Genes, environment, and behavior

The way that you carry a conversation, respond to failure, form relationships with others, and generally behave is in part related to your genetics - but your world and life experiences also shape your attitudes and behaviors. This combination of your genetics and experiences ultimately form your personality, identity, and influences your behavior.
Consider this example: Jennifer and Karen are identical twins. If both girls have genetic information (genes) associated with obesity, does this mean that they will have no control over their weight? Or, if the girls are adopted by different families and have different life experiences, will their environment dictate how much they will ultimately weigh?

How do genes influence who you are and what you do?

Genes are instructions that dictate how a person’s body is made, in the same way that blueprints are instructions to build a house. Information from genes let the body know what characteristics a person will have, like if they will have hairy or hairless ears and/or a small or big chin. These instructions come from our parents; when their genes are mixed together, our genes are formed. This is why we often look like a mixture of our parents! I have my mother’s blue eyes, but my father’s stature. Almost everyone has different information in their genes, which makes sense given how much diversity there is in how people look and act.
Cartoon showing how a son acquires eye color and height from parents.
Now consider the exception - identical twins. Identical twins look exactly the same because each twin shares the same genes as their identical sibling. Why? When a mother is pregnant, the egg holds the mixture of genes from both the mother and father. Occasionally this egg splits into two eggs with the exact same mixture of genes. This results in two identical people who are similar to another in the way they look and behave.
Cells undergoing meiosis that result in twin cells.
Genes can carry instructions that can make it more likely for you to develop certain illnesses or conditions. For example, Jennifer and Karen both have genes associated with obesity. Their genes could tell their body to:
  • increase the size of their fat cells or dictate how they use fat in their body
  • release chemicals (like hormones) which control hunger and appetite
  • influence behavior as Jennifer and Karen interact with their environment. For example, if Karen begins to gain weight, she may seek out fewer opportunities to exercise because going to the gym makes her feel uncomfortable.

How do your life experiences influence your genes?

Suppose Jennifer is raised by wealthy parents who have access to the best, healthiest foods. Her parents cook nutritious meals like vegetable risotto and lentil soup and limit the amount of sugar, salt, and fat their daughter consumes. Jennifer learns to love fruits and vegetables, and doesn’t crave excessively salty or sweet foods. Her parents have a significant amount of time to play with her and teach her to live an active lifestyle. By eating nutritious foods and staying physically active, the genes increasing her chance of developing obesity are not expressed and she never develops obesity.
Karen however is raised by low-income parents who live in an area where fresh, healthy food is scarce and expensive. Because her parents can’t afford fruits and vegetables, Karen eats a lot of frozen, packaged meals and fast food, which are higher in sugar, fat, and salt. Her parents both work multiple jobs, and Karen spends her time alone in front of the television. Karen’s eating and physical activity habits enhance the expression of her genes for obesity.
Flowchart of two twins demonstrating how their environment (the food they eat) influences their genetics which determines whether they will be obese.
The characteristics (physical traits and behaviors) that you are born with and what you experience throughout your life are both important. Your characteristics can impact your experiences and your experiences can impact your characteristics. Karen and Jennifer’s life experiences influenced when and how their genes were expressed. The relationship between your genes and behavior can change over time as you have new experiences. In some situations, genes play a larger role in determining your behavior; in other situations, environment plays a larger role in influencing your behavior. If you had a whole different set of experiences over your lifetime your genes may be expressed in different ways, and you may behave differently than you do now. If Jennifer and Karen had grown up in the same environment, their health behaviors would probably be more similar to one another.

Is there a way to tell how much of an influence genes have on a behavior?

Scientists can look at the influence of genes on behavior by using a mathematical formula called a heritability estimate. Heritability estimates give information about how much of an impact genes have on a behavior in a certain environment. Think about blood type as an example - in your group of friends, there is probably some variation in your blood types. If differences in blood type are mostly influenced by genes then the heritability estimate would reflect that. Heritability estimates can range from 0 to 1; when the estimate is higher (closer to 1), this means that genes have a larger influence on the behavior of interest, as it would be with blood type. When the estimate is lower (closer to 0), it reflects a larger impact of the environment on the behavior. To study heritability, scientists use information from identical twins that were separated at birth, like Karen and Jennifer. They do this because the genetic material of identical twins is almost exactly the same, which makes it easier to determine the relative influence of the environment.
Diagram showing blood type heritability from parents to children.
To better understand heritability estimates, consider Karen and Jennifer again. In Karen’s town where healthy food is scarce - many of her neighbors do not get enough to eat and are malnourished. The athletic ability of Karen and her neighbors is decreased and the heritability of athleticism is closer to 0, because their experiences in a financially poor environment have inhibited genes associated with athleticism. On the other hand, Jennifer and her neighbors eat plenty of healthy foods are well nourished. If we look at the athletic ability of Jennifer’s neighbors, the heritability estimate is closer to 1 because their experiences in financially well off environment have enabled their athleticism genes to be expressed.
A heritability estimate doesn’t tell us about a person’s chances for inheriting a certain genetic trait - Jennifer doesn’t have a greater chance of being athletic because she lives in a well off neighborhood. Heritability estimates give us more information about groups of people, like Jennifer and her neighbors. The varying levels of athleticism among the people in her neighborhood can be explained by differences in their genetic makeup and differences in their life experiences. When we calculate a heritability estimate, it gages how much of the differences can be explained by each factor.

Consider the following:

  • Why do you think heritability estimates cannot be generalized, or applied to different populations? Consider exactly what a heritability estimate measures - the relative influence of genes and the environment. If there was a change in the environment, the heritability estimate would change as well; an estimate from Jennifer’s neighborhood would not be applicable in Karen’s neighborhood. Each estimate is very specific to one group of individuals and their environment, which means that it could not be generalized. However, we can look at large groups of people and develop a range of estimates to tell us more about a particular trait of interest. The range allows for interpersonal and small group differences that are influenced by specific environments, but still gives us important information about the differences in people’s traits.
  • The interactions between your genes and your environment are especially important during your early development. For example, exposure to toxins during and immediately after pregnancy can produce lasting effects on a baby’s health - children exposed to pesticides at a young age have a higher risk of developing mental health problems later in life.
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