Introduction
Research in personality is a heavily concentrated field, and newer studies are continuously being conducted. Personality psychology research can be dated back all the way to the 1930s in which by that time personality psychology officially was recognized as a discipline in social sciences. Although for some time prior to that personality psychology was considered to be a part of experimental psychology (Hall & Lindzey, 1957). During the era many well-known researchers eagerly began bouncing ideas of one another designing studies to be tested. The field began to emerge and blossom and following closely behind was a string of different theories. Skepticism was common in the United States because just the thought of behavioral traits being heritable was unfathomable. The reason behind the denials was that inherited behaviors went against peoples egalitarian ideal (Holden, 1987).
Although despite the dismay there have been successful research in behavioral genetics that is promising. Behavioral genetics aims to quantify the relationship between environments and genetics. The different measurements to assess those differences will consist of questionnaires and objective tests. Although there are many different novel questions from exceptional researchers, the main focus of this paper will be on biological process (DNA, Blood type, etc.,) playing a key role in determining specific personality traits. The relationship between DNA and personality type has been studied through a variety of different methods, but the most common are twin studies. Twin studies have shown that 40- 60% of variance resulting from the Big Five personality traits is heritable (Bouchard, McGue, 2003; Vernon, Martin, Schermer, Mackie, 2008). In recent decades research in adoptees and twins have been used to differentiate environmental influences from genetics influences.
Genetics play a role in occurrence of events that are classified as environmental. For example, there was a study called the Colorado adoption project that looked at the heritability of television watching. The results showed that the amount of television a child watches is indeed a trait that is inherited (Plomin, Corley, DeFries, and Fulker, 1990). Looking more closely at Genetics, it holds a key to a vast amount of information some of which has not yet been explored. Genetics carries more than just information regarding individuals, but it also has strong rooted genes that have been passed down for centuries. This paper will highlight the importance of DNA and why it can be valuable for distinguishing between characteristics. Also, the paper will provide background on various studies that were done on DNA and how the information was used when determining personality types of individuals, and animals. Lastly, the paper will close by supporting the argument of biological processes being a huge factor in determining personality traits in individuals.
Biological Processes in Humans
Personality traits in humans have significant ontogenetic and heritable components that greatly influenced important life outcomes in social and reproductive functioning as well as health (Nettle, 2005). Behavioral genetics provides a strong set of tools for understanding individual differences (Plomin, DeFries, McClearn, & McGuffin, 2001; Turkheimer, 2000). A typical analysis sections off observed variations into three sections. The sections are shared environmental influences, nonshared environmental influences, and genetic influences. Genetics can cover all aspects of a person’s life down to their political affiliation. There is a modern approach to studying political interest in adults and children. Most people believed that a child’s political interest lies within their parents’ beliefs, which is false. In fact, researcher have showed that there is little significance in parental and child’s political interest. Thus, researchers decided to take a different approach and look at genetic components for significance in decision makings. Through countless research regarding political interest and genetics they found a strong correlation between the two concluding that political affiliation is inherited. In connection with the previous statement researchers looked further into inherited affiliations and found a significant link between personality traits and political affiliation. For example, there was a study conducted on the British population’s aggressiveness, and it showed a strong positive effect on political interests. Also, the results showed that rigidity had a negative impact on interests as well.
Behavioral traits are thought to be partly heritable and having influence on life history traits. Heritable traits look at the genetic variation (Vg) in relation to the phenotypic variation (Vp) (Falconer & Mackay, 1996). The two types of heritability’s once sorted are distinguished into either nonadditive or broad sense. Many studies looked further into heritability and found that the broad-sense component is in the Big Five personality traits in humans (Openness, agreeableness, conscientiousness, extraversion, and neuroticism) (Bouchard & Loehlin, 2001). Researchers conducted studies on primates and humans, what they found was in altered brains serotonin activity was associated with anxiety, depression, dominance, aggression, and hostility. Expanding further into the topic recent studies reported that 5-HTTLPR which is a human serotonin transporter gene has been associated with neuroticism and other neurotic traits. Scientist looked at the relationship between neuroticism and dopamine receptors and found that it yielded mixed results. The gene exist in long and short form, the longer form producing more protein and serotonin mRNA transporter. Whereas the short form is dominate over the long form. Thus, individuals who possess the dominate allele show increased scores on neuroticism and other related traits such as avoidance which were measured on the Revised NEO Personality Inventory (NEO-PI-R) (Costa, McCrae, 1992). They also measured the traits using the TPQ (Katsuragi, Kungi, Sano, Tsutsumi, Isogawa, Nanko, et al, 1999). Genetic proportions account for a significant amount of variance in neuroticism estimating 27-31 % (Loehlin & Nichols, 1976; Floderus-Myrhed, 1980; Rose, 1988; Eaves,1989, 1998, 1999; Loehlin, 1992; Lake, 2000). Research on the biological processes of personality is typically limited to one specific gene “loci”. Gene coding for receptors, enzymes, and transporters of neurotransmitter systems is of interest in several studies due to the nature of their personality theories. For example, those personality theories serve as a basis for a biochemical base in major personality traits (Cloninger, 1987). Personality traits in humans is very complex and it is associated with somatic and psychiatric disorders (Bienvenu, Hettema, Neale, Prescott, Kendler, 2007; Distel, Trull, Willemsen, 2009; Goodwin, Stein, 2003; Sutin, Terracciano, Deiana, et al, 2010). Looking further into genetics from a psychological perspective it focuses on relationships between physical features that are largely hereditary and behavioral (Cattell, 1950). Blood type also play a role in determining specific characteristics of humans mainly because it is free of environmental modifications. Studies have looked closely at blood type B/AB in smoking addiction. They found that blood type B was made up of nonsmokers, whereas AB primarily consisted of those who smoked occasionally (Cohen and Thomas, 1962).
