Gene. Rather than being a group of independent genes, the human genome consists of many genes that collaborate both with each other and with nongenetic factors inside and outside the body. The collaboration operates at many points. For example, the cellular “machinery” mixes, matches, and links small pieces of DNA to reproduce the genes, and that machinery is influenced by what is going on around it (Moore, 2015). Whether a gene is turned “on”—that is, working to assemble proteins—is also a matter of collaboration. The activity of genes (genetic expression) is affected by their environment (Gottlieb, 2007; Moore, 2015). For example, hormones that circulate in the blood make

their way into the cell, where they can turn genes “on” and “off.” And the flow of hormones can be affected by environmental conditions such as light, day length, nutrition, and behavior. Numerous studies have shown that external events outside of the original cell and the person, as well as events inside the cell, can excite or inhibit gene expression (Lickliter & Honeycutt, 2015; Moore, 2015). Recent research has documented that factors such as stress, exercise, nutrition, respiration, radiation, temperature, and sleep can influence gene expression (Craft & others, 2014; Dedon & Begley, 2014; Donnelly & Storchova, 2015; Giles & others, 2016; Lindholm & others, 2014; Ma & others, 2015; McInnis & others, 2015; Mychasiuk, Muhammad, & Kolb, 2016; Turecki & Meaney, 2016). For example, one study revealed that an increase in the concentration of stress hormones such as cortisol produced a fivefold increase in DNA damage (Flint & others, 2007). Another study also found that exposure to radiation changed the rate of DNA synthesis in cells (Lee & others, 2011). And recent research indicates that sleep deprivation can affect gene expression in negative ways such as increased inflammation, expression of stress-related genes, and impairment of protein functioning (da Costa Souza & Ribeiro, 2015).

Genes and Chromosomes Genes are not only collaborative; they are enduring. How do they get passed from generation to generation and end up in all of the trillion cells in the body? Three processes are central to this story: mitosis, meiosis, and fertilization.

Mitosis, Meiosis, and Fertilization All cells in your body, except the sperm and egg, have 46 chromosomes arranged in 23 pairs. These cells reproduce through a process called mitosis.During mitosis, the cell’s nucleus—including the chromosomes—duplicates itself and the cell divides. Two new cells are formed, each containing the same DNA as the original cell, arranged in the same 23 pairs of chromosomes. However, a different type of cell division—meiosis—forms eggs and sperm (which also are called gametes). During meiosis, a cell of the testes (in men) or ovaries (in women) duplicates its chromosomes but then divides twice, thus forming four cells, each of which has only half of the genetic material of the parent cell (Johnson, 2017). By the end of meiosis, each egg or sperm has 23 unpaired chromosomes. During fertilization, an egg and a sperm fuse to create a single cell, called a zygote. In the zygote, the 23 unpaired chromosomes from the egg and the 23 unpaired chromosomes from the sperm combine to form one set of 23 paired chromosomes—one chromosome of each pair from the mother’s egg and the other from the father’s sperm. In this manner, each parent contributes half of the offspring’s genetic material. Page 41

Figure 3 shows 23 paired chromosomes of a male and a female. The members of each pair of chromosomes are both similar and different: Each chromosome in the pair contains varying forms of the same genes, at the same location on the chromosome. A gene that influences hair color, for example, is located on both members of one pair of chromosomes, at the same location on each. However, one of those chromosomes might

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