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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.

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, individuals 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.

• Tay-Sachs syndrome—recessive trait that produces progressive loss of nervous function and death in a baby.

• Albinism—recessive trait that produces lack of pigment and involves quivering eyes and inability to perceive depth with both eyes.

• Phenylketonuria (PKU)—recessive trait that results in severe, irreversible brain damage unless the baby is fed a special diet low in phenylalanine.

• Huntington’s disease—dominant gene defect that involves degeneration of the nervous system, characterized by tremors, jerky motions, blindness, and death.

• Sex-linked traits—recessive genes located on the X chromosome with no corresponding gene on the Y chromosome, which result in expression of recessive trait, more frequently in males.

• Color blindness—sex-linked trait with which individual cannot see certain colors, most often red and green.

CHAPTER 8

Sensation and Perception

IN THIS CHAPTER

Summary: If you had to give up one of your senses, which one would it be? Most people choose the sense of smell or taste; no one ever chooses sight or hearing. What we see and hear are more essential for our survival than what we smell and taste. Vision is the most studied sense, and has the largest area of our cerebral cortex devoted to it of all of our senses.

All species have developed special sensory mechanisms for gathering information essential for survival. Sensation is the process by which you detect physical energy from your environment and encode it as neural signals. Perception is the process that organizes sensory input and makes it meaningful. What you perceive is influenced by your memory, motivation, emotion, and even culture. The study of sensation and perception is rooted in physics. Psychophysics is the study of the relationship between physical energy and psychological experiences. Psychophysics asks questions about our sensitivity to stimuli.

This chapter focuses on the conversion of sensations to perceptions that give you your view of your world.

Key Ideas

Thresholds

Vision

Hearing (Audition)