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The goal of this chapter is to help every reader grasp the complex interaction between genotype and phenotype. This is not easy. For decades, in many nations, millions of scientists have struggled to understand this complexity. Each year brings advances in statistics and molecular analysis as well as new data to uncover various patterns, all resulting in hypotheses to be explored.

Here we examine only two complex traits: addiction and visual acuity, in two specific manifestations, alcohol use disorder and nearsightedness. These two illustrate a general truth: Every trait is affected by both nature and nurture, and understanding the progression from genotype to phenotype has many practical implications.

People everywhere discovered fermentation thousands of years ago. Alcohol has been declared illegal (as in the United States from 1919 to 1933) or considered sacred (as in many Judeo-Christian rituals). And for thousands of years, people have considered the abuse of alcohol and other drugs to be a moral weakness, a social scourge, or a personality defect. Those with alcohol use disorder have been jailed, jeered, or even burned at the stake.

We now know that inherited biochemistry affects alcohol metabolism. The disorder is as much biological as cultural. Genes create an addictive pull that can be overpowering, extremely weak, or somewhere in between, as each person’s biochemistry reacts to alcohol by causing sleep, nausea, aggression, joy, relaxation, forgetfulness, sexual urges, or tears.

There is no “alcoholic gene,” but alleles that make alcohol use disorder more likely have been identified on every chromosome except the Y (Epps & Holt, 2011). Although easy availability of alcohol may awaken genetic tendencies, punishing those with the genes does not stop addiction.

Metabolism allows some people to “hold their liquor” and therefore drink too much. Others (including many East Asians) sweat and become red-faced after just a few sips, a physiological response that may lead to a psychological reaction—embarrassment and abstinence. This inherited “flushing” tendency not only makes alcohol addiction rare, but it also improves metabolism (Kuwahara et al., 2014).

Sex (biological—either XX or XY) and gender (cultural) also affect the risk of the disorder. For biological reasons (body size, fat composition, metabolism), women become drunk on less alcohol than men, and women who are heavy drinkers double their risk of mortality compared to men (Wang et al., 2014).

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Many cultures encourage men to drink but not women (Chartier et al., 2014). For example, in Japan, both sexes have the same genes for metabolizing alcohol, yet women drink only about one-tenth as much as men. When women of Japanese ancestry live in the United States, their alcohol consumption increases.

Apparently, Americans of Asian descent try to adopt the drinking patterns of their new culture (Makimoto, 1998). In this and many other ways, drug addiction is a combination of nature and nurture, biology and culture.

All three factors—age, genes, and culture—affect vision as well. The effects of age are easy to notice. Newborns focus only on things within 1 to 3 feet of their eyes; vision improves steadily until about age 10; the eyeball changes shape at puberty, increasing nearsightedness (myopia), and again in middle age, decreasing myopia. The effects of genes and culture are more complex than age, as you will see.

HERITABLE? A study of British twins found that the Pax6 gene, which governs eye formation, has many alleles that make people somewhat nearsighted (Hammond et al., 2004). Heritability is almost 90 percent, which means that if one monozygotic British twin was myopic (nearsighted), the other twin is almost always myopic, too.

However, heritability indicates only how much of the variation in a particular trait within a particular population and in a particular context and era can be traced to genes. For example, the heritability of height is very high (about 95 percent) when children receive good medical care and nutrition, but it is low (about 20 percent) when children are malnourished. Thus, the 90 percent heritability of nearsightedness among the British may not apply elsewhere.

Indeed, it does not. In some African communities, vision heritability is close to zero because severe vitamin A deficiency makes sight depend much more on diet than on genes. If a child has no vitamin A, that child may become blind, even if the genotype is programmed for great vision.

Scientists and public health workers seek to develop and distribute a genetically modified strain of maize (the local staple) that is high in vitamin A. If they succeed, heritability will increase as vision improves (Fiedler et al., 2014).

What about children who are well nourished? Is their vision entirely inherited? Cross-cultural research suggests that it is not (Seppa, 2013).

A NEW EPIDEMIC “We are going down the path of having a myopia epidemic,” according to the head of a vision program in Australia (Sankaridurg, quoted in Dolgin, 2015, p. 276). The problem is particularly acute in China, where 90 percent of the teenagers and young adults are nearsighted compared to 10–20 percent 60 years ago (Dolgin, 2015).

The first published research on this phenomenon appeared in 1992, when scholars noticed that, in army-mandated medical exams of all 17-year-old males in Singapore, 26 percent were nearsighted in 1980 but 43 percent were nearsighted in 1990 (Tay et al., 1992). Between the early 1970s and the early 2000s, nearsightedness in the U.S. population increased from 25 to 42 percent (Vitale et al., 2009).

