Behavioral geneticists—study 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.
Zygote—fertilized egg.
Studies of twins help separate the contributions of heredity and environment.
Identical twins—also called monozygotic twins; two individuals who share all of the
same genes/heredity because they develop from the same zygote.
Fraternal twins—also called dizygotic twins; siblings that share about half of the same
genes because they develop from two different zygotes.
Heritability—the proportion of variation among individuals that is due to genetic
causes.
When twins grow up in the same environment, the extent to which behaviors of
monozygotic twins are behaviorally more similar than dizygotic twins reveals the contri-
bution of heredity to behavior.
If monozygotic twins are separated at birth and raised in different environments
(adoption studies), behavioral differences may reveal the contribution of environment to
behavior; similarities reveal the contribution of heredity.
In adoption studies, 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.
Gene—each DNA segment of a chromosome that determines a trait.
Chromosome—structure in the nucleus of cells that contains genes determined by
DNA sequences.
Human cells contain 23 pairs of chromosomes, 23 of which come from the sperm of
the father and 23 of which come from the egg of the mother at fertilization. If the father
contributes a Y sex chromosome, the baby is male; otherwise the baby is female.
Errors during fertilization can result in the wrong number of chromosomes in cells of
a baby.- Turner’s syndrome—females with only one X sex chromosome who are short, often
sterile, and have difficulty calculating; - Klinefelter’s syndrome—males with XXY sex chromosomes;
- Down syndrome—usually with three copies of chromosome-21 in their cells, indi-
viduals who are typically mentally retarded and have a round head, flat nasal bridge,
protruding tongue, small round ears, a fold in the eyelid, and poor muscle tone and
coordination; - Genotype—the genetic make-up of an individual;
- Phenotype—the expression of the genes;
- Homozygous—the condition when both genes for a trait are the same;
- Heterozygous—also called hybrid, the condition when the genes for a trait are different;
- Dominant gene—the gene expressed when the genes for a trait are different;
- Recessive gene—the gene that is hidden or not expressed when the genes for a trait
are different;
82 ❯ STEP 4. Review the Knowledge You Need to Score High