- Explain the role chromosomes play in carrying genetic information.
- Explain the relationship between genes and environment including types of gene-environment correlations.
- Discuss the role of twin and adoption studies in investigating genetic influences on mind and behavior.
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. But our genes and chromosomes interact with environmental factors to determine our actual physical and behavioral traits (our phenotype). Teasing out the relative effects of genetic and environmental influences on the phenotype is often difficult because of the complex interactions between them. All traits, including psychological and behavioral traits, are determined by complex interactions between genetic and environmental factors.
DNA and the Genetic Code
DNA, or deoxyribonucleic acid, determines whether our eyes are blue or brown, how tall we will be, and even our dispositions for certain types of behavior. DNA holds 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 encode a particular characteristic.
Each chromosome is made up of a single DNA molecule coiled around histone proteins. Research dating back to the 1800s shows that every living creature has a specific set of chromosomes in the nucleus of each of its cells.
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.
As science advances, the ability to manipulate genes and chromosomes is becoming increasingly sophisticated. Cloning is an example of taking chromosomal and genetic material and creating a new animal, and was first successfully achieved in the famous example of Dolly the sheep. There is much controversy surrounding the manipulation of chromosomes in human beings, with many people believing it to be unethical.
Gene-Environment Correlations: Nature or Nurture
Our genetic destiny is not necessarily written in stone. Genetic expression can be influenced by the environment, including various social factors, as well as physical environmental factors, ranging from light and temperature to exposure to chemicals (see Section 3.7 on Epigenetics).
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 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 that neither genes nor environment work alone; rather, the two work in tandem to create the people we ultimately become. However, because of gene-environment correlations, discriminating the effects of environment from the effects of genes can be very difficult.
We now know that genes can be turned on and off. 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 therefore different individuals may be differently affected by exposure to the same environment in medically significant ways. For example, some people become very ill from exposure to peanuts, while others are entirely unaffected. Another example is exposure to sunlight. 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 environment also affects these genes in different ways.
Gene-environment Correlations by Type
Gene-environment correlations, represented as rGE, occur when genetic factors influence environmental exposure. Genes can affect environmental exposure indirectly via behavior. For example, high IQ parents may have more books in their homes than lower IQ parents leading to greater exposure to books and ideas in homes with high IQ parents compared to homes with lower IQ parents. In such a case, children of high IQ parents may read earlier and have greater facility with a wider range of ideas partly indirectly through the influence of genes for higher IQ on the selection of environments which facilitate exposure to books and ideas during childhood. However, the relation between early exposure to books and intellectual ability later in life may be misinterpreted as evidence of a purely environmental relationship and therefore support the view that nurture is the predominant influence. Such an interpretation would be mistaken because it missed the significant influence of the genes of the parents on selection of the home environment, along with direct genetic effects on intelligence, that led to later intellectual abilities of their children.
Gene-environment correlations, rGEs, can be causally related or non-causally related and can be categorized as passive, evocative, or active.
Passive Gene-Environment Correlations
In 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 significantly 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 misinterpreted as a relationship between environment and psychological 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 because both genes and environment affected the child's development and many aspects of the child's environment were affected by the parents' genes when the home environment was created by the parents. Thus, correlations between environment and a child's cognitive characteristics may be misinterpreted as evidence for environmental influence that is really due more to genetic factors involved in selection of the environment, factors which have been missed and therefore left out of the analysis of causation.
Evocative Gene-Environment Correlations
Evocative gene-environment correlation happens when an individual’s (heritable) behavior evokes an environmental response. For example, the association or correlation between marital conflict (arguing) and depression may reflect or be due to the tensions that arise from interacting with a depressed spouse rather than a causal effect of marital conflict on risk for depression. In other words, marital conflict may not cause increased risk for depression, but instead a genetic disposition for depression in one marriage partner may cause tensions between partners that lead to marital conflict. Therefore, what first appears as an environmental factor in depression may actually have genetic roots. Genes cause a disposition for depression, and depression in one partner leads to increased marital conflict. Therefore a correlation between marital conflict and depression may not be evidence for the influence of environment on depression, but the influence of genetics for depression on the environment (marital conflict).
Active Gene-Environment Correlations
In 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. Thus, a correlation between spending time at libraries and shyness may not indicate the influence of libraries, an environmental factor, on shyness, but instead reflects the effect of genes for shyness on selection of libraries as a place to spend time.
Adoption and Twin Studies in the Nature vs. Nurture Debate
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. Identical 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 adoptive siblings vs. how similar the child is to his or her biological parents and siblings. Adoption studies and twin studies make a strong case for genetic influence. However, it is now generally accepted among scientists, that all traits are influenced by both genetic and environmental differences among individuals.
- 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).
- 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.