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| ====== Biological Basis of Sexuality PowerPoint ====== | ====== Biological Basis of Sexuality PowerPoint ====== | ||
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| ====== Overview of Human Sexuality ====== | ====== Overview of Human Sexuality ====== | ||
| Human Sexuality is the way in which we experience and express ourselves as sexual beings. Your awareness of yourself as a female or male is a part of sexuality as well as the capacity to have erotic experiences and responses. Knowledge of gender roles in culture also influences sexuality. Human sexuality is studied by biologists, medical researchers, sociologists and psychologists (Pearson Education, n.d.). All fields make a contribution because sexuality is influenced by the interaction of biological, psychological, social, economic, political, cultural, legal, historical, religious and spiritual factors. Human sexuality is a central aspect of being human throughout life and encompasses sex, gender identities, gender roles, sexual orientation, eroticism, pleasure, intimacy and reproduction. Sexuality is both experienced and expressed in thoughts, fantasies, desires, beliefs, attitudes, values, behaviours, practices, roles and relationships. Sexuality can include all of these dimensions, not all of them are always experienced or expressed (World Health Organization, 2018). | Human Sexuality is the way in which we experience and express ourselves as sexual beings. Your awareness of yourself as a female or male is a part of sexuality as well as the capacity to have erotic experiences and responses. Knowledge of gender roles in culture also influences sexuality. Human sexuality is studied by biologists, medical researchers, sociologists and psychologists (Pearson Education, n.d.). All fields make a contribution because sexuality is influenced by the interaction of biological, psychological, social, economic, political, cultural, legal, historical, religious and spiritual factors. Human sexuality is a central aspect of being human throughout life and encompasses sex, gender identities, gender roles, sexual orientation, eroticism, pleasure, intimacy and reproduction. Sexuality is both experienced and expressed in thoughts, fantasies, desires, beliefs, attitudes, values, behaviours, practices, roles and relationships. Sexuality can include all of these dimensions, not all of them are always experienced or expressed (World Health Organization, 2018). | ||
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| ====== Genes and Sexuality ====== | ====== Genes and Sexuality ====== | ||
| - | There has been extensive research done to determine if there are genes that lead to different phenotypic traits of sexuality. Most studies have focused on homosexuality in males, mainly because sexuality in females is typically more fluid, and thus males are easier to research. There are two main hypotheses that look into genes for homosexuality in males. | + | There has been extensive research done to determine if there are genes that lead to different phenotypic traits of sexuality. Most studies have focused on homosexuality in males, mainly because sexuality in females is typically more fluid, and thus males are easier to research. In addition, many researchers have concluded that male homosexuality is genetic and likely passed on from the maternal chromosomes. These conclusions have led to the following two main hypotheses that look into genes for homosexuality in males. |
| ==== Man-Loving Genes Hypothesis ==== | ==== Man-Loving Genes Hypothesis ==== | ||
| - | This hypothesis was first thought of by an Italian researcher named Andrea Camperio-Ciani. He and his colleagues found that there is "evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity" (Camperio-Ciani, Corna, & Capiluppi, 2004). This "Man-loving gene" is inherited by the son through the X chromosome of the mother, and is found to code for attractiveness to males. The gene survives as a result of increasing female's fitness, as she is more attracted to males, and therefore creates more offspring (Camperio-Ciani, Corna, & Capiluppi, 2004). In some cases, this gene is passed onto her son and leads to homosexuality. Other supporting evidence for this hypothesis shows that homosexual men have more relatives in general on the mother's side, suggesting her increased fitness again (Camperio-Ciani, Corna, & Capiluppi, 2004). | + | This hypothesis was first conceived by an Italian researcher named Andrea Camperio-Ciani. He and his colleagues found that there is "evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity" (Camperio-Ciani, Corna, & Capiluppi, 