The Emperor's New Chromosomes?

It took Henry VIII three wives and nearly 30 years before he got what he wanted: a son. Five-hundred years later, the same desperate desire for a prince is evident in Japan. Old habits die hard.

Although it's been strictly ceremonial since the end of World War II, the Imperial Household of Japan is the longest uninterrupted monarchy in history, spanning more than 1,500 years, perhaps more than 2,500. Lately, however, it's hit the same dry spell that plagued Henry, having failed to produce a male successor in more than 40 years.

When last year a government panel recommended that the current law be revised to allow females -- namely, the Emperor's granddaughter, young Princess Aiko -- to ascend the throne, the uproar was palpable. A majority of the Japanese population would be fine with Empress Aiko, but the issue remains divisive: a Tokyo rally against a potential female monarch drew 10,000 last month.

Meanwhile, some conservative politicians have appealed to biology. They believe that something quintessentially imperial would be lost if inheritance of the throne were opened to the other sex. Is there any foundation in genetics for this concern?

In principle, yes.

First, a word about the Y chromosome, that signature of maleness all boys receive from their fathers. The Y gets little respect: it is small in stature (and may be getting smaller) and harbors just 78 of males' 22,000-plus genes.

Because the Y is transmitted strictly from father to son, it is conceivable that the Y carried by the current Emperor's sons could date back to the presumptive Emperor Jimmu (circa 660 BC). Jimmu, it should be noted, was thought by some to have descended directly from the Sun Goddess (no Y chromosome there).

This idea of the Y as heirloom is not new. Several years ago, one of us (David Goldstein) studied the Y lineage of the ancient Jewish priests ("Cohanim"). Traditionally, Jewish priestly status was -- like the Japanese throne -- passed down from father to son. The Bible records Moses's brother Aaron as a high priest. When contemporary self-identified "Cohanim" were typed for unique DNA markers on their Y chromosomes, more than half of the presumptive priestly chromosomes were identical to one another, versus less than 10 percent of Jews who did not claim priestly descent.

At the time the study was published, many reports proclaimed that "Aaron's Y chromosome has been found." Oh really? Suddenly a modest genetic finding became "proof" of the scriptures.

Those who favor the status quo in Japan start from the somewhat more plausible premise that the Y chromosomes inhabiting the palace today truly are more or less the same as those belonging to the first Emperor. But even if we were to exhume ancient Emperor bones and extract their DNA, genetics would be hard pressed to prove complete continuity of the imperial father-to-son line.

In any case, as geneticists we would counsel caution. Sure, many of us feel a special connection to a historical figure or event because the quirks of chance have left us with our own personal molecular signature. How else to explain the bull market for molecular genealogy, where people send their DNA and hundreds of dollars to find out which African village, Lithuanian shtetl or Viking staging ground (supposedly) harbored their ancestors.

But does inheritance equate with character?

If the Japanese government is concerned with preserving the Emperor's Y chromosome, it's not for us to be outraged. But we would point out that since the Y accounts for less than one half of one percent of male DNA, it's hardly a real measure of who we are, even on a genetic level. Nor would we discount the social question of whether male-only succession has hurt the cause of women in Japan.

So where does this leave the Imperial Household? It was recently announced that Princess Kiko, 39, wife of the Emperor's second son, is pregnant. As the couple does not wish to know the sex of the child, a nation now holds its collective breath.

In September, if Princess Kiko somehow delivers a princess instead of a prince, a Y chromosome may be lost, but it seems to us that the Chrysanthemum throne would survive, maybe even enriched. One of the challenges for modern societies is to negotiate contemporary realities, maintain ties to the past, and yet somehow avoid the artificial seductions of genetic determinism.

Mel Brooks famously said, "It's good to be the King." Might it also be good to be the Queen? Maybe it's time to find out.