The study of hereditary factors that affect personality and behavior is called behavior genetics. Much of our knowledge of behavior genetics comes from lab-oratory studies in which the researchers control which males and females mate with one another. Studies have shown, for example, that heredity determines to a large extent the degree of savageness in laboratory rats. The white rat is tamed easily; the gray rat is not. When white rats are mated with gray rats, some of the offspring show the tame disposition of the white parent, and others exhibit the ferocity of the gray parent.
To understand evolution more fully, one needs to understand some basic principles of genetics, the biological mechanisms by means of which hereditary characteristics are transmitted from one generation to the next. You’ll notice that the word genetics contains the word gene. Genes, tiny structures within the nuclei of the body’s cells, carry the messages of heredity. Genes are segments of a long, stringy molecule called deoxyribonucleic acid—DNA, for short—which serves as the carrier of genetic information.
When a child is conceived, the genes received from the sperm cell of the father and the egg cell of the mother jointly provide chemical instructions to the developing organism. These genetic blueprints affect the physical characteristics of the child: it is because of such messages that a child can have “her father’s nose” or “his mother’s eyes.” In addition, genes influence the child’s developing brain and nervous system in ways that can affect intellectual capacities, personality, and behavior.
Behavioral Genetics;Do Genetics Affect Behavior?
If parents’ characteristics are passed on to their children through their genes, why aren’t children exact copies of their parents? One reason is that genes usually act in pairs consisting of one gene from each parent. A random combination of two sets of characteristics often leads to a result that resembles neither parent very closely. A second reason is that genetic information is only indirectly reflected in visible characteristics.
We must distinguish between a person’s genotype, or genetic make-up, and the person’s phenotype, or outward expression of the genetic make-up. For example, Jerry’s genes for eye color may be one brown (say, from his father) and one blue (from his mother). So Jerry’s genotype for the characteristic of eye color is brown-blue. But this doesn’t mean that Jerry will have one brown eye and one blue eye. In fact, he will have brown eyes, because the gene for brown eyes prevails over the gene for blue eyes in the determination of eye color (the phenotype). In this case, the gene for brown eyes is said to be a dominant gene, and the gene for blue eyes is said to be a recessive gene.
Identical twins develop from a single fertilized egg and have identical sets of genes. Identical twins are more similar in many respects than are fraternal twins, who develop from two different fertilized eggs and are no more similar genetically than any other pair of siblings.
Although our genes influence our psychological characteristics, they are clearly not the only influences. To the contrary, more of the variation between people in their intellectual characteristics and personality traits is produced by differences in people’s environments than by differences in their genes (Plomin, 1989). Thus, if you want to know why Tom is more adventurous than Ray, you need to consider not only Tom’s and Ray’s genetic inheritance but also the ways in which they were brought up, their past experiences with taking risks, and their present life circumstances.
In acknowledging that our genes influence psychological characteristics, we must also emphasize that these effects are indirect. Genes don’t produce behavior people do. There is no gene for fear of heights, for intelligence, or for any other psychological trait. Rather, genes exert their effects on the development of bodily structures, including the brain and nervous system, and on the body’s complex chemical processes. These physical and chemical legacies play a part in determining our intellectual capacities, personalities, and behavior—but only in complex interaction with the environment in which we grow up.