If what sociologist Shyamal Kumar Das and his colleagues at Minot State University in the US found in their initial study is true, it might take just a drop of blood to get a broad hint as to whether a child will be straight or gay later in life.

The study — involving 7,000 college students in the US and Canada — picked up possible predictive connections between sexual orientation and blood type. The researchers found that the chances of gay men having the blood type A are less than in heterosexual populations, whereas in the case of lesbians it is higher. It was also found that the proportion of those with Rh negative blood was unusually high among homosexuals.

However, both Das and his senior colleague Lee Ellis are very guarded about their findings which appeared in the February issue of the Archives of Sexual Behavior.

“I would take caution before generalising, since this study is not replicated. It would thus be premature to come to a conclusion right now,” said Das when asked whether the blood type and Rh factor can partly point to a person’s sexual orientation.

“Our interest in the blood type and Rh factor was mainly because these are genetically determined. We suspected that sexual orientation is also genetically influenced,” observed Ellis.

“And we reasoned that maybe we would find that sexual orientation is linked to the blood type and/or Rh factor.”

As the genes responsible for the blood type and Rh factor are found on chromosomes 9 and 1, respectively, the scientists concluded that some genes that influence one’s sexual preference may also be located on these chromosomes.

The findings seem to present yet more evidence — albeit to a modest degree — on the role of genes in the nature versus nurture debate vis-à-vis the origins of sexual orientation.

A couple of years ago, Brian Mustanski, a researcher at the University of Illinois in Chicago, along with his co-workers from the US National Institutes of Health, found that a large number of homosexual males had identical stretches of DNA on three other chromosomes — chromosomes 7, 8 and 10. “There is no single gay gene. Sexual orientation is a complex trait, and so it’s not surprising that we found several DNA regions involved in its expression,” Mustanski had said.

Mustanski, too, has a paper in the latest issue of the Archives of Sexual Behavior which focuses on the subject of sexual orientation. The paper — co-authored with psychologist Stacie Miller of Binghamton University, US — looks at the correlation between what scientists call developmental instability (differences in the size of organs that occur in pairs) and male sexual orientation. For example, those between the length or breadth of the right and left ears, between the breadth of the ankles, and the ratio between the lengths of two fingers — the index finger and the ring finger — which is often called digit ratio or 2D:4D ratio. The study showed that, at least in men, if there are three or more such asymmetries, there is a tendency towards homosexuality.

In most people, the index finger is slightly shorter than the ring finger, but at least in the right hand, the difference is accentuated by higher levels of androgens — male sex hormones like testosterone — during foetal development. Typically, in women, the two fingers of the right hand are nearly of the same length. In men, the index finger is obviously shorter.

Though the connection between digit ratio and sexual preference was speculated on for long, it was in 2000 that Marc Breedlove, then at the University of California, Berkeley, showed that lesbians on an average have a higher masculine digit ratio than straight women do. Though it was questioned in the beginning, many labs confirmed the link subsequently. The reason, as it turned out, was that lesbians are exposed to more prenatal testosterone than are heterosexual women.

Another paper in the journal, authored by Heino F.L. Meyer-Bahlburg of Columbia University and her colleagues, reports that women who suffer from a deficiency of the enzyme 21-hydroxylase are seen to have an excess production of androgen while still in the mother’s womb. This is because when the enzyme — which is crucial for two other major hormones, cortisol and aldosterone — is deficient, “the precursor hormones for cortisol and aldosterone go into a third ‘production line’ of the adrenal glands (two endocrine glands above the kidneys), which yields another male sex hormone called androstenedione, which subsequently gets converted into testosterone,” says Meyer-Bahlburg.

The role of genetics in sexual preference may be deepening, but researchers are still not prepared to attribute it all to genes. “There is no single gene — or even a set of genes — that decides whether a person will be straight or gay. In fact, there are many social, psychological as well as biological factors that influence sexual preference,” said one expert.

Having said that, it can perhaps be said that there may be at least some individuals who are gay because of genetic factors leading to things like higher foetal androgen levels.