2004). In other words, they hypothesized that women who are very sexually active have a man-loving gene which makes them more sexually attracted to men and results in more offspring. This "Man-loving gene" is inherited by the son through the X chromosome of the mother, and is found to code for attractiveness to males. They believe this gene to be X-linked because of the observation that homosexual sons had more homosexual male relatives on their mother's side of the family compared to their father's. The gene survives as a result of increasing female's fitness, as she is more attracted to males, and therefore creates more offspring (Camperio-Ciani, Corna, & Capiluppi, 2004). In some cases, this gene is passed onto her son and leads to homosexuality. Other supporting evidence for this hypothesis shows that homosexual men have more relatives in general on the mother's side, suggesting her increased fitness again (Camperio-Ciani, Corna, & Capiluppi, 2004). The "Man-loving gene" would be a disadvantage in terms of reproductive fitness for the son, since he will not be passing on his genes to his offspring. However, the researchers believe that the benefit to the mother's fitness outweighs the cost to the son's fitness, so the gene is able to persist. |
| ==== Chromosomes Xq28 and 8 ==== | ==== Chromosomes Xq28 and 8 ==== | ||
| - | <box 45% round | >{{ :chrom_x_8.png?400 |}}</box| Figure 1: Text > | + | <box 45% round | >{{ :chrom_x_8.png?400 |}}</box| Figure 1: Chromosome regions of interest: Xq28 and 8q12 (Sanders et al., 2015).> |
| - | A 2015 study by Sanders et al. looked at chromosomes X and 8 (Figure 1) and whether or not they may influence the male phenotypic trait of homosexuality. The study consisted of 351 families, specifically 908 genotyped family members. Of these 908 members, 793 were homosexual brothers, 33 were heterosexual brothers, 49 were mothers and 33 were fathers. Among these individuals, Sanders and colleagues calculated allele frequency estimates using a deCODE genetic mapping system to discover common SNPs among these individuals (Sanders et al., 2015). It was discovered that common SNPs throughout homosexual brothers were found on chromosomes X and 8, specifically regions Xq28 and 8q12 respectively. Therefore, "it is likely that genes contributing to variation in male sexual orientation reside in these regions" (Sanders et al., 2015). It was important in the study to look at the genotype of mothers and fathers, as it was noted that the Xq28 SNPs were more common among mothers than fathers, suggesting a correlation with the Man-loving genes hypothesis (Sanders et al., 2015). | + | A 2015 study by Sanders et al. looked at chromosomes X and 8 and whether or not they may influence the male phenotypic trait of homosexuality. The study consisted of 351 families, specifically 908 genotyped family members. Of these 908 members, 793 were homosexual brothers, 33 were heterosexual brothers, 49 were mothers and 33 were fathers. Among these individuals, Sanders and colleagues calculated allele frequency estimates using a deCODE genetic mapping system to discover common SNPs among these individuals (Sanders et al., 2015). It was discovered that common SNPs throughout homosexual brothers were found on chromosomes X and 8, specifically regions Xq28 and 8q12 respectively (Figure 1). Therefore, "it is likely that genes contributing to variation in male sexual orientation reside in these regions" (Sanders et al., 2015). Figure 2 illustrates the logarithm of odds (LODs), which shows the likelihood of SNPs on a chromosome being inherited together, and the corresponding chromosome. The two chromosomes of interest - chromosomes 8 and 23 (or chromosome X) - show distinct peaks, indicating that the SNPs on the respective chromosomes will be inherited together. It was important in the study to look at the genotype of mothers and fathers, as it was noted that the Xq28 SNPs were more common among mothers than fathers, suggesting a correlation with the Man-loving genes hypothesis and strengthening the association between homosexuality and the X chromosome (Sanders et al., 2015). |
| <box 50% round | >{{ {{ :lod.png?400 |}}</box| Figure 2: LOD values depicting the common SNPs inherited at chromosomes 8 and 23 (Sanders et al., 2015) > | <box 50% round | >{{ {{ :lod.png?400 |}}</box| Figure 2: LOD values depicting the common SNPs inherited at chromosomes 8 and 23 (Sanders et al., 2015) > | ||
