Do you have the genetic makeup to excel in sprint events? Are you genetically programmed to be addicted to nicotine? Send off a mouth swab, cross someone's palm with silver and they will be eager to tell you. But are such predictions of any value whatsoever, or is this sort of genetic testing no more than entrails gazing dressed up as science. Could it be - whisper it - that genetic testing is about to replace astrology?

Let us be clear here about the type of genetic testing in question. We are not talking of the type offered by genetics clinics up and down the land, identifying single gene defects or defining tumour markers. No, this is an altogether different beast, the preserve of the internet and small ads - but one poised to be as addictive as Mystic Meg's predictions of a Sunday, and as lucrative for its exponents.

The tests offered are based on fact. So, for example, the gene ACTN3 is linked to sporting potential. Those with a normal variation produce a protein found in fast-twitch muscle fibres giving them an advantage in sprint events. Those with a variation called R577X are more suited to endurance events. And the dopamine gene (actually the D2 receptor gene, DRD2) has been linked with tobacco addiction. Here endeth the known and factual.

Compare and contrast with astrology. There are planets. Their position does shift about the sky a bit, I'm told. Moons come and go in a predictable way. Hereafter, fact is as water in sand.

Let me illustrate. You are told that because Uranus is rising, you, a Gemini, will be bowled over by a tall dark stranger. Intrigued, you spend your day searching out tall dark men, mentally filing them under T (as in To be bowled over by). Thus it is that you meet the swarthy loitering lothario of your dreams. Your confidence in the power of astrology as a predictor is increased, for we would rather share one spooky tale of success with our friends than a thousand of failure. Similarly, someone says you have tested positive for a gene variant for sporting ability. Encouraged, you buy new kit, practise every day instead of only when you have nothing better to do, and sporting prowess becomes a self-fulfilling prophecy. You are so pleased with yourself, you tell all your friends, reinforcing gene testing as a predictor in your and others' minds.

Almost every week someone pops up with a gene discovery: the shopping gene, the gay gene or even the maths gene. There might be some ghost of fact here but the problem with genes is their profound interaction with environment. What your genes might predict on paper may never be borne out in practice, it's the chalk and cheese of genotype and phenotype. Oh all right, I'll go quietly, the brie and cheddar of G & P.

Even if you do have a particular "genetically demonstrable" ability, what use is that knowledge when suggestion alone is enough to negate it. Consider for a moment that oft quoted maths gene. The work of social psychologist Claude Steele from Stanford University showed that when men and women did tests together involving difficult calculations, women performed less well than men because they assumed that their maths abilities were under fire. Told that gender would not affect their scores, these mathematically-accomplished women performed as well as men. It's called stereotype threat. Or in plain language, fear of confirming that which you fear most.

So suggestion rules your genes - never mind all the other interactions that may come between you and an A* at maths A-level. Like preferring music because Miss who teaches it is a babe, or taking up English just because you always liked reading.

One could dismiss these tests as worthless, but perhaps there is the basis here of a new industry harnessing the power of suggestion. The way it might work is this: compulsory genetic testing at age seven, aimed at identifying children who carry the maths gene. If my analogy with astrology works, it might mean that those who are told they don't have it give up. Those who have it find maths a whole lot easier.

So it's simple, isn't it? Test the nation but switch the results. Anyone without a hint of Pythagoras in their genes will be empowered, taking a "can do" attitude, whereas the ones with, manage anyway. My cunning plan might falter if enough people fail, despite clutching their certificate of certain ability. Or it might fall apart if someone reveals the truth about what's going on.

Oops. Have I just blown that one? Should have read my horoscope. "Your ruler has turned retrograde." I'll say.