In Real Life, the causes of homosexuality are unclear. Though one hypothesis, as counterintuitive as it may seem, posits the existence of a "gay gene" that increases the chance of someone being gay.
If fiction, of course, it's up to the writer. Fairly often, you may see gay characters who have relatives who are also gay. In particular, if a gay character has kids with The Beard or via Homosexual Reproduction those kids are more likely to be gay than not.
Anime and Manga
- Tomoyo of Cardcaptor Sakura has an unrequited crush on her best friend and second cousin Sakura. It's later revealed that her mother had the exact same relationship with Sakura's mother.
- In the autobiographical comic Fun Home, lesbian Alison Bechdel comes to wonder if this was the case with her and her closeted gay father.
- America Chavez is a lesbian, much like her mothers. Her grandmother is also Ambiguously Gay.
- One old joke has a client going to a doctor about his attraction to men, and how his father, his brothers, his uncles, and his cousins also have that. When the doctor asks if there is anyone in his family who are not attracted to men, the client replied that his sister isn't.
- In Another Gay Movie Andy is gay and his father is either gay or bisexual (and his mom is a drag queen although the character is apparently female). Played for squick when the two nearly hook up in a public men's room. The sequel gets squickier when the two had anonymous sex, finding out the truth only after.
- In Cyteen Jordan is heavily implied (and outright stated in the sequel) to have a gay relationship with his male azi assistant Paul, his son Justin has a similar relationship with an azi named Grant. Of course, Justin is also a clone of his dad.
- Volle is completely gay but has two sons from an Arranged Marriage. Volyan the older son turns out straight but Yilon, the younger one, shares his father's preferences.
- In the Heralds of Valdemar series, Vanyel the last Herald-Mage, a notorious historical figure, was gay. He, however, had four children via being a Chosen Conception Partner. One of the gay men of the main time period is descended from Vanyel's first child.
- All descendants of the elves in Labyrinths of Echo tend to be bisexual, which includes the current ruling dynasty of Echo. The protagonist of the series is very surprised to learn that His Majesty Gurig VIII has both male and female "favorites".
- A Song of Ice and Fire: Prince Oberyn is openly bisexual and his second-eldest daughter, Nymeria, is attracted to women (it's not clear whether she's bisexual or a lesbian).
- In Call Me by Your Name, Elio's father not only susses out that Elio is bisexual and in a secret relationship with Oliver, but hints pretty strongly that he recognised the signs because he's also bisexual.
- Lightly parodied in Simon vs. The Homo Sapiens Agenda: after coming out to his family, Simon tries to deflect attention by accusing his sister Alice of having a secret boyfriend at college. She responds by asking him if he's sure she isn't hiding a girlfriend instead. She's joking - she's just worried about her family embarrassing her in front of her new boyfriend.
- The crew from Law & Order: Special Victims Unit once investigated the deaths of two HIV-positive gay men and discovered that one of them had infected a partner who later died from AIDS. They later discover that a gay activist was the likely culprit because his brother was the partner who died. One detective comments that it's likely their homosexuality was hereditary.
- One episode of Seinfeld reveals that Susan's father once had an affair with John Cheever. After she and George break up, she starts dating women.
- This trope was discussed in one episode of Louie; after Louie and his brother learn that their mother came out as a lesbian, his brother becomes anxious about it, as he believes that he might have inherited "gay genes" from his mom.
- Degrassi: The Next Generation: Paige's older brother, Dylan, is gay. Paige herself later enters a long-term relationship with a woman, although it's not really clear if this is a case of being bisexual or If It's You, It's Okay.
- Downplayed on Game of Thrones. Loras Tyrell is a known homosexual. His sister Margaery is Ambiguously Bi.
- Michael on Queer as Folk learns that his father is a drag queen.
- Will & Grace:
- Vince is a gay man whose sister is a lesbian.
- In The Revival, Jack's grandson is sent to "conversion therapy" when his ultra-religious mother fears that he's gay, partly due to her anxiety over this trope.note The kid's certainly got campy interests and mannerisms, but is really too young to say with certainty yet.
