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MCAT Content / Biological Bases Of Behavior / Behavioral Genetics

Behavioral Genetics

Topic: Biological Bases Of Behavior

Behavioral genetics studies heritability of behavioral traits, and it overlaps with genetics, psychology, and ethology (the scientific study of human and animal behavior).

Genetics plays a large role in when and how learning, growing, and development occurs. For example, although environment has an effect on the walking behavior of infants and toddlers, children are unable to walk at all before an age that is predetermined by their genome. However, while the genetic makeup of a child determines the age range for when he or she will begin walking, environmental influences determine how early or late within that range the event will actually occur.

Chromosomes are structures in the nucleus of a cell containing DNA, histone protein, and other structural proteins. Chromosomes also contain genes, most of which are made up of DNA and RNA.

DNA, or deoxyribonucleic acid, determines whether our eyes are blue or brown, how tall we will be, and even our preference for certain types of behavior. Known as our “genetic code,” it is shaped like a double helix, made of sequences of nucleic acids attached to a sugar phosphate backbone. Genes are subsections of DNA molecules linked together that create a particular characteristic.

Each chromosome is made up of a single DNA molecule coiled around histone proteins. Human chromosomes are divided into two types—autosomes and sex chromosomes. Some genetic traits are linked to a person’s sex and therefore passed on by the sex chromosomes. The autosomes contain the remainder of a person’s genetic information. All human beings have 23 pairs of chromosomes by which genetic material is developed and characteristically demonstrated; 22 of these are autosomes, while the remaining pair (either XX, female, or XY, male) represents a person’s sex chromosomes. These 23 pairs of chromosomes work together to create the person we ultimately become.

Chromosomal abnormalities can occur during fetal development if something goes wrong during the replication of the cells. Common abnormalities include Down syndrome (caused by an extra chromosome #21), Klinefelter syndrome (caused by an extra X chromosome), and Turner syndrome (caused by a missing X chromosome). Genetic counseling is available for families in order to determine if any abnormalities exist that may be passed along to offspring. Many chromosomal abnormalities are of psychological importance, with substantial impacts on mental processes; for example, Down syndrome can cause mild to moderate intellectual disabilities.

Genetic expression can be influenced by various social factors, as well as environmental factors, from light and temperature to exposure to chemicals. Our genetic destiny is not necessarily written in stone; it can be influenced by several factors, such as social factors, as well as environmental influences among which we live, including anything from light and temperature to exposure to chemicals. The environment in which a person is raised can trigger the expression of behavior for which a person is genetically predisposed, while the same person raised in a different environment may exhibit different behavior.

Long-standing debates have taken place over the idea of which factor is more important, genes or environment. Is a person destined to have a particular outcome in life because of his or her genetic makeup, or can the environment (and the people in it) work to change what might be considered “bad” genes? Today, it is generally agreed upon that neither genes nor environment work alone; rather, the two work in tandem to create the people we ultimately become.

Environmental elements like light and temperature have been shown to induce certain changes in genetic expression; additionally, exposure to drugs and chemicals can significantly affect how genes are expressed. People often inherit sensitivity to the effects of various environmental risk factors, and different individuals may be differently affected by exposure to the same environment in medically significant ways. For example, sunlight exposure has a much stronger influence on skin cancer risk in fair-skinned humans than in individuals with an inherited tendency for darker skin. The color of a person’s skin is largely genetic, but the influence of the environment will affect these genes in different ways.

Gene-environment correlations, known as rGE, can be explained in 3 particular ways—passive, evocative, or active:

  • Passive gene-environment correlation: An association exists between a person’s genetic makeup and the environment in which he or she is raised. In other words, the person’s environment, particularly in the case of children, is largely determined by the parent’s genetic characteristics. Parents create a home environment that is influenced by their own heritable characteristics. When the children’s own genotype influences their behavior or cognitive outcomes, the result can be a misleading relationship between environment and outcome. For example, an intelligent parent is likely to create a home environment rich in educational materials and experience. Since intelligence is moderately heritable, it can be argued that intelligence in the child is inherited rather than a factor of the home environment created by the parents. It is relatively unclear whether the genetic or environmental factors had more to do with the child’s development.
  • Evocative gene-environment correlation: Happens when an individual’s (heritable) behavior evokes an environmental response. For example, the association between marital conflict and depression may reflect the tensions that arise when engaging with a depressed spouse rather than a causal effect of marital conflict on risk for depression.
  • Active gene-environment correlation: The person’s genetic makeup may lead them to select particular environments. For example, a shy person is likely to choose quiet activities and less boisterous environments than an extroverted individual. He or she may be more likely to spend time at the library than at a dance club.

Adoption and twin studies can help make sense of the influence of genes and the environment. Studies of adult twins are used to investigate which traits are heritable. Identical twins share the same genotype, meaning their genetic makeup is the same. Twins raised apart tend to be similar in intelligence and, in some cases, life events and circumstance, when studied years later, even when raised separately.

However, researchers have discovered that the phenotype (or the observable expression of a gene) of identical twins grows apart as they age. In adoption studies, identical twins raised by different families can give insight into the nature-versus-nurture debate. Since the child is being raised by parents who are genetically different from his or her biological parents, the influence of the environment shows in how similar the child is to his or her adoptive parents or siblings. Adoption studies make a strong case for the influence of environment, whereas twin studies make a strong case for genetic influence.


Key Points

• Chromosomes are structures in the nucleus of a cell containing DNA coiled around histone proteins.

• All animals have some number of chromosomes, which transmit genetic material. Human beings have 46 chromosomes (23 pairs).

• Humans have two types of chromosomes: autosomes and sex chromosomes.

• Chromosomal abnormalities can result in genetic conditions such as Down syndrome.

• Today it is generally accepted that nature and nurture work in tandem to create the people we ultimately become.

• Adoption and twin studies show that both nature and nurture are factors in human development.

• The environment in which a person is raised can trigger expressions of behavior for which that person is genetically predisposed; genetically identical people raised in different environments may exhibit different behavior.

• Three types of gene -environment correlations (rGE) exist: passive (ambiguous correlation), evocative (one factor invokes a response in the other), and active (one factor influences a preference for another).


Key Terms

chromosome: a structure in the cell nucleus that contains DNA, histone protein, and other structural proteins.

gene: a unit of heredity; a segment of DNA or RNA transmitted from one generation to the next, carrying genetic information such as the sequence of amino acids for a protein.

autosome: any chromosome that is not a sex chromosome

gene-environment correlation: A relationship in which exposure to environmental conditions correlates with an individual’s genotype

phenotype: the observable expression of a gene

histone: a protein that provides structural support to a chromosome

down syndrome: a condition in which a person has an extra chromosome causing learning difficulties

Klinefelter syndrome: a set of symptoms that result from two or more X chromosomes in males. The primary features are infertility and small poorly functioning testicles



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