Is homosexuality genetic? Yes, this is a controversial topic which makes exploring it all the more interesting.
One of the main characteristics of living organisms is reproduction. The process of reproduction may vary from simple binary fission (in case of bacteria) or fragmentation (in Oscillatoria) to the complex process of sexual reproduction in higher multicellular plants and animals. In case of higher animals, sexual reproduction occurs by the process of fertilization which involves mating of the male and the female animal. In humans (and other mammals), internal fertilization occurs which requires a process of sexual intercourse in which the male sheds its sperms (in semen) in the female vagina which then travels through the female reproductive tract to fertilize the egg. Therefore, a sexual attraction between a male and a female is necessary prior to the intercourse for fertilization and subsequent reproduction. This sexual attraction in between a male and a female is referred to as "heterosexuality".
Homosexuality involves same sex attraction, i.e. attraction of a male for a male (gay) or a female for another female (lesbian). It is taken to be "unnatural" in a hetero-oriented society and is a much debated topic throughout the world, but study reveals that it's a natural phenomenon and is observed in many other animals as well. Dr. Alfred Kinsey and his colleagues (1948) developed a Heterosexual-Homosexual rating scale based on interviews of thousands of individuals about their sexual histories, which is commonly known as Kinsey scale. It is a seven point scale ranging from zero to six. Individuals scoring zero are exclusively heterosexual while those scoring six are exclusively homosexual, all the rest have varying degrees of bisexual characteristics1. Thus sexual orientation appears to vary as a gradient amongst individuals. A study in April 2011 revealed that 1.8 percent of the American population identified themselves as gay or lesbian and 1.7 percent were bisexuals2. The percentage seems to be even greater as obviously we do not expect everyone to be liberal enough to disclose it that openly.
Sexual orientation is very much dependent the genetic constitution of an individual. Every characteristic of a human is dependent on the expression pattern of several genes which rely on the interaction of genes and its environment and homosexuality goes without an exception. Homosexuality can be referred to as a polygenic trait and several phenotypic attributes are characteristic to homosexual men and women. Sex hormones in prenatal life is a prime determinant of homosexuality. For example, females born with Congenital Adrenal Hyperplasia (CAH) possess unnaturally elevated levels of male sex hormones and are very often lesbians. A heterosexual (straight) male being raised as a girl does not preclude his attraction towards a woman in adulthood which makes the social theory of sexual orientation really weak. There are differences in organization of the brain in between homosexual and heterosexual individuals. The psychology and cognitive abilities of gay men is similar to that of heterosexual females and that of lesbians is similar to heterosexual males3. Sexual orientation is determined at birth, it only gets expressed at adolescence when all other primary and secondary sex characters develop.
Tuck C. Ngun (2015) performed a study on the molecular basis of differences in sexuality among 37 pairs of identical male twins and found differences in the methylation patterns in nine different non-coding regions of the human genome revealing that these differences are "epigenetic"4.
A detailed study deploying 800 gay men showed that homosexuality is linked to five Single Nucleotide Polymorphisms (SNPs) located in two distinct regions of the human genome - one on the X-chromosome (locus Xq28) and the other on the pericentromeric region of chromosome 85. Xq28 was long known to be a marker influencing sexual orientation in males, often referred to as the "gay gene"6. Homosexuality is not inherited following Mendelian genetics and also do not occur in family histories as studied so far which is indicative that these epigenetic differences are sporadic and might appear during early development of the embryo. There is also an evidence of the involvement the mother's immune system in influencing homosexuality of the male fetus. Successive fraternal birth leads to progressive immunization of the mother with Y-linked minor histocompatibility antigens called H-Y antigens inducing the production of anti-H-Y antibodies that can pass through the placental barrier into the developing male fetus in subsequent pregnancies affecting various aspects of sexual differentiation in the brain triggering homosexuality7. Sexual attraction is an integral part of life, but a very little is known about its genetic and molecular basis which still needs a lot of investigation before we can come to a generalized definite conclusion.