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Principles of Genetics

Behavior genetics is a branch of psychology that examines the genetic base of behavior and personality differences among people. An understanding of genetics begins with the following basic concepts:

  • A vast number of cells make up the human body. Each cell has forty-six chromosomes, which come in twenty-three pairs. The only exceptions are sex cells.
  • Sex cells are sperm in males and eggs in females. Each sex cell has only twenty-three chromosomes.
  • Chromosomes are made up of thin strands of deoxyribonucleic acid (DNA). Each chromosome pair contains thousands of genes.
  • Genes are segments of DNA that function as hereditary units. Genes are carried on chromosomes.
  • DNA is made up of units called nucleotides. There are only four different nucleotides, labeled A, C, G, and T. Long strings of nucleotides make up genes.
  • Genes get translated into proteins, which carry out various functions in our bodies. For instance, some proteins serve as the building blocks of cells. Other proteins function as enzymes or hormones.

Who Shares Genes?

No two people share the exact combination of genes unless they are identical twins. However, all family members share some genes with one another. The closer the biological relationship between individuals, the more genes they share. The chart below shows the percentage of genes any person shares with his or her close relatives:

Percentage of Shared Genes
Identical twin 100 percent
Parent 50 percent
Brother or sister 50 percent
Nonidentical twin 50 percent
Grandparent 25 percent

Monogenic and Polygenic Traits

Some characteristics or traits are controlled by a single gene, which means they are monogenic. A single gene, for example, can be part of what brings about alcoholism or schizophrenia.

Most traits are controlled by the actions of several genes, which means they are polygenic. For example, a person’s intelligence is linked to the combination of several genes.

The environment also shapes traits, and later in this chapter we will discuss how genes interact with the environment to produce psychological traits.


In a group of people, a particular psychological trait, such as intelligence, usually varies a lot. Differences in groups may be due to genes or the environment, and researchers use a statistic called heritability to see which has the largest influence. Heritability is a mathematical estimate that indicates how much of a trait’s variation can be attributed to genes. There are three important principles of heritability:

  • Heritability estimates don’t reveal anything about how much genes influence a person’s traits. These figures tell us only to what extent trait differences between people can be attributed to genes.
  • Heritability depends on the similarity of the environment for a group of people. In a group of people who share similar environments, heritability of a particular trait may be high. However, that same trait may have low heritability in a group of people who operate in different environments.
  • Even if a trait is highly heritable, it can still be influenced by environmental factors.

Example: Imagine that ten people live in identical environments. Somehow, they experienced identical prenatal environments while in their mothers’ wombs, were raised in identical homes by parents who were identical in every way, and had all the same childhood and adulthood experiences. Suppose that these ten people turn out to be different with respect to one trait, such as the rate at which they can wiggle their ears. Since both genes and environment can influence traits, these differences would have to be genetic, since they could not be due to differences in environment. In such a case, heritability of the ear-wiggling trait would be close to 100 percent. Now suppose some of these ten people enter different ear-wiggle training camps. The camps vary in effectiveness, so the subjects in some camps increase their ear-wiggling rates, while other subjects remain the same. After the camp training, environment would account for some of the differences among the ten people in ear-wiggling ability. A smaller proportion of the differences would be due to genes alone. Therefore, heritability would be lower.