It's all about pleiotropy, baby!
May 02, 2002

I just came across No Watermelons Allowed site, and his statement on how he doesn't see that homosexuality could be genetic in origin. "... I suppose we could find some genetic component, but it's difficult to see just how something like this would be transmitted genetically," he writes.

Actually it�s not that difficult.

We're taught in high school biology the difference between recessive genes and dominant ones. Generally, recessives hang around in a personal genome until, when passed on to a child, they either get a matching gene from the other parent and wind up with enough strength to be dominant or are stuck all alone, waiting another generation for another chance. (Sometimes though only one copy is enough to be dominant.)

What�s not really well taught is that there is also a penetrance factor to certain genes. Not all genes are equally dominant� some are more dominant than others. Often, it requires many copies of a certain gene or groups of genes before their effect slips through the morass and is well known. If a gene is 5% penetrant, then there�s a 5 percent chance that the characteristic is codes for will be expressed. If you were to receive two or three or four copies of these genes, you can boost the chance of its effects being known accordingly. Or if you think about paint colors in a painting, a hint of blue can be masked but many more strokes of red or yellow or green. The blue is still there, and if you look hard enough you can see it, but the other colors are much brighter and more pronounced, essentiality "hiding" the color within its folds.

Penetrance is one of the reasons why identical twins can be discordant for traits, such as left handedness or diabetes. The odds didn't work out right in one sibling to have the characteristic expressed.

There are also epistatic effects that must be considered. Sometimes a group of genes work together to express a certain characteristic. All of them have to be �on,� active and working in order for the characteristic to appear. If something doesn't work out along the way, then the effect isn't seen. You also do have the possibilities of epigenetic characteristics � ones where both the genes and the environment contribute together to force a characteristic to express itself. Some researchers have suggested that schizophrenia is epigenetic, where people can be preloaded to develop the disease, but depending on the stability of the family they are born into the effects might never be known. There is also the effects of pleiotropy to be considered. These are genetic side effects, where genes that code for and express one certain characteristic have the effect of producing another characteristic that wasn�t the original intent.

Homosexuality can be the evolutionarily desirable characteristic that was selected for over time, sticking around in families until enough copies show up in an individual to wind up active. It could also be the side effect from some other evolutionarily advantageous gene or gene grouping. It could require a group of genes, freely available in the human genome, to line up together just right in order to appear. Or it could be a characteristic that hangs in the background until some outside factor triggers its activation. In any event, as with most everything about us, homosexuality is genetic in origin. Whether that etiology is simple dominant/recessive, epigenetic, epistatic, pleiotropic or penetrant, it has yet to be determined.

Posted by Jody at May 2, 2002 10:40 AM

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