Child Development From Infancy to Adolescence. Laura E. Levine
which behaviors, traits, and abilities are linked to our genetic inheritance. As scientists have learned more about specific genes and how they function, it has become clear that gene expression is affected by the environment and the experiences that the individual with those genes has, especially early in life.
How the Environment Shapes Gene Expression
One of the major findings in recent research on genes is that the environment can influence gene expression. The concept of environmental influence on gene expression is described as canalization and one mechanism by which this happens is called epigenesis.
Canalization
There is considerable variability in how strongly genes affect different traits or characteristics. The degree to which the expression of a gene is influenced by the environment is captured in the concept of canalization proposed by Conrad Hal Waddington (1942). Imagine yourself looking down from the top of a steep hill. You see that water and weather have carved gullies (or canals) into the hillside. Some are deep and narrow, and others shallow and wide. If you begin rolling balls down the hillside, some will travel down the deep, narrow pathways and end up in about the same place every time, but others will travel down the shallow, wide pathways and may end up on one side or the other of the gully by the time they reach the bottom of the hill (see Figure 3.8).
Canalization: The degree to which the expression of a gene is influenced by the environment.
Figure 3.8 Canalization.
In a similar way, we can think of some traits as being deeply canalized. Genes for deeply canalized traits have a self-righting tendency that produces the expected developmental outcome under all but the most extreme environmental conditions (Black, Hess, & Berenson-Howard, 2000). For example, across a wide spectrum of environmental conditions, almost all infants reach the early motor milestones, like sitting up and walking. In contrast, a trait such as intelligence is much more variable in its outcome. This genetic pathway is less constrained or less deeply canalized, so it is more influenced by the landscape of the child’s environment. Children raised in the relatively enriched environment of middle- and upper-income families are likely to express the levels of intelligence provided by their genetic inheritance, whether that is high, low, or average. However, genetic inheritance plays a smaller role for children raised in low-income families because scarcer high-quality educational opportunities and a host of other risks in their environment limit their development (Blair & Raver, 2012; Tucker-Drob & Bates, 2016).
Children from low-income families can certainly overcome these limitations and express the full range of possibilities provided by their genetic inheritance, but this is more likely to happen if they are provided with environmental resources such as good educational opportunities and healthcare. The concept of canalization thus gives us insight into the complex interaction between our genetic endowment and the influence of the environment. We next describe one mechanism by which canalization occurs: behavioral epigenetics.
Environmental influence on gene expression. In Arlington, VA, a trolley was used to take children from low-income families to their community libraries during the summer. Although poverty can restrict the expression of genes that contribute to intelligence and academic achievement, this range can be expanded with educational opportunities such as these.
The Washington Post / Contributor via Getty images
Behavioral Epigenetics
What determines the way individual genes are influenced by environmental experiences? One answer comes from behavioral epigenetics, which is the study of the chemical reactions that activate and deactivate parts of the genetic material of an organism, as well as the factors that influence these chemical reactions. Behavioral epigenetics has been described as the bridge between nature and nurture (Zhang & Meaney, 2010). All living things have chemical tags that can turn a gene’s activity on or off (see Figure 3.9). While the structure of the genes remains the same, the way each gene is expressed may be very different depending on where and when these chemical tags are “turned on” or expressed, and this can be influenced by events or circumstances in the individual’s environment (Szyf & Bick, 2013).
Behavioral epigenetics: The study of the chemical reactions that activate and deactivate parts of the genetic material of an organism, as well as the factors that influence these chemical reactions.
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The experiences you have in your life can change the structure of your genes. False
Figure 3.9 Epigenetics.
Source: Courtesy of the National Human Genome Research Institute.
One example of how behavioral epigenetics works has been demonstrated by Michael Meaney, a researcher at McGill University, who studied rat mothers and their offspring. He and his colleagues found that rat babies reared by mothers who ignored them and did not touch them were more fearful and stressed by environmental events later in their lives, shown both by the babies’ behavior and by the levels of stress hormones they produced. These researchers were able to link this behavior with a particular gene that was active in babies reared by nurturing mothers but which had been “turned off” in the neglected babies.
To be sure that this effect was due to the mother’s behavior and not to her genes, the researchers switched babies between nurturing and non-nurturing mothers at birth, and the results were the same. In those babies who had been reared by non-nurturing mothers, the gene had been “turned off” by these early experiences even though their biological mothers were very nurturing (Diorio & Meaney, 2007). In an evolutionary sense, it appears that baby rats that do not experience adequate mothering “reprogram” their genes. The result of this reprogramming is that the babies respond more quickly to stress. The evolutionary advantage is that their increased responsiveness enables them to respond quickly when facing danger, rather than waiting for unresponsive mothers to protect them.
Similar results have been found in humans. Turecki and Meaney (2016) reviewed 40 studies (13 animal studies and 27 human studies) on the relation between early stress and later epigenetic markers in the genome. The human studies examined a history of trauma in early childhood, including child abuse and parental death. They found strong evidence for the effect of early stress on changes in the epigenome, which consists of all the chemical tags that can turn genes on and off, in parts of the brain that control reactions to stress. Individuals with these changes are more reactive even to lower levels of stress and more prone to depression and anxiety as a result.
Epigenetic response to stress. Highly stressful events early in life, such as the death of a parent, can result in changes in the individual’s epigenome that contribute to a higher incidence of depression and anxiety later in life.
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