Prenatal hormones and the biological basis of sexual orientation

¶ … sexual development and maturity begins with the fertilization of the ova by the sperm. In a unique process, the DNA chain of the ova and sperm separates and connects with one another, creating an entirely new chromosome mixture. Determining the sex of the child happens in the last pair of the chromosome chain, with two Xs resulting in a girl and an XY resulting in a boy. Sexual maturation begins at puberty, when gonadotropin-releasing hormones secreted by the hypothalmus. These hormones are the primary controller of sexual dimorphism. However, what actually affects sexual orientation is an often hotly contested topic for debate. Although some believe it is nurture rather than nature, previous research has found that sexual orientation, such as homosexuality, instead may be due to prenatal exposure to androgens resulting in different brain structures.

The Process of Sexual Development and Maturity

Carlson (2007) states that sexual development and maturity begins with the fertilization of the ova by the sperm. In this process, the baby's chromosomal sex is determined. All human cells have twenty-three chromosomal pairs. Reproduction in these cells occurs by duplication of the chromosome chains exactly. However, when gametes are created they duplicate in an entirely unique way. The DNA chain of the ova and the sperm split into two and then combine with one another to make an entirely new strand of DNA, and a new individual. It is the combination of the last pair of chromosomes on the end of this new chain of DNA that determines if the infant will be a male or female.

Males have an X and a Y chromosome in their DNA. Females have two X chromosomes. When the DNA splits, half of the male's chain contains the X chromosome; the other half contains the Y chromosome. If the half with the X chromosome fertilizes with the female's X chromosome half, an XX sex chromosome is created and a female is produced. If the Y chromosome half is used, an XY chromosome is created and a male is produced (Carlson, 2007). In some instances, however, the process becomes corrupted.

Akodon azarae vole mice have occasionally demonstrated females with a chromosome sex pair that is indistinguishable from males, according to Bianchi (2002). These were XY sex reversed females. Nine Akodon species have shown this tendency for fertile heterogametic females. These heterozygous females, interestingly, exhibit a better reproductive performance than their XX counterparts, as a means of compensating for the non-viable YY zygotes.

The first of the three types of sex organs to develop are the gonads. The gonads not only produce sperm or ova, but they also are charged with hormone secretion. It isn't until a fetus is about six weeks old that differentiation between the sexes begins to occur. At that time, for males, a gene on the Y chromosome produces a protein that causes the males to develop testes. Hormones also affect the development of the fetus's sex organs, as well as their brain development, which will later be activated when the fetus begins maturing sexually, according to Carlson (2007). The internal sex organs, Wolffian system for boys and Mullerian system for girls, starts to form when the fetus is approximately three months old. Which system develops is dependent on whether or not the testes secretes hormones. If the male hormone dihydrotestosterone is present then the external genitalia becomes male. If there is no dihydrotestosterone secreted then the external genitalia becomes female.

The next step in sexual development happens at puberty. When the child reaches puberty, the hypothalmus secretes gonadotropin-releasing hormones. This stimulates the anterior pituitary gland to produce gonadotropic hormones. The gonads then produce luteinizing hormone and folicale-stimulating hormone that facilitate sexual maturing. Although the gonadotropic hormones are the same for both sexes, males primarily produce testosterone in the testes and females primarily produce estrodiol in the ovaries (Carlson, 2007).

The Interaction between Hormones, the Body, and Behavior, Including Sex

Hormones are critical to not only the sexual development, sexual maturation, and body development, as discussed earlier, but they also have an important role to play when it comes to behavior, including sex. Once again, it is hormones that once the gender is determined by chromosomes that instructs the body on how to proceed to produce sexual dimorphism in a species. Internal and external sexual organs are all determined by hormones. As also discussed, it is hormones that turn on and guide the sexual maturation process. Although both genders produce a small amount of the other's hormone, it is the gender-specific hormone that creates the masculinity of males and the femininity of females. It's not surprising that it is hormones that also control behavior, including sex.

Carlson (2007) discusses the three categories of male mammal sexual behavior -- intromission, pelvic thrusting, and ejaculation. Each of these behaviors is affected by hormones. Oxytocin facilitates orgasm. Prolactin is the hormone that affects the refractory period in males. The hormones of progesterone and estradiol, according to Carlson, as well as Agmo, Choleris, Kavaliers, Pfaff, and Ogawa (2008), affect a females receptivity and proceptivity to sex, as well as their attractiveness to males.

Impact of Environment on Sexual Orientation

The debate of nurture vs. nature when it comes to sexual orientation has waged for decades. Quoting previous research by Bell, Weinberg, and Hammersmith, Carlson (2007) notes that their research did not find any correlation between an individual's environment and their sexual orientation. Coghlan (2008) confirms these findings with his study that found that homosexual men had symmetric brains, like heterosexual women, and homosexual women had asymmetrical brains, like heterosexual men. As research into this topic continues, more and more studies support the fact that environment has very little, if anything at all, with sexual orientation. Instead, it is the physiological differences in the brain that are present from birth.

Conclusion

Sexual development and orientation are fascinating areas of study, as they effect every single living being on the face of the planet. From the moment the two strands of DNA combine to form a new identity and determine the gender of a new life, sexual development begins. Hormones are the instigator, organizer and conductor of the process. From the development of physical organs to the catalyst for sexual behavior, it is hormones that are behind it all. Even sexual orientation may be affected by hormones. Rather than environmental factors, as some may believe, instead hormonal effects on brain structure development may actually be the key to understanding sexual orientation.