Biological Processes in Animals
Through decades of research on animal personality it has been proven that over a hundred different species of animals regardless of sex or age differ greatly in their underlying physiology and behavior in any setting (Carere et al., 2010). Although, research was not always as efficient as it is now. In the past those differences among species were rarely looked at as biological variation, in fact it was often interpreted as either inaccurate measurement, or non-adaptive variation among adaptive means (Wilson, 1998). When analyzing animal personalities, researchers used the same reliability and validity scores that are regularly used in statistical analysis. For example, in human personality reliability falls within the range of .70- .85. Whereas in animals’ reliability is averaged at .73 (Gosling, 2001). Validity in accurate measurements is harder to study in both humans and animals, but the average score for both of them is .30- .50 (Locurto, 2007). Although, despite all of the challenges they are faced with studying animal personality it is actually high in demand in contemporary behavioral biology due is integrative and holistic approach (Sih et al., 2004). The importance of researching animal personalities has several different reasons. One of them is it takes an interdisciplinary approach that combines mechanisms with evolution and ecology. Another reason to study animal personality is because it brings out implications in evolutionary theory. Some of the implications are that correlated behaviors do not evolve in isolation, instead they evolve as an integrated pattern that generate trade offs and boundaries to endless plasticity (Sih et al., 2004; Wolf et al., 2007; Wolf et al., 2008).
The study of heritable traits is not specific just to humans, animals have shown a genetic preference in traits as well. Heritable traits in animals range from the Big Five to trait-oriented approaches. For example, a study was conducted using rodents and the basis of the research was to look for genetic components in personality traits. Researchers looked at locomotor activity, aggression, and explorative behavior. The results of the study concluded that the rodents showed a strong response to the selections in the experiment which indicated a strong genetic influence (Wimer & Wimer, 1985). Through the years genetic studies conducted on wild species are seemingly rare, but there are exceptions. One of the scientific exceptions is chimpanzees. Chimpanzees’ personality factors are those that are scored on the Big Five which are commonly studied in humans, with the exception of dominance (King & Figueredo, 1997). Dominance found in chimpanzees make up 63% heritability that is scored using family relationships pedigree (Weiss et al, 2000). Several studies were conducted that focused on the well-being in chimpanzees using the subscale of human emotionality (Tellegen et al., 1988), the results concluded a 40% heritability factor (Weiss et al., 2002). Alike the chimpanzees there is another species of wild animals typically found in parts of North America and Asia called the great tit (Perrins, 1965). The great tit is a monogamous passerine that is quite territorial and common in those parts. They reside in wooded areas, secondary holes, and artificial nest boxes. Studies were conducted in both the laboratory and in the wild. The results showed that there was a difference in physiological and behavioral parameters in both non-social and social environments (Groothuis & Carere, 2005). Thus, these previous studies have given tremendous information regarding the existence of personalities and behavioral traits in animals. Animals who experience exploratory behaviors actually experience a combination of two personality traits that make up one. The two traits are exploration towards an object and boldness towards the object. Thus, according to studies on the great tit they found that 30% of variance found in exploratory behavior is contributed to their wild caught parents (Drent et al., 2003). In phenotypic variations about 20-50% found in quantitative studies shows that animal personality traits have a strong genetic base (Van Ores, 2008; Van Ores & Sinn in press). Recent studies in animal personality and behavioral aspects have been gradually increasing over the years. Scientist interested in animal personality traits have peaked mainly because it signifies how they cope with the stressors of their environment which can be both predictable and stochastic. The fascinating complex structure of a nonhuman brain piqued the interest of scientist significantly, that multiple studies in various departments in psychology have been conducted.
