Invitation to Psychology

Chapter 2 Theories of Personality 55

People differ in part because they carry dif-

ferent mutations (variant forms of genes) in their

genetic code. Mutations may be inherited or

may arise before or after birth. Some occur as a

result of environmental factors, as when ultra-

violet radiation from the sun causes mutations

that lead to skin cancer. But individuals also

differ for another reason: Stable changes in the

expression (activity) of genes, and thus of traits

associated with those genes, can occur without any

changes in a gene’s DNA. One of the most excit-

ing developments in genetics is a specialty called

epigenetics, which studies the molecular bases and

consequences of these changes in genetic activity

(Berger et al., 2009). Epigenetic changes affect

behavior, learning and memory, and vulnerability

to mental disorders (Zhang & Meaney, 2010).

Thus, genes do not provide a static blueprint of

development.

Watch the Video Special Topics: Epigenetics:

A Revolutionary Science at mypsychlab

Epigenetic changes may help explain why

one identical twin might get a disease and the

other not get it, and why identical twins and

even cloned, genetically identical animals living

in exactly the same environment may differ con-

siderably in appearance and behavior (Raser &

O’Shea, 2005). Yes, you read that right: Even

clones can differ. The study of epigenetics is dem-

onstrating that the timing and pattern of genetic

activity are critical not only before birth but also

throughout life (Feinberg, 2008). And just like

epigenetics The study

of stable changes in the

expression of a particular

gene that occur without

changes in DNA; the

Greek prefix epi- means

“on top of” or “in addi-

tion to.”

could write virtually any message on the blank

slate of human nature: “Give me a dozen healthy

infants, well-formed, and my own specified world

to bring them up in and I’ll guarantee to take

any one at random and train him to become any

type of specialist I might select—doctor, lawyer,

artist, merchant-chief and yes, even beggar-man

and thief, regardless of his talents, penchants,

tendencies, abilities, vocations, and race of his

ancestors.”

Today, however, almost all psychologists

would say the nature–nurture debate is over.

Biology and experience, genes and environment,

are interacting influences, each shaping the other

over time (Johnson et al., 2009). In this section

and the next, we will examine the interlaced influ-

ences of nature and nurture on personality.

How can heredity affect personality? genes,

the basic units of heredity found in every cell of

the body, are made up of elements of DNA (deoxy-

ribonucleic acid). These elements form chemical

codes for the synthesis of proteins. Proteins, in

turn, affect virtually every aspect of the body, from

its structure to the chemicals that keep it running.

Genes can affect the traits we call “personality”

through their effects on an infant’s developing

brain and nervous system. They can also affect

the functioning of an adult’s brain and nervous

system, directly and also indirectly, by switching

other genes on or off.

genes The functional

units of heredity; they are

composed of DNA and

specify the structure of

proteins.

Genes are not destiny. In part because of epigenetic

influences on gene expression, even identical twins and

cloned animals can differ. The first cat ever cloned was

named CC, for “carbon copy,” but she’s not really a

carbon copy of her genetically identical mother (shown

here nuzzling her grown daughter). The two have

different coat patterns and different personalities.

Sidney Harris/ScienceCartoonsPlus.com