Expert: Dana Krempels, Ph.D. - 5/5/2010

Question
Hi Dana,

I'm a physics undergrad. student. I have written to you once before too. I have layman understanding of biology.

What I really want to talk about is homosexuality and bisexuality. Do we really understand it scientifically?
And do we have enough empirical evidence for it?

Also, do we understand the evolutionary reasons for homo or bisexuality?

Are such sexualities found in other sexually reproducing animals too?

Thanks

Answer
Dear Metalrose,

Homosexuality and bisexuality in humans is not fully understood, but it is now fairly well established that this is a biological, at least partly genetic phenomenon, and not a personal choice.  A great deal of literature has been published on the topic of the origins and reasons for homosexuality, but there is so far no consensus, and certainly no one has discovered a "gay gene" that makes someone gay.  Homosexuality is likely something that is not only polygenic (i.e., affected by several different genes), but also--to some extent--determined by environment.

Some interesting trends that are now well-established:

1.  A man who has a maternal uncle who is gay is significantly more likely to be gay than a man who does not have a gay maternal uncle.

2.  The more sons a woman has, the more likely it is that her younger sons will be gay.

These suggest not only a genetic basis for male homosexuality in humans, but also possibly an *epigenetic* component, with the in-utero environment of a mother with many sons possibly determining the packaging of the DNA in her later sons.  But so far, this is hypothetical, and no one has yet discovered the genes responsible.

In case you are not familiar with genetic terminology, I'm going to insert a brief primer here.  If you already know this, just skip to the point ending with the double brackets:

[[A Genetics Primer

The physical expression of genes results in an organism's PHENOTYPE:  what it looks like, what enzymes it makes, how it behaves...essentially everything it is.  The phenotype is a result of the GENOTYPE, the genes it has in every nucleus of all its nucleated cells (i.e., its genome, which should be identical in every cell, though not all genes are expressed in every cell), interacting with its environment.

Every animal (including human) carries two copies of every gene.  Scientists now estimate that a human has about 30,000 genes in his/her GENOME (i.e, full complement of genes), and every human has two copies of that genome:  one from mom, and one from dad.  The two versions of each gene (called *alleles*) may be the same in a single person, or they may be different.  What does that mean?

A brief example.  Let's say that there's a human gene that codes for the shape of the forehead hairline.  There are two versions of the gene.  One, which we'll call "W", codes for a small "V" of hair to point down onto the forehead (Widow's Peak).  The other version, which we'll call "w", codes for a straight hairline.  In this case, the W allele of the gene masks the expression of the w allele.  The W is said to be *dominant*, and the w is said to be *recessive*.  So if every person has two copies of this gene, then the possible combinations are:

WW - Widow's peak
Ww - Widow's peak
ww - straight

Traits such as the color of hair, skin and eyes (and even more complex things, such as behaviors) are often not that simple.  Instead of being controlled by only one gene, such traits are each controlled by *several* different genes, each with two or more versions (alleles).  This means that the different versions can combine and interact in unpredictable ways to produce a wide range of phenotypes (physical appearance).

A trait that is controlled by several genes is called a POLYGENIC TRAIT. A polygenic trait is the expression of a single phenotypic trait that is affected by the action of more than one gene.

There are too many examples to list, since most traits are--at least to some degree--polygenic.  But human hair color, eye color, and skin color are among them.

One cute, easy-to-see example of a polygenic trait is the inheritance of fruit color in bell peppers, and it is a bit analogous to the human traits just named. There are at least three genes involved here, which we'll abbreviate as:

 * Y - timing of chlorophyll elimination (Y - early; y - normal)
 * R - color of carotenoids (R - red; r - yellow)
 * C - regulation of carotenoid deposition
  (C - normal; c1, c2 - lowered concentration)

(The capital letters indicate the dominant alleles; the lower case indicate various versions of recessive alleles.)

This leads to a few possible genotypes producing interesting phenotypes:
       o Y- rr c1c2 - pale yellow
       o Y- rr Cc2 - darker yellow
       o yy rr CC - green
       o Y- R- CC - red
       o yy Rr CC - purple
       o Y- Rr Cc2 - pale yellow

You can see what these look like here:

http://www.bio.miami.edu/dana/pix/bellpeppers.jpg

See?  It is a little bit like human color, but in this case there are only *three* genes involved.  Imagine how complicated things get when there are more than three genes, as there are in human hair, eye, and skin color!

The more genes involved in the expression/appearance of a trait, the more possible variations there are, and the more difficult (perhaps impossible) it becomes to guess what a baby will look like, especially if you don't know the exact genetics of the parents.  (Knowing the grandparents' phenotypes can help, but usually not very much.)]]

*****

As far as the evolutionary "reason" are concerned...those also hypothetical.  Some have suggested that the genetic components determining homosexuality may result in homosexuality when inherited in recessive homozygous condition (i.e., all the alleles are recessive), but may confer greater fecundity when inherited in heterozygous condition. Others have suggested a kin selection component, with the younger males serving as nurturers to their siblings while not competing with their older brothers for mates.  But no one really knows.

And yes, there are other species of vertebrates that engage in homosexual behavior, though we cannot really say what their *gender* is.  (The term "gender" is used for the sexuality one identifies oneself as, whereas "sex" is physical and biological--male, female or, rarely, intersex)  Most vertebrates that engage in homosexual behavior don't do it as a "lifestyle".  It's intermittent, and situational.  It's different in humans, who form long-lasting pair bonds, and are highly social animals.

Long answer to a short question, and still not many real answers!  But that's about where we are.

Hope that helps a little.

Dana