APPENDIX I
Behavioral Genetics

1. THE BIOLOGY OF GENETICS AND HEREDITY
   Genetics is the biology of inheritance. Chromosomes are long, thin structures composed of genes. Genes, which are made up of deoxyribonucleic acid (DNA), provide instructions for the synthesis of proteins, which are the basis of physical traits, such as eye color. Male sperm cells and female egg cells are formed by meiosis, which leaves them with twenty-three single chromosomes. The twenty-three single chromosomes are combined in conception to form a new cell, called a zygote. Many traits are polygenic-that is, influenced by many genes. The forty-six chromosomes contain the genes, which are the individual's genotype. An individual's phenotype is the set of observable characteristics resulting from an interaction between heredity and environment.
2. A BRIEF HISTORY OF GENETIC RESEARCH IN PSYCHOLOGY
   Sir Francis Galton first suggested family, twin, and adoption studies be used to study the link between heredity and human behavior. Galton referred to genetic influences as "nature" and environmental influences as "nurture." Although a genetic contribution to intelligence was supported by studies in 1924, research on genetic influences was inhibited by behaviorism and the association of genetics research with the views of the Nazis. The work of Arthur Jensen and Richard Herrnstein angered people because of its focus on ethnic and class differences.
3. THE FOCUS OF RESEARCH IN BEHAVIORAL GENETICS
   Behavioral genetics is the study of differences in a given trait in a population, not the study of the development of that trait for an individual. Average differences in height in a specific population are influenced primarily by genetics, for example; however, we cannot say that a specific person's height was influenced primarily by genetics. Genes can affect a trait without completely determining whether or not it will appear.
4. THE ROLE OF GENETIC FACTORS IN PSYCHOLOGY
   1. Genetic Influences Over the Life Span
      Genetic influences on mental ability apparently increase throughout the life span. A possible reason for this is that inherited predispositions may lead people to choose environments that are favorable to the development of this ability.
   2. Genes Affecting Multiple Traits
      Genes that influence one characteristic may also influence others. Anxiety and depression may be affected by some of the same genetic factors.
   3. Identifying Genes Related to Behavior
      Huntington's disease, which leads to a loss of motor control, personality changes, and forgetfulness, is caused by a single dominant gene. Several genes have now been linked to Alzheimer's disease. These and other advances flowing from the Human Genome Project will continue to help identify genes related to human behavior. Thus far, the most surprising finding of the project is that the number of genes in the human genome is much smaller than expected. New techniques help to illuminate gene-environment interactions.
5. BEHAVIORAL GENETICS AND ENVIRONMENTAL INFLUENCES
   While genetic causes of schizophrenia have been supported with evidence from behavioral genetics research, environmental factors can be just as important. Often fifty percent or more of the variance among individuals is due to nongenetic factors. One goal of psychologists is to study the "nonshared" aspects of the environment to see how the different environments of family members influence the appearance of traits. Events and others' behavior can be experienced differently by children in the same family.
   
   A current hypothesis to be investigated is that genetic differences between siblings can contribute to differences in how they are treated. Research so far indicates that parents' responsiveness is related to children's mental ability and that children choose playmates partly based on genetically influenced traits such as temperament. Nature and nurture, therefore, are interacting to influence a person's characteristics.