Genetics and Behavior

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Photo by: Zsolt Biczó

Genetics (je-NE-tiks) is the study of how traits are passed from parents to children. Behavior is a person's observable activity. The study of how genetics affects behavior is called behavioral genetics.

Jennifer's Story

At 13 years old, Jennifer is fit and trim. Her mother, however, has been told by her doctor that she is morbidly obese ^* and must keep to a strict diet and exercise plan to control her weight. Jennifer notices that her aunt and her grandmother are also severely overweight. Jennifer is not sure if her family members are overweight because they do not exercise much and like to eat fatty foods or if there is a problem with the way that their bodies process food.

In school Jennifer has learned that children inherit many traits from their parents. She wonders if she is destined to have a weight problem later in life, since so many of the women in her family do. She decides to ask her doctor if there are diet and exercise programs she can start now to help her avoid having her mother's problem when she gets older.

What Is Behavioral Genetics?

Genetics is the study of how traits are passed from parent to child. The information that children get from their parents is contained in chemical packets of information called genes. These genes make up larger units called chromosomes (KRO-mo-somz). Humans have 46 chromosomes; 23 come from the father, and the other 23 come from the mother. The human genome is the term for all the genetic information contained in humans.

Variations in human behavior are seen every day. Often people are kind and thoughtful. Sometimes people behave in ways that are considered antisocial ^* or hurtful to themselves. Behavioral genetics is a branch of science that tries to find out what role inherited factors play in how people act and what kinds of genes may lead to different patterns of behavior.

Environmental factors also can influence human behavior. Behavioral geneticists work to discover how much of people's behavior is determined by the genetic information they inherited from their parents and how much is caused by their living conditions, learning choices, and other influences from the world around them.

What Is the Nature Versus Nurture Conflict?

People have long wondered what makes humans act as they do and what produces the differences in human behavior that can be seen every day. Scientists have been especially interested in finding out what causes types of behavior that are considered antisocial or abnormal. For a long time there were two schools of thought. Some scientists believed that everyone is destined to act the way they do because of the genes they inherit from their parents. These scientists believed for the most part that the environment that a person grows up in has little or nothing to do with the way that a person behaves. This idea is called the "nature school of thought," because it was believed that nature has given each person certain genes whose traits cannot be escaped.

Scientists of the "nurture school of thought" believed that humans are not inclined from birth to any certain forms of behavior. They believed that the genes children get from their parents do not matter. The environment that a child grows up in was considered the most important influence on that child's behavior.

Today, however, most scientists agree that both genes and environmental factors are important in forming behavior. There is still debate as to how much nature (genes) and how much nurture (environment) affect behavior. It is generally accepted that for each trait, nature and nurture work together in different proportions. For example, scientists estimate that in deciding how tall a person will be, inherited genes have nine times more influence than the environment a person grows up in (which may influence such factors as a person's nutrition). However, researchers have estimated that in determining whether or not a person will become depressed, genes and the environment are about equally important.

Charles Darwin (1809-1882) was a naturalist, or a scientist who believed that all events could be explained by scientific laws. He was best known for his theory on evolution, which maintained that it was genetic makeup that determined a being's success in its environment.

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Why Is Behavioral Genetics So Hard to Study?

To study how likely it is that a certain kind of behavior runs in a family, we must be able to separate the genetic factors leading to the behavior from the environmental ones. This is often very difficult. Many times studies on genetics focus on identical twins that are raised in separate homes or on babies of parents with a certain disorder who are adopted into families who do not have the disorder. It is difficult to find cases like these to study, and even when scientists can find them, the results often do not prove anything. Frequently there are many different genetic and environmental factors that could contribute to the disorder, so pinpointing the factors with the most influence can be difficult or impossible.

Some scientists have begun trying to link certain antisocial types of behavior to specific genes. This is also very difficult, but for different reasons. It is estimated that there are more than 50,000 gene pairs in the human genome. Very few have been linked specifically to any particular trait. It is especially hard to link genes to most types of behavior, because most are complex and are determined by more than one gene.

What Are Some Behavioral Disorders
That Genes Influence?

Schizophrenia

One of the most serious disorders that causes altered behavior that is linked to a person's genes is schizophrenia (skitz-o-FREE-nee-a). Schizophrenia is a mental disorder in which people have an altered sense of reality. People with the disorder may have hallucinations ^* , such as hearing voices or seeing things that are not really there, or delusions ^* , such as believing the FBI is "after them" when they are law abiding. This often causes them to behave in abnormal or odd ways.

