Being gay being genetic is (obviously) possible

I am not a geneticist. I do, however, have both a GCSE in Biology and an interest in the propagation of sense.

Some people think that being gay can’t be genetic because it’s counter-evolutionary to want to have sex with people you can’t have babies with. Even some people who know quite a lot about genetics don’t always, when confronted, give a quick and simple explanation of how it’s possible for being gay to be maintained at a level of 10% in a population, even if:
• gay people have fewer children than non-gay people
• being gay has a purely genetic basis.

This post is my attempt at such a quick and simple explanation. I am not claiming that the proposed model reflects reality – it is radically simplified, uses the word "gene" where often "gene or combination of genes" would be more appropriate, it's highly symmetrical and it has very few variables. However, if I can come up with a simple model that works with relatively little effort, maybe it will help convince people that nature, which is quite complicated, can come up with a method that works too.

Right, my model is very simple and relies on two basic assumptions (as well as the aforementioned conditions):
• being gay is a result of someone having two particular genes
• these genes, when someone only has one of them, cause that person to have more children than if they don’t have either.

Genes having different effects when they are differently combined is well known and can be demonstrated by the following (also very simple) example: If there were a gene for getting lairy and a gene for being strong (both quite feasible), then people with both would get into fights and win them and people with just the lairy gene would get into fights and lose them. The lairy gene can therefore lead to both increased and diminished survival rates, depending on whether the person also has the strong gene or not.

In my model, I am assuming:
• a gene for being fighty (F) contrasting with a gene for being genial (G)
• a gene for being dirty (D) contrasting with a gene for being clean (C).

I apologise slightly for using stereotypical gay traits (being genial and clean), but this is not intended to offend and should help keep the model clear.

Given these genes, people can have one of four gene combinations, three of which lead to the person being straight and one of which causes the person to be gay. Additionally, some of the straight people are more attractive than the others.
In summary:
• clean and genial (CG) = gay
• clean and fighty (CF) = straight and normally attractive
• dirty and genial (DG) = straight and normally attractive
• dirty and fighty (DF) = straight and less than normally attractive.

Now, based on this setup, I want to see if a population can maintain a rate of 10% gay people (CG). As my dirty fighty people (DF) are also less attractive, there can be less of them as well, so for simplicity let’s say there are 10% of them too. This leaves 80%, which we can split 50/50 between CF and DG, in this case making 40% of each.
So to start with we have:
• CG – 10% (gay)
• CF – 40%
• DG – 40%
• DF – 10% (straight but not very attractive because dirty and fighty).

The next step is an obvious one. It translates the percentages into real numbers by assuming a population of 100 people so that we can test to see what happens when they start mating.

If we’re assuming gay people have fewer children than straight people, and the dirty fighty (DF) people also have fewer children than people who are only dirty or fighty, then most children will be had by CF and DG amongst themselves. And we want to find out what the proportions of parent combinations are within a self-sustaining population, which means remaining the same size (100) and with the same distribution (10, 40, 40, 10).

You could do this with simultaneous equations but I threw a spreadsheet together and fiddled with the parameters because I found it more fun.

I found that if each CF or DG person has 2.25 children with another CF or DG person, then this gives 80*2.25/2 children, = 90. If people have no preference for CF or DG, then:
• One third will have CF and CF parents
• One third will have DG and DG parents
• One third will have one CF and one DG parent.

This makes 30 of each. All CF-CF parents will produce CF children. All DG-DG parents will produce DG children. The CF-DG parents will produce:
• One quarter (7.5) CF children
• One quarter (7.5) CG children (gay)
• One quarter (7.5) DF children
• One quarter (7.5) DG children

So the 90 kids of CF and DG mating amongst themselves turn out as:
• 37.5 CF
• 37.5 DG
• 7.5 CG (gay)
• 7.5 DF

For the population to sustain itself, it needs 10 more children. This doesn’t seem unreasonable, as some gay people have children and some dirty fighty people might have children too. As the CF and DG people are the most attractive, we can assume that one of them will still be involved in the remaining 10 reproductions.

So, keeping things simple, we have 2.5 kids from each of the following four parent combinations:
• CG and CF
• CG and DG
• DF and CF
• DF and DG.

These 2.5 kids in each combination are then distributed as follows.
• CG-CF gives 1.25 CG (gay) and 1.25 CF
• CG-DG gives 1.25 CG (gay) and 1.25 DG
• DF-CF gives 1.25 DF and 1.25 CF
• DF-DG gives 1.25 DF and 1.25 DG

In total, that gives us 10 children, of whom:
• 2.5 are CF
• 2.5 are DG
• 2.5 are CG (gay)
• 2.5 are DF

Added to the children from earlier who had solely CF and DG parents, this gives us:
• 40 CF
• 40 DG
• 10 CG (gay)
• 10 DF

This is exactly the same distribution as we started with. So it can be done!

Clearly, this model is very simple and probably very different from how nature has managed to maintain its rate of having 10% gay people (or quite possibly more) within the human population.

However, I hope that my demonstration of how staggeringly simplistic such a system could be will finally put a stop to suggestions that it is entirely impossible, or even paradoxical, for being gay to have a genetic basis.

Because it’s so very possible that it’s actually true.

PS: It’s quite obvious, but worth a PS to mention that my model also shows that you don’t have to have gay parents to be gay yourself. Just like in real life.

PPS: I've just realised that it might have  been a bit silly to choose the word "genial" in a piece about genes. Hope no-one got too confused.