An article in the leading British medical journal suggests that, although genes are to blame for most cases of severe myopia, “any genetic differences may be small” for the common nearsightedness of Asian school children (I. Morgan et al., 2012, p. 1739). Nurture must somehow be involved. But how?

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One suggested culprit is indoor study. As Chapter 7 notes, contemporary East Asian children are amazingly proficient in math and science. As their developing eyes focus on their books, those with a genetic vulnerability to myopia may lose acuity for objects far away—which is exactly what nearsightedness means.

However, it does not seem that studying itself is the problem. American children who study less but spend many hours watching television or playing video games also have rising rates of nearsightedness.

Data from the United States on children playing sports have led ophthalmologists to suggest that the underlying cause of myopia among Americans is inadequate time spent in daylight (I. Morgan et al., 2012). An ophthalmologist comments that “we’re kind of a dim indoors people nowadays” (Mutti, quoted in Holden, 2010, p. 17).

One suggestion is that even sitting outside and reading could reduce nearsightedness. That seems too simplistic, according to some experts who believe it is not only outdoor light but also the eye adjusting to images at various distances that reduces myopia (Dolgin, 2015).

In any case, evidence from many places finds that in previous decades, genetically vulnerable children did not necessarily become nearsighted, but now they do. Perhaps if children spent more time outside playing, walking, or relaxing, fewer would need glasses.

Since genes affect every disorder, no one should be blamed or punished for inherited problems. However, knowing that genes never act in isolation allows prevention. For instance, if alcohol use disorder is in the genes, parents can keep alcohol out of their home, hoping their children become cognitively and socially mature before imbibing. If nearsightedness runs in the family, parents can play outdoors with their children every day.

Of course, abstention from alcohol and outdoor play are recommended for all children, as are dozens of other behaviors, such as flossing, saying “please,” getting enough sleep, eating vegetables, and writing thank-you notes. No parent can enforce every recommendation, but awareness of genetic risks can guide priorities.

Awareness of genetic risks alerts parents to set priorities and act on them, and it helps professionals advise pregnant women. Some recommendations should be routine (e.g., prenatal vitamins including folic acid) because it is impossible to know who is at risk. Others are tailored to the individual, such as weight gain for underweight women.

Care must be taken to keep pregnancy and birth from being an anxious time, filled with restrictions and fears about diet, diseases, drugs, and other possible dangers. Anxiety itself may reduce sleep, impair digestion, and raise blood pressure—all of which hinder development—which in turn may make birth complicated and postpartum depression likely.

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Indeed, stress reduces the chance of conception, increases the chance of prenatal damage, and slows down the birth process. A conclusion from every page of this chapter is that risks can be minimized if everyone—fathers as well as mothers, professionals as well as community members, grandparents as well as college students—does what is needed to ensure that newborns begin life eager and able to live 80 more healthy years.

WHAT HAVE YOU LEARNED?

Question 2.30

For thousands of years, people considered the abuse of alcohol and other drugs to be a moral weakness, a social scourge, or a personality defect. We now know that the disorder is as much biological as cultural. Genes create an addictive pull that can be overpowering, extremely weak, or somewhere in between, as each person’s biochemistry reacts to alcohol differently.

Question 2.31

For biological reasons (body size, fat composition, metabolism), women become drunk on less alcohol than men, and women who are heavy drinkers double their risk of mortality compared to men. Many cultures encourage men to drink but not women.

Question 2.32

One study of British twins found that the Pax6 gene, which governs eye formation, has many alleles that make people somewhat nearsighted. Heritability is almost 90 percent, which means that if one monozygotic British twin was myopic (nearsighted), the other twin is almost always myopic, too.

Question 2.33

Many feel that nearsightedness is more prevalent than it used to be because of the amount of time we spend inside. National data collected of children playing sports have led ophthalmologists to suggest that the underlying cause of myopia among Americans is inadequate time spent in daylight.

Question 2.34

Many East Asians sweat and become red-faced after just a few sips of alcohol, a physiological response that may lead to a psychological reaction—embarrassment and abstinence. This inherited “flushing” tendency not only makes alcohol addiction rare, but it also improves metabolism. Also, East Asian children are amazingly proficient in math and science. As their developing eyes focus on their books, those with a genetic vulnerability to myopia may lose acuity for objects far away.

Question 2.35

Newborns are only able to focus on things within one to three feet of their eyes. Vision improves steadily until about age 10. The eyeball changes shape at puberty, increasing myopia, and again in middle age, decreasing myopia.

Question 2.36

Awareness of genetic risks alerts parents to set priorities and act on them, and it helps professionals advise pregnant women. Care must be taken to keep pregnancy and birth from being an anxious time, filled with restrictions and fears about diet, diseases, drugs, and other possible dangers. Indeed, stress reduces the chance of conception, increases the chance of prenatal damage, and slows down the birth process.