In the 1940s, Robert Tyson succeeded in breeding both a “maze-bright” colony and a “maze-dull” colony of rats that varied greatly as to their ability to learn the correct path through a maze. Additionally he discovered that the offspring of the maze-bright rats learned even faster than their parents did and that the offspring of the maze-dull rats were even poorer at negotiating mazes than their parents.
The genes involved appeared to be very specific to this type of behavior as the two groups of rats did not demonstrate any other differences in performance behavior, including running a completely different type of maze. Continuing studies and experiments have revealed that behavior clearly has a heritable component.
How Can Genes Affect Behavioral Actions?
How can genes, which code only for proteins, result in behavioral actions and responses? Recent research into mutations associated with particular behavioral abnormalities has provided much greater detail as to the connection between genes and behavior.
In the fruit fly Drosophila, individuals that possess alternate alleles for a single gene vary greatly in their feeding success as larvae and a variety of mutations that affect almost every aspect of courtship behavior in the fly have been identified.
In 1996 scientists discovered a new gene in mice, fosB, that seems to determine whether or not female mice will properly nurture their young. Females with mutated fosB genes initially investigate their young but then ignore them. Females with normal fosB genes display caring and protective maternal behavior.
The genetic details of this behavior have been worked out and it seems as if the cause of this inattentiveness results from a chain of events: (1) When mothers of new babies initially inspect them, sensory information from auditory (sound), olfactory (smell), and tactile (touch) receptors is transmitted to the hypothalamus gland. (2) In the hypothalamus gland the fosB genes are activated. (3) Gene activation in turn results in the activation of enzymes and other genes that mold the neural circuitry within the hypothalamus. (4) These modifications within the brain cause the female to behave in a nurturing maternal manner to her offspring. In contrast, females with mutated fosB genes do not produce the proper activation proteins, the brain’s neural circuitry is not properly wired, and normal maternal behavior does not result.
Nature or Nurture?
The connection between environmental influences and genes in determining behavior quickly raised the ongoing and often controversial question of “nature vs nurture.” That is, which plays the greater role in molding the existence of an individual, its nature (genes) or its nurture (environment)? Scientists attempt to answer that question in humans through the use of twin studies in which identical twins that were separated at birth and raised in different settings are investigated. Since identical twins have the same genome, any differences between them should be the result of being reared in different environments.
While the sample size of such investigations is relatively small and as such one must be cautious about drawing definitive conclusions, these studies have shown striking and even eerie similarities in personality, temperament, leisure-time activity preferences, and choices of everything from toothpaste to color and style of clothing. However, to present, the relative importance of genetics versus environment and the role each component play in behavior remains highly controversial.
Behavioral geneticists struggle to understand and explain how a specific configuration at the genetic level translates into behavior at the organism level, the interaction and importance of nature and nurture on the behavior of an individual, and the role of behavioral genetics in the evolution of species.
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