Genetics - An Introduction for Dog Breeders
Genetics-An Introduction for Dog Breeders
By Jackie Isabell
Excerpt:
Nature or nurture which determines behavior? The answer is neither one alone. No instinct is completely determined by heredity; no social, cultural, or learned behavior is purely environmental. Behavior always develops through the interaction of both hereditary and environmental factors.
Behavioral genetics is the study of organisms by means of both genetic and behavioral analysis; it is an interface between the two sciences. Like other hereditary traits, behavior is a phenotype that can be observed and measured. Unlike physical traits, behavioral phenotype influences the entire organism, making it the most dynamic of all hereditary traits as well as the most responsive to environmental changes.
HOW DO GENES AFFECT BEHAVIOR?
The precise way in which genes affect behavior is not yet clear, but knowledge is growing by leaps and bounds. What is known so far is tantalizing.
EFFECT ON STRUCTURAL TRAITS
The genes provide instructions for producing proteins, and some of those proteins play a role in behavior. Their first effect is to create the brain and nervous systems.
The structure of the brain and nervous system of a fruit fly is very different than that of a frog. The closer the relationship between animals, the fewer their structural differences. The brains and nervous systems of mammals such as dogs and wolves or humans and chimpanzees have many structural similarities.
The structural traits immediately set limits on the type of behavior that each creature can possibly achieve. The behavior of animals that are low on the evolutionary scale is almost completely instinctive that is, programmed by the genes. Surprisingly, even fruit flies can learn to avoid shocks and to recognize different odors. This, too, is hereditary, and some strains learn more quickly and have better memories than others. As the brain and nervous system become more sophisticated, behavioral patterns become more complex, and the potential for learned behavior increases.
THE GENETIC CALENDAR
Behavior is also attuned to a genetic time clock. Genes turn on the behavior appropriate for puberty, mating, and raising young. There are daily genes that influence the patterns of the feeding and the wake-sleep cycles. Annual cycles, such as mating and migration, respond to seasonal changes such as the length of daylight.
HORMONES AND BEHAVIOR
Genes produce many hormones that play important roles in behavior. Hormones are produced not only by the endocrine system but also by cells throughout the body. Although the endocrine system and the nervous system are anatomically distinct, the physiology of the systems are intricately interwoven. The nervous system is mediated by the endocrine system, and the endocrine system is controlled by the nervous system. The domesticated silver foxes described in Chapter 1 illustrate the complexity of these relationships.
Neurohormones are any biochemicals produced by or acting on nerve tissue are produced in the brain as well as throughout the nervous system. They interact with neurotransmitters substances that transmit nerve impulses across synapses and influence behavior by initiating or modifying the transmission of nerve impulses. The neurotransmitters act in response to the environment. Fear, for example, floods the body with fight-or-flight neurohormones. Although the interplay of neurotransmitters is inherently an automatic, involuntary response, it can be modified in several ways. If an individual is repeatedly exposed to a frightening stimulus that never actually causes harm, the response becomes less intense or disappears. Humans can exert remarkable conscious control over their neurotransmitters through biofeedback techniques. Neurotransmitters are also modified by ingested substances. This can be beneficial in the case of mental disorders, and it can be harmful in the case of cocaine.
Arons studied the genetic variability of predatory behavior toward sheep in three breeds of dogs representing different functional types using neurochemical analysis. Siberian Huskies exhibit the complete predatory behavior pattern and are consistent killers of livestock. Border Collies represented the herding dogs that exhibit an inhibited predatory response. Shar Planinetz, developed to protect livestock, have low level of interest in sheep and exhibit little, if any, predatory behavior, and they had the lowest level of catecholamines (a group of biochemicals that function as neurohormones or neurotransmitters).
Stress alters the levels of neurohormones such as nor epinephrine, dopamine, and serotonin. An example of the heredity of stress-related neurohormone levels has been identified in a strain of genetically nervous Pointers. The dogs behave normally when not under stress, but freeze and exhibit other abnormal behavior in unfamiliar (stressful) situations. Comparative biochemical assays have found that these dogs have increased levels of nor epinephrine and decreased serotonin and dopamine.
Stress plays havoc with the endocrine system and even with the bodies homeostasis (the ability to maintain physiological processes within normal limits despite varying external conditions). Furthermore, it sometimes impairs the immune system, leaving the individual vulnerable to infections that might have been easily overcome by normal body defenses. The incidence of cancer is higher during the five years following extreme stress, such as the loss of a loved one. The effect of long-term stress is less clear, but some permanent physiological damage is suspected. Many people who develop clinical depression under extreme stress must remain on antidepressants for years, sometimes for the rest of their lives. Symptoms of depression have been observed in chimpanzees, and the incidence appears to parallel that in humans, which is another indication of similar biochemistry.
Evidence that some mental disorders are related to disturbances of neurohormone levels is growing. Many drugs that are used to treat these disorders appear to act by restoring the normal balance of neurohormones. What is not clear is how much is genetically predetermined and how much is environmentally induced. Disorders such as schizophrenia and attention deficit disorder show such pronounced familial patterns that hereditary factors are suspected.