Genética e comportamento

18-06-2009 22:30

 A Genética Comportamenal estuda a relação entre o genótipo entre e o comportamento de um organismo.



1.     Genética e comportamento

Sir Francis Galton (1822-1911) foi o primeiro cientista a estudar a hereditariedade e o comportamento humano de modo sistemático.

A genética comportamental estuda os mecanismos genéticos envolvidos em diversos comportamentos humanos.

Importante ressalvar que não existe um único gene que determine um comportamento específico, como em 1999, quando se publicou um estudo acerca da inteligência e a sua relação com a sobreexposição de um gene, a imprensa denominou de imediato este de “gene inteligente”. A comunicação social normalmente aproveita-se desta área do conhecimento para fazer notícias sensacionalistas que muitas vezes distorcem a realidade.

 O que é certo é que o comportamento humano é extremamente complexo, envolvendo múltiplos genes e uma variedade de outros factores.


What is behavioral genetics?

Sir Francis Galton (1822-1911) was the first scientist to study heredity and human behavior systematically. The term "genetics" did not even appear until 1909, only 2 years before Galton's death. With or without a formal name, the study of heredity always has been, at its core, the study of biological variation. Human behavioral genetics, a relatively new field, seeks to understand both the genetic and environmental contributions to individual variations in human behavior. This is not an easy task, for the following reasons.

  • It often is difficult to define the behavior in question. Intelligence is a classic example. Is intelligence the ability to solve a certain type of problem? The ability to make one's way successfully in the world? The ability to score well on an IQ test? During the late summer of 1999, a Princeton molecular biologist published the results of impressive research in which he enhanced the ability of mice to learn by inserting a gene that codes for a protein in brain cells known to be associated with memory. Because the experimental animals performed better than controls on a series of traditional tests of learning, the press dubbed this gene "the smart gene" and the "IQ gene," as if improved memory were the central, or even sole, criterion for defining intelligence. In reality, there is no universal agreement on the definition of intelligence, even among those who study it for a living.

  • Having established a definition for research purposes, the investigator still must measure the behavior with acceptable degrees of validity and reliability. That is especially difficult for basic personality traits such as shyness or assertiveness, which are the subject of much current research. Sometimes there is an interesting conflation of definition and measurement, as in the case of IQ tests, where the test score itself has come to define the trait it measures. This is a bit like using batting averages to define hitting prowess in baseball. A high average may indicate ability, but it does not define the essence of the trait.

  • Behaviors, like all complex traits, involve multiple genes, a reality that complicates the search for genetic contributions.

  • As with much other research in genetics, studies of genes and behavior require analysis of families and populations for comparison of those who have the trait in question with those who do not. The result often is a statement of "heritability," a statistical construct that estimates the amount of variation in a population that is attributable to genetic factors. The explanatory power of heritability figures is limited, however, applying only to the population studied and only to the environment in place at the time the study was conducted. If the population or the environment changes, the heritability most likely will change as well. Most important, heritability statements provide no basis for predictions about the expression of the trait in question in any given individual.

What indications are there that behavior has a biological basis?

  • Behavior often is species specific. A chickadee, for example, carries one sunflower seed at a time from a feeder to a nearby branch, secures the seed to the branch between its feet, pecks it open, eats the contents, and repeats the process. Finches, in contrast, stay at the feeder for long periods, opening large numbers of seeds with their thick beaks. Some mating behaviors also are species specific. Prairie chickens, native to the upper Midwest, conduct an elaborate mating ritual, a sort of line dance for birds, with spread wings and synchronized group movements. Some behaviors are so characteristic that biologists use them to help differentiate between closely related species.

  • Behaviors often breed true. We can reproduce behaviors in successive generations of organisms. Consider the instinctive retrieval behavior of a yellow Labrador or the herding posture of a border collie.

  • Behaviors change in response to alterations in biological structures or processes. For example, a brain injury can turn a polite, mild-mannered person into a foul-mouthed, aggressive boor, and we routinely modify the behavioral manifestations of mental illnesses with drugs that alter brain chemistry. More recently, geneticists have created or extinguished specific mouse behaviors—ranging from nurturing of pups to continuous circling in a strain called "twirler"— by inserting or disabling specific genes.

  • In humans, some behaviors run in families. For example, there is a clear familial aggregation of mental illness.

  • Behavior has an evolutionary history that persists across related species. Chimpanzees are our closest relatives, separated from us by a mere 2 percent difference in DNA sequence. We and they share behaviors that are characteristic of highly social primates, including nurturing, cooperation, altruism, and even some facial expressions. Genes are evolutionary glue, binding all of life in a single history that dates back some 3.5 billion years. Conserved behaviors are part of that history, which is written in the language of nature's universal information molecule—DNA.

How is behavioral genetics studied?

