influence behavior. Evolutionary psychologistsstudy how natural selection favored behav-
iors that contributed to survival and spread of our ancestors’ genes, and may currently con-
tribute to survival and the spread of our ancestors’ genes and may currently, contribute to
our into the next generations. Evolutionary psychologists look at universal behaviors shared
by all people. They look at behaviors conserved across related species to understand how we
are adapted to maximize our success in our environments.Genetics and Behavior
Behavioral geneticistsstudy the role played by our genes and our environment in mental
ability, emotional stability, temperament, personality, interests, etc.; they look at the causes
of our individual differences. Your genes predispose your behavior. Studies of twins have
been helping to separate the contributions of heredity and environment. Identical twins
are two individuals who share all of the same genes/heredity because they develop from the
same fertilized egg or zygote; they are monozygotic twins.Fraternal twinsare siblings
that share about half of the same genes because they develop from two different fertilized
eggs or zygotes; they are dizygotic twins.Heritabilityis the proportion of variation among
individuals that is due to genetic causes. The heritability for traits of identical twins is zero.
When twins grow up in the same environment, the extent to which behaviors of monozy-
gotic twins are behaviorally more similar than dizygotic twins reveals the contribution of
heredity to behavior. Schizophrenia and general intelligence are more similar in monozy-
gotic twins than dizygotic twins. If monozygotic twins are separated at birth and raised in
different environments (adoption studies), behavioral differences may reveal the contribu-
tion of environment to behavior; similarities may reveal the contribution of heredity.
Adoption studies assess genetic influence by comparing resemblance of adopted
children to both their adoptive and biological parents. The children must have been
adopted as infants without contact with their biological parents. If the children resemble
their biological parents, but not their adoptive families, with respect to a given trait,
researchers infer a genetic component for that trait. Such constellations of behaviors as
alcoholism, schizophrenia, and general intelligence have shown both genetic and environ-
mental components.Transmission of Hereditary Characteristics
Transmission of hereditary characteristics is achieved by biological processes, including
formation of sex cells, fertilization, embryonic development, and protein synthesis.
Each DNA segment of a chromosomethat determines a trait is a gene. Chromosomes
carry information stored in genes to new cells during reproduction. Normal human body
cells have 46 chromosomes, except for eggs and sperms that have 23 chromosomes.
Males have 44 chromosomes, plus X and Y. Females have 44 chromosomes, plus X and X.
At fertilization, 23 chromosomes from the sperm unite with 23 chromosomes from the
egg to form a zygote with 46 chromosomes. If the male contributes an X chromosome,
the baby is female; if the male contributes a Y chromosome, the baby is male. The presence
of a Y chromosome makes the baby a male. All of the cells of the embryo/baby have the
same 23 pairs of chromosomes, which carry genes for the same traits. Fertilization that
includes a sperm or egg with the wrong number of chromosomes results in a zygote,
and subsequently an individual, with chromosomal abnormalities. Turner’s syndrome
females have only one X sex chromosome (XO). Girls with Turner’s syndrome are
typically short with a webbed neck, lack ovaries, and fail to develop secondary sex
characteristics at puberty. Although usually of normal intelligence, they typically evidence74 ❯ STEP 4. Review the Knowledge You Need to Score High