Abnormal Psychology

Understanding Psychological Disorders: The Neuropsychosocial Approach 43

that formulated the core ideas of Mendelian inheritance, or the transmission of

traits by separate elements (which turned out to be genes). The two most important

aspects of Mendelian inheritance are that:

1. Each parent transmits a distinct “element” to its offspring, which specifi es each
   trait.


2. One element may dominate the other in the expression (or nonexpression) of
   the trait in the offspring. The element that is not dominant is called recessive.
   The offspring will show the effect of the recessive element only if neither parent
   transmits the dominant element, leaving the offspring with two copies of the
   recessive element, one from each parent.

In the middle of the 20th century, James Watson

and Francis Crick famously identifi ed the “elements”

that Mendel hypothesized as genes, which correspond

to segments of DNA that control the production of par-

ticular proteins and other substances (see Figure 2.6).

For many traits, gene variants—referred to as alleles—

determine how the trait is manifested. The sum of an

organism’s genes is called its genotype. In contrast, the

sum of its observable traits is called its phenotype, and

many aspects of the phenotype are determined by how

the genotype is expressed in a particular environment.

At one time differences in eye color were used as

an example of the effects of having different alleles,

with brown being dominant and blue recessive. This

example has mostly been dropped from textbooks

for a simple reason: Eyes come in a huge range of

colors, not just two (if you need to be convinced of

this, check out the eye colors of your friends). Thus,

Mendelian inheritance doesn’t really explain differ-

ences in eye color. In fact, most traits do not arise

from simple combinations of dominant and recessive

genes. Rather, for most traits, many genes work to-

gether to cause particular effects. Such sets of genes give rise to traits, such as

height, that are expressed along a continuum, and the joint actions of these genes

producecomplex inheritance (Plomin et al., 1997). Traits that arise from complex

inheritance cannot be linked to a few distinct genes, but rather emerge from the

interactions among the effects of numerous genes. Almost all psychological disor-

ders that have a genetic component, such as schizophrenia and depression, arise

in part through complex inheritance (Faraone, Tsuang, & Tsuang, 2001; Plomin

et al., 2003).

Behavioral Genetics

Studies that investigate the contributions of genes to mental illness rely on the meth-

ods of behavioral genetics, which is the fi eld that investigates the degree to which

the variability of characteristics in a population arises from genetic versus environ-

mental factors (Plomin et al., 2003). With regard to psychopathology, behavioral

geneticists consider these questions: What is the role of genetics in causing a par-

ticular mental disorder? What is the role of the environment? And what is the role

of interactions between genes and the environment?

Throughout this book, we discuss the relative contributions of genes and the

environment to the development of specifi c mental disorders. We must always keep

in mind, however, that any conclusions about the relative contributions of the two

infl uences are always tied to the specifi c environment in which the contributions are

measured. To see why, consider the following example (based on Lewontin, 1976).

Imagine three situations in which we plant two apple trees of the same variety, one

of which has genes for large apples and one of which has genes for small apples.

Mendelian inheritance

The transmission of traits by separate

elements (genes).

Genes

Segments of DNA that control the production

of particular proteins and other substances.

Genotype

The sum of an organism’s genes.

Phenotype

The sum of an organism’s observable traits.

Complex inheritance

The transmission of traits that are expressed

along a continuum by the interaction of sets

of genes.

Behavioral genetics

The fi eld that investigates the degree to

which the variability of characteristics in

a population arises from genetic versus

environmental factors.

2.6 • DNA

Figure 2.6

26 • DNA

Gene

DNA molecule

Proteins and other substances

Amino acid

amino acids

combine to form

codes for

combine to produce

Physical and

psychological traits