Traditional research strategies in behavioral genetics include studies of twins and adoptees, techniques designed to sort biological from environmental influences. More recently, investigators have added the search for pieces of DNA associated with particular behaviors, an approach that has been most productive to date in identifying potential locations for genes associated with major mental illnesses such as schizophrenia and bipolar disorder. Yet even here there have been no major breakthroughs, no clearly identified genes that geneticists can tie to disease. The search for genes associated with characteristics such as sexual preference and basic personality traits has been even more frustrating.

Genetics and molecular biology have provided some significant insights into behaviors associated with inherited disorders. For example, we know that an extra chromosome 21 is associated with the mental retardation that accompanies Down's syndrome, although the processes that disrupt brain function are not yet clear. We also know the steps from gene to effect for a number of single-gene disorders that result in mental retardation, including phenylketonuria (PKU), a treatable metabolic disorder for which all newborns in the United States are tested.

In general, it is easier to discern the relationship between biology and behavior for chromosomal and single-gene disorders than for common, complex behaviors that are of considerable interest to specialist and nonspecialist alike. So the former are at the more informative end of a sliding scale of certainty with respect to our understanding of human behavior. At the other end of the scale are the hard-to-define personality traits, while somewhere in between are traits such as schizophrenia and bipolar disorder—organic diseases whose biological roots are undeniable yet unknown and whose unpredictable onset teaches us about the importance of environmental contributions, even as it reminds us of our ignorance.

What implications does behavioral genetics research have for society?

Researchers in the field of behavioral genetics have asserted claims for a genetic basis of numerous physical behaviors, including homosexuality, aggression, impulsivity, and nurturing. A growing scientific and popular focus on genes and behavior has contributed to a resurgence of behavioral genetic determinism—the belief that genetics is the major factor in determining behavior.

Are behaviors inbred, written indelibly in our genes as immutable biological imperatives, or is the environment more important in shaping our thoughts and actions? Such questions cycle through society repeatedly, forming the public nexus of the "nature vs. nurture controversy," a strange locution to biologists, who recognize that behaviors exist only in the context of environmental influence. Nonetheless, the debate flares anew every few years, reigniting in response to genetic analyses of traits such as intelligence, criminality, or homosexuality, characteristics freighted with social, political, and legal meaning.

What social consequences would genetic diagnoses of such traits as intelligence, criminality, or homosexuality have on society? What effect would the discovery of a behavioral trait associated with increased criminal activity have on our legal system? If we find a "gay gene," will it mean greater or lesser tolerance? Will it lead to proposals that those affected by the "disorder" should undergo treatment to be "cured" and that measures should be taken to prevent the birth of other individuals so afflicted?

There are several scientific obstacles to correlating genotype (an individual's genetic endowment) and behavior. One problem is in defining a specific endpoint that characterizes a condition, be it schizophrenia or intelligence. Another problem is in identifying and excluding other possible causes of the condition, thereby permitting a determination of the significance of a supposed correlation. Much current research on genes and behavior also engenders very strong feelings because of the potential social and political consequences of accepting these supposed truths. Thus, more than any other aspect of genetics, discoveries in behavioral genetics should not be viewed as irrefutable until there has been substantial scientific corroboration.

How do genes influence behavior?

No single gene determines a particular behavior. Behaviors are complex traits involving multiple genes that are affected by a variety of other factors. This fact often gets overlooked in media reports hyping scientific breakthroughs on gene function, and, unfortunately, this can be very misleading to the public.

For example, a study published in 1999 claimed that overexpression of a particular gene in mice led to enhanced learning capacity. The popular press referred to this gene as "the learning gene" or the "smart gene." What the press didn't mention was that the learning enhancements observed in this study were short-term, lasting only a few hours to a few days in some cases.

Dubbing a gene as a "smart gene" gives the public a false impression of how much scientists really know about the genetics of a complex trait like intelligence. Once news of the "smart gene" reaches the public, suddenly there is talk about designer babies and the potential of genetically engineering embryos to have intelligence and other desirable traits, when in reality the path from genes to proteins to development of a particular trait is still a mystery.

With disorders, behaviors, or any physical trait, genes are just a part of the story, because a variety of genetic and environmental factors are involved in the development of any trait. Having a genetic variant doesn't necessarily mean that a particular trait will develop. The presence of certain genetic factors can enhance or repress other genetic factors. Genes are turned on and off, and other factors may be keeping a gene from being turned "on." In addition, the protein encoded by a gene can be modified in ways that can affect its ability to carry out its normal cellular function.

Genetic factors also can influence the role of certain environmental factors in the development of a particular trait. For example, a person may have a genetic variant that is know to increase his or her risk for developing emphysema from smoking, an environmental factor. If that person never smokes, then emphysema will not develop.