Lifespan Development. Tara L. Kuther

yellow fur, are extremely obese, are shaped much like a pincushion, and are prone to diabetes and cancer. When agouti mice breed, most of the offspring are identical to the parents—yellow, obese, and susceptible to life-shortening disease. However, a groundbreaking study showed that yellow agouti mice can produce offspring that look very different (Waterland & Jirtle, 2003). The mice in the photo above both carry the agouti gene, yet they look very different; the brown mouse is slender, is lean, and has a low risk of developing diabetes and cancer, living well into old age.

      Why are these mice so different? Epigenetics. The epigenome carries the instructions that determine what each cell in your body will become, whether heart cell, muscle cell, or brain cell, for example. Those instructions are carried out by turning genes on and off.

      In the case of the yellow and brown mice, the phenotype of the brown mice has been altered, but the DNA remains the same. Both carry the agouti gene, but in the yellow mouse, the agouti gene is turned on all the time. In the brown mouse, it is turned off. In 2003, Waterland and Jertle discovered that the agouti female’s diet can determine her offspring’s phenotype. In this study, female mice were fed foods containing chemicals that attach to a gene and turn it off. These chemical clusters are found in many foods such as onions, garlic, beets, soy, and the nutrients in prenatal vitamins. Yellow agouti mothers fed extra nutrients passed along the agouti gene to their offspring, but it was turned off. The mice looked radically different from them (brown) and were healthier (lean, not susceptible to disease) even though they carried the same genes.

      Another example supports the finding that the prenatal environment can alter the epigenome and influence the lifelong characteristics of offspring. Pregnant mice were exposed to a chemical (bisphenol-A or BPA, found in certain plastics). When female mice were fed BPA 2 weeks prior to conception, the number of offspring with the yellow obese coat color signaling an activated agouti gene increased (Dolinoy, 2008). When the pregnant mice were exposed to BPA plus nutritional supplementation (folic acid and an ingredient found in soy products), the offspring tended to be slender and have brown coats, signaling that the agouti gene was turned off. These findings suggest that the prenatal environment can influence the epigenome and thereby influence how genes are expressed—and that nutrition has the potential to buffer harm.

      The most surprising finding emerging from studies of epigenetics, however, is that the epigenome can be influenced by the environment before birth and can be passed by males and females from one generation to the next without changing the DNA itself (Soubry, Hoyo, Jirtle, & Murphy, 2014; Szyf, 2015). This means that what you eat and do today could affect the epigenom—the development, characteristics, and health—of your children, grandchildren, and great-grandchildren (Bale, 2015; Vanhees, Vonhögen, van Schooten, & Godschalk, 2014).

      What Do You Think?

      1 Much of the research on epigenetics examines animals, but there is a growing body of work studying humans. In what ways, if any, might you expect research findings based on people to differ from the findings of animal research, described previously? Explain.

      2 What might you do to “care for” your epigenome? Identify activities and behaviors that you think might affect the health of your genome today and tomorrow.

      Thinking in Context 2.4

      To answer the following questions, begin by thinking about how your own development reflects interactions among your genes and sociocultural context. Then, describe a skill, ability, or hobby in which you excel.

      1 How might a passive gene–environment correlation account for this ability? For example, in what ways has the context in which you were raised shaped this ability?

      2 In what ways might this ability be influenced by an evocative genetic environment correlation?

      3 Provide an example of how this ability might reflect an active gene–environment correlation.

      4 Which genetic environment correlation do you think most accurately accounts for your skill, ability, or hobby?

      5 How might you apply the epigenetic framework to account for your ability?

      Apply Your Knowledge

      Strapped in and buckled in the rear seat of her mother’s bicycle, 1-year-old Jenna patted her helmet as her mother zoomed along the bike path to the beach. There she giggled and kicked her legs as her mother whooshed her through the water. As a child, Jenna loved to be outside and especially in the water. Jenna practiced swimming at the local YMCA nearly every day and became quite skilled. Jenna’s proud mother encouraged her daughter’s athleticism by enrolling her in swim classes. As a teenager, Jenna decided that if she were going to become an exceptional swimmer, she would have to go to a summer swimming camp. She researched camps and asked her mother if she could attend. Jenna further honed her skills as a swimmer and won a college scholarship for swimming.

      Many years later, Jenna was surprised to learn that she had a twin sister, Tasha. Separated at birth, Jenna and Tasha became aware of each other in their early 40s. Jenna was stunned yet couldn’t wait to meet her twin sister. Upon meeting, Jenna and Tasha were surprised to find that they were not exactly the same. Whereas Jenna was athletic and lithe, Tasha was more sedentary and substantially heavier than Jenna. Unlike Jenna, Tasha grew up in a home far from the beach and with little access to outdoor activities. Instead, Tasha’s interest was writing. As a child, she’d write stories and share them with others. She sought out opportunities to write and chose a college with an exceptional writing program. Both Jenna and Tasha excelled in college, as they did throughout their education, and earned nearly identical scores on the SAT.

      Jenna and Tasha look very similar. Even the most casual observer could easily tell that they are sisters as both have blond hair, blue eyes, and a similar facial structure. Tasha’s skin, however, is more fair and unlined. Jenna’s face is sprinkled with freckles and darker spots formed after many days spent swimming outside. Jenna and Tasha both are allergic to peanuts, and they both take medication for high blood pressure. The more that Jenna and Tasha get to know each other, the more similarities they find.

      1 Considering Jenna and Tasha, provide examples of three types of gene–environment correlations: passive, evocative, and active.

      2 Do you think Jenna and Tasha are monozygotic or dizygotic twins? Why or why not?

      3 What role might epigenetic influences play in determining Jenna and Tasha’s development?

      Descriptions of Images and Figures

      Back to Figure

       Mitosis:

      1. Interphase. Chromosomes replicate.

      2. Prophase. Chromosomes become visible.

      3. Metaphase. Chromosomes line up individually at metaphase plate.

      4. Anaphase. Sister chromatids separate, creating two identical daughter cells with 2n chromosomes.

       Meiosis:

       Meiosis I:

      1. Interphase. Chromosomes replicate.

      2. Prophase I. Homologous chromosomes undergo synapsis and crossing over occurs.

      3. Metaphase I. Chromosomes line up by homologous pairs at metaphase plate.

      4. Anaphase I. Homologs separate, creating daughter cells of Meiosis I.

       Meiosis II:

      5. Anaphase II. Sister chromatids separate, creating daughter cells of Meiosis II. Each daughter cell has 1n chromosome.

      Back to Figure

      The father has an X and a Y chromosome. The mother has two X chromosomes. Each son has one X