A trait may be defined as a distinguishing quality or attribute of an organism that is expressed by genes and possibly modified by environmental influences (Trait). Variation in a trait may be due to genetic differences (such as alleles) or the result of environmental pressure, or both (Griffiths). The underlying genetic bases for some traits and their variants are known while others remain elusive.
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Pedigree analysis is a useful tool for investigating whether a trait can be inherited. The benefits of determining inheritance and its pattern (e.g., dominant vs. recessive, autosomal vs. X-linked) include increasing our understanding of disease and how to treat or prevent it, making predictions from genotype to phenotype in varying environments, and growth potential (Korte).
Arachnophobia, or irrational fear of spiders, is one of the most common phobias. Estimates of prevalence vary, but likely 3-7% of the population suffers from arachnophobia (Arachnophobia: Why People Are Scared of Spiders). There have been studies conducted to explore whether arachnophobia is a learned behavior as the result of a terrifying experience with spiders or genetically determined or both (Buddle). Conclusions from these studies are not consistent and so present the opportunity for further study (Buddle). A better insight into underlying causes of this phobia may suggest more effective treatment and prevention strategies.
As mentioned above, some research has attempted to address whether arachnophobia is due to genetic or environmental causes; however, no definitive basis has been found. Individuals suffering from this phobia are more likely to be female, but cause of this gender bias has not been determined (Buddle). The behavior also tends to run in families, although it is difficult to rule out conditioning in people who live in the same environment (Buddle). Hettema and colleagues studied identical and fraternal twin responses to spiders and snakes. They found significant genetic contributions to phobic behavior (Hettema). In addition, Dias and colleagues have demonstrated that exposure to a smell in mice, along with an electrical shock, can condition the animals to exhibit a phobic response to the smell. This response was shown to be heritable, due to epigenetic changes that were able to be passed down through generations (Dias).
The Bliss Family pedigree, spanning four generations, was established to investigate the heritability of arachnophobia. The hypothesis employed was if arachnophobia is inherited (passed down from parents to offspring), then it is autosomal recessive.
A pedigree for the Bliss family that included four generations was drawn. All members were identified by generation and number, and affected individuals were indicated by a shaded symbol. Arachnophobia was defined for the purposes of this study as an extreme fear of being near, touching, or manipulating spiders under any circumstances starting at least one year ago and continuing to the present time. In all cases, the affected individuals expressed a phobia that began very early in life and continued for many years, if not decades. The data were analyzed to determine trait inheritance, and if present, pattern of inheritance.
Figure 1 shows a family pedigree that spanned four generations. Individual IV 3 was affected by arachnophobia, but her maternal ancestry contained no members who suffered from the behavior. On her paternal side, five additional members, all female, were identified as being affected as shown in Figure 1. These individuals were found in every generation and appeared in a ratio of 1:2 to 1:3 (affected:unaffected).
The hypothesis that was being tested was if arachnophobia is inherited (passed down from parents to offspring), then it is autosomal recessive. Examination of this pedigree supports this hypothesis overall. It is reasonable to conclude from the pedigree that the individuals in generation IV who were arachnophobic (individuals 2 and 3) possessed the trait at least in part due to inheritance from the common grandmother II 4. Also, individual IV 3 was affected, but her parents were not, lending weight that it is recessive.
While the pedigree does support heritability of the trait, it does not show a perfect pattern of autosomal recessive inheritance (affected males and females approximately equal in number). The presence of the trait only in females is interesting. It is very likely that the trait is not X-linked recessive since X-linked recessive traits are only expressed in males or homozygous females. However, gender bias in expression of the trait in this pedigree does raise questions about whether the trait is influenced by some other sex-specific factor, such as a hormonal influence on gene expression, or other epigenetic influences.
The method for determining the presence or absence of the trait in any individual involved only asking about attitudes toward spiders (the members assessed themselves). This subjective assessment is a potential source of bias since one person’s understanding of what it means to have a fear of spiders could be very different from another’s. A more objective measure of arachnophobia would provide more reliable data regarding the trait in members of the pedigree. This might involve use of a standardized set of questions regarding fear of spiders or measurement of physiologic responses such as heart rate in people as they are exposed to spiders (Zsido and Knoft). Also, for those deceased members of the pedigree, presence or absence of the trait in those individuals was determined by the recollections of descendants which is arguably less accurate than if every individual of the pedigree were able to be assessed quantitatively for the presence or absence of the trait.
The results of this analysis are consistent with what has been published about the population as a whole regarding gender bias and the tendency for arachnophobia to run in genetically related groups (Buddle). Furthermore, Figure 1 supports the hypotheses that arachnophobia can be inherited and is autosomal recessive. Only females in the pedigree were affected which suggests potential future avenues of research such as exploring epigenetic influences.
Arachnophobia: Why People Are Scared of Spiders. (2014, July 21). Retrieved from https://www.spring.org.uk/2013/11/scared-of-spiders-5-psychological-insights-into-arachnophobia.php
Buddle, C. (2015, May 20). Why are we so afraid of spiders? Retrieved from https://www.independent.co.uk/life-style/health-and-families/features/why-are-we-so-afraid-of-spiders-10263450.html
Dias, B. G., & Ressler, K. J. (2014, January). Parental olfactory experience influences behavior and neural structure in subsequent generations. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/24292232
Griffiths, A. J. (1999, January 01). Human Pedigree Analysis. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK21257/
Hettema, J. M. (2003, July 01). A Twin Study of the Genetics of Fear Conditioning. Retrieved from https://jamanetwork.com/journals/jamapsychiatry/fullarticle/207570
Knopf, K., & Pessel, P. (2009, January). Individual response differences in spider phobia: Comparing phobic and non-phobic women of different reactivity levels. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/18781450
Korte, A., & Farlow, A. (2013, July 22). The advantages and limitations of trait analysis with GWAS: A review. Retrieved from https://plantmethods.biomedcentral.com/articles/10.1186/1746-4811-9-29
Trait. (n.d.). Retrieved from https://www.merriam-webster.com/dictionary/trait
Zsido, A. N., Arato, N., Inhof, O., Janszky, J., & Darnai, G. (2018, March). Short versions of two specific phobia measures: The snake and the spider questionnaires. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/29306023
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