Genetics is the structure and the function of genes and the way in which genes are passed from one generation to another. Genetics also involves the study of how genetic make up of a person influence its physical and behavioral characteristics. Genetics related are the principle of heredity which is the sum of the traits and tendencies inherited from one's parents and other biological ancestors.
The biological structure is intimately related to the human behaviour that is why genetic inheritance which shapes the structure may have much to do with behaviour. Most physical characteristics such as height, hair color, blood type and eye color are largely shaped by heredity. What about other characteristics such as intelligence, moodiness, impulsiveness and shyness? To what extent does human behaviors is influenced by genetics? These question focus on the behavioral genetics - as a field that studies the influence of genetic factors on behavioral traits. As noted by one of the most prolific researcher in this field, Robert Plomin, behaviour genetics is intimately involved with providing an explanation of why people differ (Plomin 1997).
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Every person must exhibit certain behaviours that are critical for life, and the species must maintain a pool of individuals that contains behaviours consistent with the production of offspring. Those behaviours include the ability to pro c u re meals successfully in a competitive environment; eating; avoidance of obvious mortal danger; and, for at least a subset of the population, finding a mate and re producing. It is beneficial to the species for those behaviors to occur naturally. Even a newborn infant knows without being taught how to suckle to obtain milk. As development progresses, feeding behavior becomes more complex and is subjected to a range of individual experiences and environmental influences. Thus, even though all of us experience the urge to eat, each of us maintains substantial control over the process of choosing to eat a particular meal. Clearly biology and environment (including experience) both play roles in this critical behavior. Biology's role in behavior is obvious because all behavior is controlled by the brain and the nervous system. Genes choreograph the development of the brain through transcription and translation of DNA into proteins. Through those processes, genes affect the molecular structure of the brain at every level, including brain anatomy, neurotransmitter levels and receptors, and the processes that control the development of interconnections among neurons. Environment also plays a role by modifying or disrupting genetically encoded actions. Variation in the genes that control brain development may result in variation of behavior. It is not necessary to consider anything as complex as brain anatomy to see that genes may influence complex behavior. Consider a simple, metabolic process .Variation in the gene that codes for aldehyde dehydrogenase, an enzyme that functions in metabolizing alcohol, can result in an inactive enzyme. When individuals who have only the mutated form of this gene drink alcohol, they cannot break down a toxic alcohol metabolite. As a result, every time they drink alcohol, they get sick. As a further consequence of this mutation, they are less likely to drink alcohol than people without this genetic variant. In other words, a gene influences their drinking behavior.
Although genes are responsible for the biological substrates of behavior, behavior also is influenced by experience and other aspects of the environment. When we talk about the environment, we mean a person's culture, experience, and interactions with family and friends, as well as anything else that is non-genetic, including prenatal exposure to toxins, a brick falling on the person's head, and an infection with a pathogenic agent that has some effect on the central nervous system. Using drinking habits as an example, some individuals never even try alcohol because of religious beliefs or cultural influences. Because exposure to alcohol is necessary for behaviors related to alcohol consumption to be expressed, these people never express an entire set of behaviors. Some cultures provide greater opportunity for exposure than other cultures. For example, regular consumption of alcohol with meals is the norm in some cultures or families but is discouraged in others. At a different level of environmental influence, alcohol can act as a prenatal environmental toxin when consumed by pregnant women during a specific period of fetal development. One possible outcome is reduced cognitive capacity (intelligence) of the child. Another example of an environmental toxin is lead exposure, which is a commonly recognized problem for inhabitants of older buildings in which lead-based paint has been used or lead is used in the plumbing. At certain stages of development, lead exposure also can lead to reduced cognitive capacity. In both of those examples, toxins prevent the brain from developing normally, which results in effects on behavior. Although we have discussed how genetics and environment influence behavior and, to some extent, form the substrates of behavior, it is rare for a particular outcome in a particular situation to be wholly specified by these factors. That is, the influences of the environment and genetics do not supplant volition, and behavior is not determined by experiences or genes.
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The search for genetics factors of behaviour has been active since 1970s. The goals of research in behavioral genetics are to answer questions about the existence and nature of genetic and environmental influences on behavior. Questions arise at a variety of levels of inquiry, and there are different methods that have been developed to answer diff e rent types of questions.
