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Home : Other Subjects : Psychology Study Guides : 101 : Evolution and Genes : Principles of Genetics
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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:
| 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.
Heritability
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.
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