Genetics and Behavior
6 Biological Bases of Behavior
STEP 4 Review the Knowledge You Need to Score High
Behavioral geneticists study the role played by our genes and our environment in mental ability, emotional stability, temperament, personality, interests, and so forth; 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 twins are siblings that share about half of the same genes because they develop from two different fertilized eggs or zygotes; they are dizygotic twins. Heritability is the proportion of variation among individuals in a population that is due to genetic causes. For the special case of identical twins, you might want to say that the heritability for traits of identical twins is zero, but that is not exactly correct. Like evolution, heritability is a concept applied to the population rather than the individual. 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 contribution of heredity to behavior. Schizophrenia and general intelligence are more similar in monozygotic twins than dizygotic twins. 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 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 environmental 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 chromosome that 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 called autosomes, plus the sex chromosomes, 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 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. Females with Turner syndrome have only one X sex chromosome (XO). Girls with Turner 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 evidence specific cognitive deficits in arithmetic, spatial organization, and visual form perception. Males with Klinefelter’s syndrome arise from an XXY zygote. The syndrome becomes evident at puberty when male secondary sex characteristics fail to develop, but breast tissue does. Males with Klinefelter’s tend to be passive. The presence of three copies of chromosome 21 results in the expression of Down syndrome. Individuals with Down syndrome typically have intellectual disability and have a round head, a flat nasal bridge, a protruding tongue, small round ears, a fold in the eyelid, and poor muscle tone and coordination.
The genetic makeup for a trait of an individual is called its genotype. The expression of the genes is called its phenotype. For traits determined by one pair of genes, if they are the same (homozygous), the individual expresses that phenotypic characteristic. If the genes are different, the expressed gene is called the dominant gene; the hidden gene is the recessive gene. Numerous recessive genes are responsible for syndromes in the homozygous condition. Tay-Sachs syndrome produces progressive loss of nervous function and death in a baby. Albinism arises from a failure to synthesize or store pigment and also involves abnormal nerve pathways to the brain, resulting in quivering eyes and the inability to perceive depth or three-dimensionality with both eyes. Phenylketonuria (PKU) results in severe, irreversible brain damage unless the baby is fed a special diet low in phenylalanine within 30 days of birth; the infant lacks an enzyme to process this amino acid, which can build up and poison cells of the nervous system. Thus, heredity and environment interact to determine a trait. Huntington’s disease is an example of a dominant gene defect that involves degeneration of the nervous system. Progressive symptoms involve forgetfulness, tremors, jerky motions, loss of the ability to talk, personality changes such as temper tantrums or inappropriate accusations, blindness, and death. Recessive genes for color blindness are located on the X chromosome with no corresponding gene on the Y chromosome. As a result, males show sex-linked traits like color blindness much more frequently than females. Behaviors and diseases may have variations, only some of which are genetically based. A form of familial Alzheimer’s disease has been attributed to a gene on chromosome 21, but not all cases of Alzheimer’s disease are associated with that gene.