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| Research has shown that the fetal brain during the intrauterine period initially develops towards the male direction. This direction of development in directly influenced by testosterone on the developing nerve cells. According to this evidence, our gender identity (belonging to the male or female gender) and sexual orientation should be programmed into our brain while still within the womb (Savic, Garcia-Falgueras & Swaab, 2010). However other research have shown that sexual differentiation of genitals does not take place until the first and second months of pregnancy and that the sexual differentiation of the brain does not start until the second half of pregnancy. This means that both of these events can be influenced independently resulting in transsexuality. This also means that in the event of ambiguous sex at birth, the degree of masculinisation by the brain may not be reflected by the genitals (Hughes et al., 2006; Swaab, 2004). | Research has shown that the fetal brain during the intrauterine period initially develops towards the male direction. This direction of development in directly influenced by testosterone on the developing nerve cells. According to this evidence, our gender identity (belonging to the male or female gender) and sexual orientation should be programmed into our brain while still within the womb (Savic, Garcia-Falgueras & Swaab, 2010). However other research have shown that sexual differentiation of genitals does not take place until the first and second months of pregnancy and that the sexual differentiation of the brain does not start until the second half of pregnancy. This means that both of these events can be influenced independently resulting in transsexuality. This also means that in the event of ambiguous sex at birth, the degree of masculinisation by the brain may not be reflected by the genitals (Hughes et al., 2006; Swaab, 2004). | ||
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| ==== Programmed Gender Identity is Irreversible ==== | ==== Programmed Gender Identity is Irreversible ==== | ||
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| It has been previously proposed that homosexuality could be maintained through kin selection. Instead of reproducing directly, homosexual individuals would invest in the children of close relatives through resource provision and child care, thereby increase their kin's chance of survival and reproduction as a result. This would enable them to indirectly pass on their genes through their kin. However, when the kin selection hypothesis of homosexuality was tested by David Bobrow and Michael Bailey (2001) and once again by Rahman and Hull (2005), it was not supported. Among the different measures of kin altruism, Bobrow and Bailey (2001) and Rahman and Hull (2005) used similar variables, such as: familial affinity, willingness to provide financial and emotional resources, and benevolent tendencies (such as the willingness to babysit). <box 35% round left|>{{:samoa.jpg?350|}}</box|Figure 6: Samoan fa'afafine > Instead of the anticipated findings of homosexual male relatives providing significantly more support to their closely-related relatives kin, they instead found no significant differences between heterosexual and homosexual men (Bobrow & Bailey, 2001; Rahman & Hull, 2005). These findings contradict the theory of homosexuality being maintained through kin selection. | It has been previously proposed that homosexuality could be maintained through kin selection. Instead of reproducing directly, homosexual individuals would invest in the children of close relatives through resource provision and child care, thereby increase their kin's chance of survival and reproduction as a result. This would enable them to indirectly pass on their genes through their kin. However, when the kin selection hypothesis of homosexuality was tested by David Bobrow and Michael Bailey (2001) and once again by Rahman and Hull (2005), it was not supported. Among the different measures of kin altruism, Bobrow and Bailey (2001) and Rahman and Hull (2005) used similar variables, such as: familial affinity, willingness to provide financial and emotional resources, and benevolent tendencies (such as the willingness to babysit). <box 35% round left|>{{:samoa.jpg?350|}}</box|Figure 6: Samoan fa'afafine > Instead of the anticipated findings of homosexual male relatives providing significantly more support to their closely-related relatives kin, they instead found no significant differences between heterosexual and homosexual men (Bobrow & Bailey, 2001; Rahman & Hull, 2005). These findings contradict the theory of homosexuality being maintained through kin selection. | ||
