For the past decade you have been buying a product you perhaps know little about and may not feel you want. That product is a map of the human genome, and although some private funding is involved, a fair estimate of government funding for the Human Genome Project is about $6 billion.

Now, that's not near the cost of even a "small" war, and only a small fraction of the tax dollars going into the half-finished clutter of orbiting junk known as the International Space Station (I digress; that's for another column). Even so, as the late Senator Everett Dirksen used to say, a billion here, a billion there, and pretty soon you're looking at real money. Six billion dollars may become more real if I point out that you, along with every other man, woman, and child in this country, have a $24 investment in mapping the human genome. So it might be nice to know what it is, and just what you are getting for your money.

Before we can reach that point we have to back up a little. The word "gene" was first used in biology about a century ago, and most of us have a good working idea of what a gene does, even if we don't use the word itself. We say, "he has his mother's eyes," or, "she gets her height from her father," and we have in mind that certain characteristics can be passed along, like an intact unit, from parent to child.

Biologists have over the years discovered the mathematical rules (the "laws of genetics") that tell exactly how the things that are passed on can vary over the generations. However, 50 years ago no one knew what a gene was. It was just a shorthand word for something responsible for a particular physical property and the way that property was transmitted from parent to offspring. No one knew what a gene was made of, but knew it was needed as something without which you could not make sense of observed parent/offspring relationships.

We have come a long way since then. The basis for heredity is known to be DNA, a word familiar to everyone who has not been lost on Gilligan's Island for the past 30 years. The DNA that completely defines your physical structure is present in almost every cell of your body. It has the form of an enormously long molecular thread, shaped like a spiral, on which four kinds of molecular beads are strung. These beads are rather simple chemicals, thymine (T), adenine (A), cytosine (C), and guanine (G), and the varying order of the T, A, C, and G sites along the DNA molecule decides the nature of an organism.

Human DNA contains about 3.3 billion of these sites. Bacteria can have as few as five million, a flowering plant maybe a hundred million. In humans the complete string happens to be divided up into 46 pieces, called chromosomes, but for our purposes that doesn't matter. Nor does it matter that the spiral thread is actually a double spiral, with a matching T for every A, and a matching C for every G. What does matter is that we can, with a great deal of effort, read what is on the whole sequence of sites. If we did it for you, the result - the string of more than three billion A, T, C, G molecules in a particular order - would be your genome, unique to you.

And what is a gene? It is a particular piece of the whole DNA string, containing from 1,000 to 50,000 A, T, C, G molecules. Most importantly, a gene is a piece of the DNA molecule, which is passed on entire, intact, and unchanged from parent to offspring. Today we not only know what genes are made of, we can determine their precise composition.

The mapping of the whole DNA sequence, including every gene, is the goal of the Human Genome Project. It should be finished in the next couple of years, and it will not be the genome of a particular individual, but the genome of an amalgam drawn from several people.

With the mapping so far along, you might think that we would know the exact number of genes. We don't. The estimates range from less then 30,000 to more than 100 thousand. There is even a sweepstakes on the number, with a winner expected to be announced in 2003. The reason for the uncertainty is that our DNA, in addition to genes, contains loads of "junk DNA," sequences serving no useful purpose or some purpose as yet undiscovered. The genes themselves comprise only about three percent of the whole DNA string.

And now for the big question: What are we getting for our money? When the genome is mapped, what will we have gained?

The conventional answer is, we will understand and be able to prevent or cure a number of hereditary conditions, such as Down's syndrome, sickle-cell anemia, and Huntington's chorea. That's certainly true, but the broader and perhaps more frightening answer is that we will at last have the tools needed to understand, and, if we choose, to modify, the entire human organism. Would you like your kids taller, smarter, healthier, more handsome - and maybe with two heads? It's your choice.

The Human Genome Project will supposedly be completed in 2003. I suspect it will then be just beginning.

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