Genetic heritability consists of several different components that can be analyzed separately. Some of the components are genetic dominance, additive genetic, genetic maternal, and sex-dependent expression (Mather & Jinks, 1971). Looking further into genetic dominance it has been reported to be a type of consequence that occurred through an interaction of alleles. Researchers deemed genetic dominance as an unimportant trait simply because the trait alone could not predict a certain selection nor its response to it (Crnokrak & Roff, 1995; Fisher, 1930). However, it can influence certain traits like additive genetic variance, or the nonadditive genetic variance (Crnokrak & Roff, 1995). Genetic maternal traits started to arise when there is a phenotypic effect on the offspring from the genotype of the mother (Mousseau & Fox, 1998). Maternal effects do have an influence on heritability due to them being transmissible from the parent to the offspring resulting in a correlation between relatives that share the same maternal environment (Mousseau & Fox, 1998).
Further studies on dopamine receptor DRD4 was looked at in not only humans but animals as well. The DRD4 receptor is thought to be a major candidate in genetic variation and gene expression (Ebstein, 1996; Dulawa, 1999; Szekely, 2004; Munafo, 2008; Flisikowski, 2009; Frieling, 2010). Reward-seeking behaviors that are associated with aggression, impulsivity, exploration and novelty-seeking is tied to the dopamine receptor DRD4. Domesticated animals such as dogs have shown to have differing DRD4 alleles. Meanwhile domesticated horses and captive monkeys show a relationship between DRD4 gene polymorphism and personality parameters (Momozawa, 2005; Bailey, 2007). Scientist looked at epigenetic personality linked disorders with the goal of finding a relationship between DNA myelination and behaviors. The results in one study concluded that in laboratory rats their resilience to stress significantly attributed to a controlled epigenetic transcription BDNF gene (Duclot & Kabbaj 2013). The second experiment conducted on laboratory rats looked at the stress levels of high and low groomed test rats. They found that rats who were groomed significantly more than puppies showed a change in myelination patterns as well as low stress (Weaver et al. 2004; Szyf et al. 2005).
Researchers have made headway in the study of animal personality that they have started paying extra attention to interindividual behavioral diversity within different species (Carere & Maestripieri, 2013). The peak of interest in the interindividual behavioral diversity is because individual differences have a great influence on ecological processes due to natural selection being affected (Wolf & Weissing, 2012). For example, some of the ecological processes that would be affected is habitat selection, life history characteristics, responses to environmental conditions, and anthropogenic pressures. Ecologist have studied the different personality traits within animals that are in captivity and in the wild. Ecologist aimed to find out why is behavioral and personality traits different among the two environments. Certain theories started to arise and it dates back to interindividual behavioral diversity and life-history trade off. Ecologist believe that there is a link between life-history trade off and personality traits (D. Reale, et al, 2007). There is evidence to support that in free living animals interindividual variation is common (Roff, Fairbairn, 2007; Roff, 2002). Animals differ in their behavioral traits compared to humans. They show less plasticity and differ in their reactions to environmental stimuli (Clark & Ehlinger, 1987; Wilson et al., 1994; Boissy, 1995; Wilson, 1998; Gosling, 2001; Greenberg & Mettke- Hofmann, 2001).
Materials
Linkage analysis methods are used when determining specific locations of genes that serve as markers for diseased genes. Thus, if the marker gene is in close proximity to the diseased gene the probability of transmitting together from parent to offspring is significantly higher which can be defined as likelihood odds ratio (LOD) (Jang, Vernon, Livesley, 2000). Although, there are several statistical analyses that could be used to test genes, the most common testing for genes that are linked to certain personality traits is qualitative trait loci (QTL). The QTL analysis looks directly for the absence or presence of alleles that are of interest to the researcher. Previous research has been conducted using the QTL analysis and they were looking for a relationship between a specific personality trait and a dopamine receptor. The results were that they successfully confirmed the relationship between novelty seeking and dopamine receptor specifically DRD4 (Cloninger, Adolfsson, Svrakic, 1996; Cloninger, Przybeck, Svrakic, 1991). Also, QTL mapping explores relationships between phenotypic variations and genetic markers that are genome wide. The QTL mapping looks at populations for traits that arise through segregation (Lynch & Walsh, 1998; Slate, 2005). Although the range of the QTL task is limited there still has been vast amount of information that was discovered since using it. Since using QTL mapping our understanding of wild/semi-wild populations underlying behavioral mechanisms has significantly increased. For example, in silver foxes there were eight loci and three pairs of epistatic loci that has been discovered. The pairs of loci have been associated with tameness and regulation of aggression (Nelson, et al. 2017).