| - | However, researchers have identified real-world evidence of kin selection within the Samoan population. Researchers have demonstrated that Samoan homosexual men (known locally as //fa'afafine//) exhibit significantly higher altruistic tendencies toward nieces and nephews compared to Samoan women and heterosexual men. Vasey & VanderLaan (2010) used money given to, and received from, oldest and youngest siblings' sons and daughters as a behavioural assay of kin altruism. This enabled them to measure the altruistic behaviour directed to nieces and nephews from their respective relatives. Once the data had been collected, they compared the results to female relatives and heterosexual male relatives. | + | However, researchers have identified real-world evidence of kin selection within the Samoan population. Researchers have demonstrated that Samoan homosexual men, known locally as //fa'afafine// (Figure 6), exhibit significantly higher altruistic tendencies toward nieces and nephews compared to Samoan women and heterosexual men. Vasey & VanderLaan (2010) used money given to, and received from, oldest and youngest siblings' sons and daughters as a behavioural assay of kin altruism. This enabled them to measure the altruistic behaviour directed to nieces and nephews from their respective relatives. Once the data had been collected, they compared the results to female relatives and heterosexual male relatives. |
| The results showed that the //fa'afafine// relatives gave significantly more money, particularly to their youngest sibling’s daughters (Vasey & VanderLaan, 2010). These finds thereby support the kin selection hypothesis for contributing to the maintenance of the phenotype of homosexuality. These three studies also illustrate the ongoing debate with regards to the validity of the kin selection hypothesis with regards to its role in the maintenance of homosexuality. | The results showed that the //fa'afafine// relatives gave significantly more money, particularly to their youngest sibling’s daughters (Vasey & VanderLaan, 2010). These finds thereby support the kin selection hypothesis for contributing to the maintenance of the phenotype of homosexuality. These three studies also illustrate the ongoing debate with regards to the validity of the kin selection hypothesis with regards to its role in the maintenance of homosexuality. | ||
| ==== Pleiotropic Genes ==== | ==== Pleiotropic Genes ==== | ||
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| <box 50% round right | >{{ :miller.png?500 |}}</box| Figure 7: Normal curve of masculine and feminine-driven development in heterosexual, bi-sexual and homosexual males with respect to the number of individuals (Miller, 2000). > | <box 50% round right | >{{ :miller.png?500 |}}</box| Figure 7: Normal curve of masculine and feminine-driven development in heterosexual, bi-sexual and homosexual males with respect to the number of individuals (Miller, 2000). > | ||
| - | To illustrate this, Miller created a normal curve with masculine and feminine traits on the x-axis and number of individuals on the y-axis. If a male inherits multiple female-driven alleles, he then becomes either homosexual or bi-sexual, depending on the number of alleles inherited. However, these alleles are most often inherited along with other genes that steer the development towards male heterosexuality. In these cases, the effect of the alleles would be increased sensitivity, empathy, tender-mindedness, and kindness (Miller, 2000). Heterosexual individuals (particularly males, in this case) with these traits would make for better caregivers and fathers, and subsequently a more attractive prospective mate (Miller, 2000), thereby resulting in an increase in their fitness. Males who inherit none of these alleles are “hypermasculine” and lack sensitivity and other traits that make them better partners which decreases fitness. | + | To illustrate this, Miller created a normal curve with masculine and feminine traits on the x-axis and number of individuals on the y-axis (Figure 7). If a male inherits multiple female-driven alleles, he then becomes either homosexual or bi-sexual, depending on the number of alleles inherited. However, these alleles are most often inherited along with other genes that steer the development towards male heterosexuality. In these cases, the effect of the alleles would be increased sensitivity, empathy, tender-mindedness, and kindness (Miller, 2000). Heterosexual individuals (particularly males, in this case) with these traits would make for better caregivers and fathers, and subsequently a more attractive prospective mate (Miller, 2000), thereby resulting in an increase in their fitness. Males who inherit none of these alleles are “hypermasculine” and lack sensitivity and other traits that make them better partners which decreases fitness. |