Many studies show that there is a strong link between heredity and schizophrenia. If one of a set of identical twins has the disorder, there is a 46 percent chance that the other twin will show symptoms of schizophrenia as well. Children who have two parents with schizophrenia have a 46 percent chance of having the disorder as well. People who come from families where no one has schizophrenia have only a very small chance (1 percent) of developing the disorder.

Genetics is not the only factor that determines whether a person will get schizophrenia. A child whose mother does not have schizophrenia but who is adopted into a family in which one of the adopted parents has the disorder has an 11 percent chance of showing schizophrenic symptoms. This means that the environment also plays a role in the development of schizophrenia.

Researchers breed lab mice to study how specific genes affect growth and behavior. The presence, absence, or alteration of even a single gene can make a noticeable difference.

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Alcoholism

For many years scientists have known that people with a family history of alcoholism are more likely to become alcoholics themselves. Using twin and adoption studies, researchers have found that genes probably play a part in determining who will become an alcoholic. As with many behaviors linked to genes, environmental factors are also very important. Researchers are trying to pinpoint genes related to alcoholism, but this is difficult, since it is thought that alcoholism may be caused by many different genes.

Obesity

Obesity is the condition of having much more body fat than is appropriate for a person's age, sex, and height. The chance of becoming obese has been linked to a person's genes. Using the methods of inheritance studies, such as investigating pairs of twins, researchers have concluded that inherited genes contribute about 40 percent and the environment about 60 percent to whether a person becomes obese. Although genes are involved in this disorder, environmental factors, such as income level, social eating habits, cultural values, and exercise also play an important role.

Depression

Depression is present when a person has feelings of sadness, despair, and hopelessness over a long period of time. It is a common condition, and it can be treated. Depression is another disorder that has been linked to the genes that a person inherits, but it is a mental disorder that is also very closely linked to environmental factors, more so than schizophrenia.

It is thought that certain events in life may bring out depression if a person has inherited genes that make it more likely that the condition will develop. People who do not have a family history of depression also can become depressed. Some of the common environmental factors leading to depression include abuse or neglect, poverty, a traumatic ^* or extremely violent episode in a person's life, and death of a parent or loved one.

What Is the Human Genome Project?

The goal of the Human Genome Project is to identify the approximately 30,000 individual genes that are contained in human DNA. DNA is a double-stranded molecule that contains the genetic code necessary to build a living being. Mapping the human genome is a long and detailed process that uses many complex biochemical ^* techniques and an enormous amount of computer power. The project was started officially in October 1990, and in January 2000 a very rough draft of the human genome was completed.

Once the gene sequence is worked out for a particular section of chromosome, research can be done to link genes to specific traits. To do this, researchers often use chemicals to remove the gene that they are studying or to make the gene inactive, and then see what kind of changes occur in the animal they are studying.

The Human Genome Project is very important to behavioral genetics. It is hard to determine how much the genetic makeup of a person is responsible for behavioral disorders and how much the environment in which that person grows up is involved. If the genes responsible for certain forms of behavior can be found, researchers can study people who have these genes and try to learn which environmental factors cause the person to act in a particular way. Someday doctors may be able to recommend more ways for people who are genetically inclined to certain behavioral disorders to make lifestyle changes that will lessen the chance that a particular disorder will occur. Doctors may be able to use gene therapy to cure, treat, or even prevent some genetic diseases by replacing or supplementing faulty genes. They might also be able to find out at birth which people have genes that might cause them to get cancer, heart disease, Alzheimer disease, and many other conditions.

Decoding the mysteries of the human genome is a landmark discovery, for other reasons as well. The possibilities for using this information are endless. DNA evidence already has been used to convict criminals and to free innocent people from prison. DNA evidence can also shed light on history. Examining DNA from the bones of people who died near Rome in the fifth century A.D. , English scientists have found signs of the disease malaria and have suggested that epidemics of malaria may have led to the fall of the Roman Empire.

There are ethical ^* considerations that limit how far researchers will be able to take this kind of experiment in humans. How will society use the information learned from the Human Genome Project? At the end of the twentieth century, scientists in Scotland cloned a sheep. Will (and should) scientists make exact replicas of human beings as well?