Successfully genetically mapping behavioral traits rarely ends with certainty, there are usually trails of confusion in the mist. The reason behind the uncertainty is that often after mapping there is not one single genetic polymorphism that is found, there usually are several that show equal strengths. The reason behind this is due to linkage disequilibrium among loci. Although, there is a way to delineate specific genomic regions and perform test that are for selection instead. The importance lies with analyzing all surrounding loci that is associated with markers at a distance where there is a decrease in linkage disequilibrium to a negligible level. There are associated markers that are at a close distance where the linkage disequilibrium decreases at a negligible level. Thus, speaking the best way to execute this would be to capture all genetic variation within a genomic region, without excluding rarities in alleles. There is an advantage to having sequencing on markers compared to genotyping. The results are haplotypes where each allelic phase of a mass of loci is discovered. This process surpasses the error-prone phase estimation through population-based methods. This method allows precise linkage disequilibrium. Also, it leads to reliable haplotype gene tree reconstruction.
Another widely used test is called the candidate gene association test. The candidate gene association test is hypothesis driven and it looks at a gene of a specific function or one that is expected to influence expressions of a particular trait. The traits are screened to assess relationships between phenotypic morphism and genetic polymorphisms. Thus, candidate genes are routinely tested in species that are of interest to the researcher to determine if the influence of genetic variance correlates with similar phenotypes (Fitzpatrick, et al, 2005). The selection of genes to study are carefully researched and handpicked because they are of interest to the researcher. Typically, when selecting a gene the goal is to select one that is likely to have a major effect on phenotypic traits (Barrett & Hoekstra, 2011). Secondly, when carefully examining a potential gene researchers look for some that are already registered in gene databases which are proven to be in biological pathways, or previously studied in literature (Fitzpatrick, et al, 2005).
In relation to the other measurements used, another popular measurement called GWAS also contributed to knowledge of genomes. The GWAS is a technique that is used to detect relationships between genetic variants within traits on unrelated groups and traits of interest (Visscher et al., 2017). Thus, the technique can be performed on organisms whose pedigree of information is unavailable and there are prior genomic relationships. Executing the task requires high-density genetic markers in use on thousands of single nucleotide polymorphisms (SNPs) which restricts the locations of the QTL mapping technique in genomic locations that have relatively small effect size (Mackay, Stone, & Ayroles, 2009). QTL mapping and GWAS is similar in performance because they both research using markers, but they differ because GWAS tends to use linkage disequilibrium between the trait of interest and markers. QTL in their research tends to focus on physical linkage between loci that segregates in pedigree and markers. Several studies of wild population were successful in identifying specific traits that are influenced by loci, life history characteristics, and morphology due to the GWAS approach. The approach is used when determining specific behavioral phenotypes in wild populations, and natural systems. For example, there was a study that found a significant amount of SNP that explained approximately 3.9% of the variance in colored fly catchers (Husby et al. 2015). In correlation to the previous study there was another one conducted on Soay sheep. Researchers found that the loci gene influenced leg length, and horn type phenotypes within that sheep population (Bérénos et al., 2015; Johnston et al., 2011).
Analyzing personality traits from an evolutionary perspective can be complex, so it is wise to break it up into two parts. The first part looks at the relationships between personality variants and fitness measures. The second part looks at analyzing genomic regions that are associated with genetic diversity patterns, genetic structure, and footprints of selection. Going further into detail about the first approach it looks more in depth with different personality variants from future evolutionary trajectories. The first approach goes more into detail about personality variants performance across for major environmental habitats for species under current study. The reason behind that is to study the interactions that could arise between environment and genotype (Van Oers et al 2005; Ellegren & Sheldon 2008; Dingemanse et al 2010). In contrast to the first approach, the second approach touches on selection history and underlying genetic variants. The second approach allows for estimation of the age of genetic variants and selection history within a specific genomic region. The approach provides information regarding selective forces and the origin which has shaped personality traits in the past.
Conclusion
In conclusion personality traits can be explored through many different outlets. For centuries the most common way was through personality measurements. Those measurements only scratched the surface on what underlies a personality trait. The study of primates awoke a different era of analyses that provided us with eye opening information. Animals are not just creatures, they are living, breathing, feeling, big ball of emotions that until recently we could not comprehend. When studying an animals personality characteristics it is very important to not limit the scope of the research to just primates. Opening the scope to which the environment they live in and their daily interactions have also proven to be important when studying their traits. The reasoning being the environment in which the grow up in can change how the react to certain situations. The same analogy related to humans as well. Human daily interactions can constantly fluctuate causing not one single person to act alike. Also, the environment someone grows up in plays a big part when determining why they may act a certain way that is different from the norm. This research paper opened my eyes to the world of science, and it filled me with more research ideas that I would love to pursue in the near future. Restating my previous theory before, I do believe that DNA plays a significant role in determining personality traits in humans, and animals.
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