Is homosexuality genetic?

The original question to which this answer was written was

-1- Is homosexuality a genetic disorder?

To that question, the answer is a definitive NO.

That question was merged into this question,

-2- Is homosexuality genetic?

To that question, the answer is a definitive YES.

I’m going to let the original answer stand. It becomes apparent in that answer that yes is the answer to this question.

A2A

Thanks for asking me, sort of. I say sort of because I’ve been asked this question in various forms many times[1][2][3][4][5][6] [7] and I was kind of over it. But I’ll take it that you haven’t been able to find those answers and genuinely want to know. So, here goes, again.

The three operative words in your question need to be addressed.

Homosexuality.
‘Homosexuality is romantic attraction, sexual attraction or sexual behavior between members of the same sex or gender. As a sexual orientation, homosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" to people of the same sex. It "also refers to a person's sense of identity based on those attractions, related behaviors, and membership in a community of others who share those attractions." Wikipedia note[1], Wikipedia note [2]’Homosexuality - Wikipedia.

I think you are using homosexuality in its ‘sexual orientation’ sense: an enduring pattern of emotional, romantic, and/or sexual attraction to members of the same sex. It’s often called same-sex sexual orientation.

From the first publication of the APA’s (American Psychiatric Association) DSM (Diagnostic and Statistical Manual) in 1953, homosexuality in it’s same-sex sexual orientation sense was listed as a mental disorder. Beginning in the late 1960s and continuing into the ‘70s, there was an intense debate within the APA over whether this was correct. In 1987, the APA removed homosexuality from the DSM-IV. The APA no longer considers same-sex sexual orientation a mental disorder. [8]
Disorder.

As you use it here, disorder means

“ An illness that disrupts normal physical or mental functions.”
disorder | Oxford Dictionaries.

Same-sex sexual orientation is not an illness. It is a normal variant of human sexuality.[9] (“Indeed, the latest and best scientific evidence shows that sexual orientation and expressions of gender identity occur naturally, and pose no threat to societies in which they are accepted as normal variants of human sexuality.”)

It is one variant on a continuum of variations from the wholly opposite-sex sexually oriented to the bisexually sexually oriented to the wholly same-sex sexually oriented person.[10]
Genetic.

Genetic as used here means

relating to genes or heredity.
genetic | Oxford Dictionaries.

There is scientific evidence that male same-sex sexual orientation is a heritable trait. It is therefore genetic.[11][12]

I’m going to quote extensively from this article[13] on the science of sexual orientation. (All the emphasis is mine)

‘Evidence points toward the existence of a complex interaction between genes and environment, which are responsible for the heritable nature of sexual orientation.
…
“[S]tudies have shown that homosexuality has both a heritable and an environmental component.
…
“Normally, all females have two X-chromosomes, one of which is inactive or “switched off” in a random manner. Researchers have observed that in some mothers who have homosexual sons, there is an extreme “skewing” of inactivation of these X-chromosomes. The process is no longer random, and the same X-chromosome is inactivated in these mothers.

“This suggests that a region on the X-chromosome may be implicated in determining sexual orientation. The epigenetics hypothesis suggests that one develops a predisposition to homosexuality by inheriting these epi-marks across generations.

“External environmental factors such as medicinal drugs, chemicals, toxic compounds, pesticides, and substances such as plasticizers can also have an impact on DNA by creating epi-marks.
…
“The academy found that a multitude of scientific studies have shown sexual orientation is biologically determined. There is not a single gene or environmental factor that is responsible for this—but rather a set of complex interactions between the two that determines one’s sexual orientation.
…
“However, more evidence is leading investigators to a specific region on the X-chromosome and possibly a region on another chromosome.

“The identification of these chromosomal regions does not imply that homosexuality is a disorder—nor does it imply that there are mutations in the genes in these regions, which still remain to be identified. Rather, for the first time, it suggests that there is a specific region on a chromosome that determines sexual orientation.”
An Evolutionary, Selected Trait.

Ben Powell’s thoughtful answer states at the end

“With respect to an environmental or evolutionary view, homosexuality is clearly aberrant, since homosexuality — unlike bisexuality — can not be an evolutionarily selected trait….”

This is incorrect. Homosexuality is not aberrant from a Darwinian evolutionary perspective. Quoting again from the same article:

“From an evolutionary perspective, same-sex relationships are said to constitute a “Darwinian paradox” because they do not contribute to human reproduction. This argument posits that because same-sex relationships do not contribute to the continuation of the species, they would be selected against.

“If this suggestion were correct same-sex orientations would decrease and disappear with time. Yet non-heterosexual orientations are consistently maintained in most human populations and in the animal kingdom over time.

“There also appear to be compensating factors in what is known as the “balancing selection hypothesis,” which accounts for reproduction and survival of the species. In this context, it has been demonstrated that the female relatives of homosexual men have more children on average than women who do not have homosexual relatives.”

Thus nature both designedly produces homosexuals and makes up for their lack of reproduction by causing female relatives thereof to be more fecund. This makes up for the progeny their homosexual siblings do not produce.