The basic genetic questions are:
Does a behavioral trait run in families?
If it does, can genes help explain family resemblance and individual difference?
If so, what is the nature of the genetic influence (is it inherited in a Mendelian pattern or is it something more complex)?
W here is the genes located?
What proteins (gene products) do the genes encode?
How does each gene product function?
With these questions in mind, there are two primary types of methods: those based on the principles of genetic epidemiology and those that employ the technology of molecular genetics.
In genetic epidemiology (the study of the clustering of specific traits in families and populations), the goal is to provide designs that permit quantification of genetic and environmental effects. In molecular genetics, the goals are to establish the biochemical basis of genetic effects.
To quantify genetic and environmental effects, methods in genetic epidemiology are designed to subdivide variability among members of a specified population into genetic and environmental components. Frequently, the environmental influences are broken down further into those that are shared by family members and those that are unique to the individual. Heritability is a commonly used term that describes the proportion of phenotypic variation among individuals in a specific population that can be attributed to genetic effects. The reason many behavioral geneticists focus on the genetic effects is that the chromosome theory of inheritance and the central dogma of molecular biology provide a theoretical context for investigating and testing genetic hypotheses. No equivalent, comprehensive, theoretical framework exists for studying environmental effects.
Family studies: An obvious place to look for genetic effects is within families. Family studies are useful because families are easy to find, and most family studies can provide information about genetic influences on a trait, mode of inheritance (for single-gene traits), and sometimes number of genes involved (for polygenic, or multiple-gene, traits). The obvious problem with relying on resemblance among family members is that they share environmental influences as well as genes. For example, if we merely determine which traits run in families and assume that all traits that run in families are genetic, we will falsely conclude that traits such as religious affiliation, wealth, and preference for cold cereal have a genetic basis. We can conclude from family resemblance that there may be genetic influences on a trait, but we need more specialized approaches to separate genetic influences from shared environmental influences.
Twin studies: Identical twins provide an experiment of nature where both members of a twin pair are entirely alike for all of their genes on average. Fraternal twins, in contrast, are genetically like non-twin siblings in that they share only half of their genes. Both kinds of twins share environmental influences to a similar degree (but greater than might be expected for non-twin siblings). Because identical twins and fraternal twins differ only in the amount of DNA that they share, greater similarity for a particular trait in identical twins than in fraternal twins is evidence for a genetic contribution to that trait. If identical and fraternal twins are similar for a particular trait, this is evidence for a share the environmental contribution to the trait. The ability to separate genetic and shared environmental influences using twins is a powerful advantage of using twins. A further advantage is that both members of a twin pair are the same age. Since many behaviors are affected by age or developmental stage, this is an important advantage over family studies, where age can differ by an entire generation. A disadvantage of relying on twins is that they are more difficult to find than families. A critical implicit assumption is that identical twins and fraternal twins share environments to the same extent, which may be an overly simplistic assumption. There are other more complicated issues that arise in the context of twin studies that are beyond the scope of discussion in this module but that must be considered by scientists when interpreting results.
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Adoption studies: Both twins and other family members share environmental influences to some extent. The study of children who have been adopted at an early age provides a unique opportunity to separate genetic effects cleanly from shared environmental effects. Any systematically observed similarity for a given trait between biological parents and adopted-away children must reflect genetic, rather than environmental, effects. In contrast, any systematically observed similarity between adoptive parents and children they have adopted must reflect shared environmental effects. The clean distinction between shared effects that are genetic in origin and shared effects that are environmental in origin makes the adoption study design appealing and powerful (although there are possible confounding factors, such as prenatal environmental influences). On the other hand, adoption studies are extremely difficult to conduct because there are very few children who are adopted in contemporary society, and there are serious issues of confidentiality that make it difficult to link adopted children to their biological parents. Furthermore, adopting parents tend to be older, wealthier, and healthier than the corresponding biological parents, who frequently are very young and usually are in difficult life circumstances.
These sampling issues raise questions about the general applicability of conclusions drawn from studies of this sort. Historically, adoption studies have been very important in psychiatric genetics, particularly for establishing a genetic basis for schizophrenia at a time when wholly environmental hypotheses were heavily favored.