- Taken Up to Eleven with Fearghal from Come Fly With Me. He's one of eleven brothers who are all air stewards; they're also all gay (except the one who's bi).
- Mac from It's Always Sunny in Philadelphia, whose Armored Closet Gay status is a longstanding Running Gag for most of the series, has two gay cousins. The first one appears as an easily forgotten Bit Character from the pilot. The other one, "Country Mac", is presented as a Foil who acts like Mac but without all his bad qualities (including being open and confident with his sexuality). Mac's father has also displayed bisexual tendencies, although that might be a result of having spent years in prison.
- Val in Dream Daddy is romantically involved with a woman and has a bisexual father (her dad, Robert, is one of the male player character's possible love interests).
- Implied to be the case in The Sims 3 with Gobias Koffi and his deceased father, Gaylord.
- Sibling protagonists Kassandra and Alexios from Assassin's Creed: Odyssey are both bisexual, as are all of their potential love interests. (Justified by the Everyone Is Bi setting of Ancient Greece.)
- Damien from Monster Prom has two Happily Married dads and is attracted to men, women, everyone in-between, and anyone who exists outside the human gender spectrum altogether. Downplayed and Justified thanks to the Everyone Is Pansexual setting of the game.
- Umlaut House
- Rick and his father are both gay and came out on the same day even.
- Jake is bi but leaning toward male preference, and his dad commented that he thought he was gay before meeting Jake's mother.
- Volair and his son Pearce are bisexual.
- One of Amanda's daughters, Alice, is bi like her. The other one, Lain, is straight.
- In an aversion, Rhonda, Rick and Jake's genetically engineered daughter, is straight.
- That Cyanide & Happiness strip where a kid comes out to his parents, who both follow suit and break up.note
- In Homestuck, Dirk Strider is gay, and his ectobiological daughter Rose is in a long term relationship with a female alien. His ectobiological son Dave also turns out to be bisexual, meaning that everyone directly related to Dirk is non-straight in some way.
- Austen of Always Human has two daddies and she's interested in women. And Word of God says she's genetically related to at least one of her dads.
- In Shortpacked!, Mike claims that legalizing gay marriage is actually part of a secret plot to exterminate gay people, because being gay is at least partially genetic, and increasing societal acceptance means that less and less gay people will feel the need to stay in the closet by marrying beards and having children, therefore homosexuality will eventually be bred out of existence. Of course, this is Mike, so he almost certainly doesn't actually believe this and is just making it up to be an asshole.
- Academy of Merlin: There are three siblings — two boys and one girl — who are all gay.
- Combined with LEGO Genetics in an episode of Family Guy where Peter volunteers for a series of gene therapy experiments. The last of which gives him "the gay gene" and makes him gay for the rest of the episode.
- In 2015 there was a study that found genetic markers that may be related to homosexuality, which was covered in media as "finding the gay gene". However, the study had little statistical power, meaning that the correlation between the genetic marker and homosexuality is likely to be due to random chance.
- A number of studies estimate that around 30% of the variance in sexual orientation is explained by genetic factors. This does not mean that there's a 'gay gene', but rather that a large number of genetic variants may marginally increase or decrease one's propensity towards homosexuality. Notably, while this degree of heritability is significant, it's much lower than the heritability of physical traits like height and weight, or of mental traits like intelligence, openness, and extraversion.
- Various studies have put forward the role of pleitropy, the ability of a gene to present in multiple ways depending on various factors, as a contributor to determining a person's sexual orientation. It has been noticed that the female relatives of gay men often have an easier than average time getting pregnant and staying pregnant, suggesting that the gene or genes that cause homosexuality when it presents in men also increase fertility when it presents in women, explaining both how it would be passed down and why it would be preserved by evolution.
- This case in which a teenager decided to leave the closet as a bisexual in front of her mother ... and she responds by confessing that she is also bisexual.