| Miller (2000) hypothesizes the occurrence of a similar effect in genes that produce homosexuality in females. However, rather than empathy and kindness, it is believed that this would result in increased willingness to engage in non-committal sexual relations; thereby contributing to a mating advantage (Zietsch et al., 2008). It is in this way that Miller (2000) hypothesizes the alleles for homosexuality have been maintained: through a type of heterozygote advantage. | Miller (2000) hypothesizes the occurrence of a similar effect in genes that produce homosexuality in females. However, rather than empathy and kindness, it is believed that this would result in increased willingness to engage in non-committal sexual relations; thereby contributing to a mating advantage (Zietsch et al., 2008). It is in this way that Miller (2000) hypothesizes the alleles for homosexuality have been maintained: through a type of heterozygote advantage. | ||
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| In a female twin study conducted by Burri et al. (2015), monozygotic and dizygotic twins were asked to complete a survey regarding sexual behaviour and sexual orientation, as well as a follow-up survey assessing CGN (childhood gender nonconformity) – a measure for sex typicality. Zygosity of the twins was established using standardized questions about physical similarity and was confirmed by multiplex DNA genotyping in cases of uncertainty. In the first survey, a Kinsey-like scale was used to score sexual attraction, ranging from 1 (only with males, never with females) to 5 (only with females, never males) with the option of “no sexual attraction” for asexual participants. As a measure of fitness, lifetime number of sexual partners was employed by the researchers. | In a female twin study conducted by Burri et al. (2015), monozygotic and dizygotic twins were asked to complete a survey regarding sexual behaviour and sexual orientation, as well as a follow-up survey assessing CGN (childhood gender nonconformity) – a measure for sex typicality. Zygosity of the twins was established using standardized questions about physical similarity and was confirmed by multiplex DNA genotyping in cases of uncertainty. In the first survey, a Kinsey-like scale was used to score sexual attraction, ranging from 1 (only with males, never with females) to 5 (only with females, never males) with the option of “no sexual attraction” for asexual participants. As a measure of fitness, lifetime number of sexual partners was employed by the researchers. | ||
| The results of the experiment showed low CGN (“masculine”) scores were associated with greater nonheterosexual attractions (score of 2-5 on the scale) and more sexual partners (Burri et al., 2015). Additionally, heterosexual women that scored “masculine” CGN were also associated with more sexual partners (Burri et al., 2015). Overall, the researchers concluded that CGN was associated with female homosexuality, more masculine heterosexual women had greater numbers of lifetime sexual partners, and that these relationships are influenced by common genetic factors via a single shared phenotype with a heritability of 40% (Burri et al., 2015). These results are consistent with the theory that the genetic variation underlying this fitness-reducing trait—female homosexuality—might be maintained through its reproductive benefits (higher number of lifetime partners) in heterosexual individuals. These results thereby corroborate the previously-proposed Miller (2000) hypothesis. | The results of the experiment showed low CGN (“masculine”) scores were associated with greater nonheterosexual attractions (score of 2-5 on the scale) and more sexual partners (Burri et al., 2015). Additionally, heterosexual women that scored “masculine” CGN were also associated with more sexual partners (Burri et al., 2015). Overall, the researchers concluded that CGN was associated with female homosexuality, more masculine heterosexual women had greater numbers of lifetime sexual partners, and that these relationships are influenced by common genetic factors via a single shared phenotype with a heritability of 40% (Burri et al., 2015). These results are consistent with the theory that the genetic variation underlying this fitness-reducing trait—female homosexuality—might be maintained through its reproductive benefits (higher number of lifetime partners) in heterosexual individuals. These results thereby corroborate the previously-proposed Miller (2000) hypothesis. | ||
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| ======References====== | ======References====== | ||
