At Milton Academy, the tony Massachusetts prep school, the longstanding “one boy, one girl” rule requiring equal representation on the student council has been scrapped. The student government voted this month to move beyond what the council’s cochairman refers to as the “archaic norms” of male and female, and instead “accept the world and the people within it the way they are now.”

To which I say: Out of the mouth of babes.

To be sure, there is nothing archaic about the labeling of human beings as either male or female. The distinction between the sexes is objective and fixed, and written into the DNA of every individual. (In rare cases, congenital irregularities can cause an infant to be born with intersex ambiguity.) Nevertheless, Milton’s well-intentioned student leaders want to be as accommodating as possible toward students who are transgender or “grappling with their identities.” Hence the vote to repeal the student council’s gender-based quota proviso.

In practical terms, nothing will change at Milton, where the student body is divided equally between boys and girls, and no one has much difficulty telling one from the other. But let’s give the kids credit for a sound insight: As a matter of fundamental fairness, quotas should not be tied to rigid classifications that cannot be clearly defined.

Do gender quotas pose that problem? No. But racial quotas certainly do.

American society is awash with race-based quotas, check-offs, preferences, and diversity policies. In countless settings — from college admissions to workplace hiring, from government contracts to legislative redistricting — opportunities and benefits are tied to racial percentages. Twelve decades after Plessy v. Ferguson, the notorious Supreme Court decision in which eight justices upheld the constitutionality of racial segregation, Americans are labeled and sorted by race more obsessively than ever. It was in Plessy that Justice John Harlan delivered his ringing dissent: “Our Constitution is colorblind, and neither knows nor tolerates classes among citizens… . The law regards man as man and takes no account of his surroundings or of his color.”

Harlan’s fierce insistence that Americans are not to be treated differently on the basis of race became the great objective of the Civil Rights movement in the 20th century. “Racial criteria are irrational, irrelevant, [and] odious to our way of life,” argued Thurgood Marshall on behalf of the NAACP in 1950. “There is no understandable factual basis for classification by race.”

Marshall’s statement was even truer than he could have imagined. Today we know for a fact what scientists in the 1950s could only have surmised: Race is not biological. It is a social construct, not a genetic reality. The DNA of blacks cannot be distinguished from the DNA of Asians or the DNA of whites. Unlike our sex, which is stamped in our chromosomes, our racial and ethnic identities are purely subjective.

“I am an African American, but in parts of Africa, I am white,” says Stanford professor Duana Fullwiley, an anthropologist of science and medicine. When research in West Africa requires her to fly from California to France to Senegal, she told Harvard Magazine in a 2008 interview, “My race changes as I cross the Atlantic.” In the United States she is black; in France she is considered métisse, or mixed-race; in Senegal, everyone regards her as white.

Of course human beings vary widely in their appearance. Populations from different parts of the world differ notably in their skin color, facial features, and hair texture. But those distinctions are superficial, not racial. They have no immutable significance. They contribute no more to “diversity” than right- and left-handedness do. To rely on such criteria when hiring employees or drawing electoral maps or assessing a corporate board is about as sensible as consulting a Magic 8 Ball.

Racial definitions change constantly. The US Census Bureau has regularly revised the categories it uses to measure race. In 1890, census enumerators divided Americans into eight racial groups: “White,” “Black,” “Mulatto,” “Quadroon,” “Octoroon,” “Chinese,” “Japanese,” and “Indian.” The most recent census, in 2010, generated data for 63 racial categories — “six single-race categories and 57 different combinations of two or more races,” as a government press release announced at the time. Orlando Patterson, the prominent sociologist, has observed that federal immigration authorities used to classify Irish, Italians, and Jews as separate races.

With millions of Americans marrying across the color line and raising biracial or multiracial children, our ubiquitous affirmative-action check-offs and diversity quotas become more nonsensical — and unjust — by the day. The present fashion for treating “Hispanic” as a quasi-racial category exacerbates the problem. Hispanic identity is not a distinctive and heritable characteristic; it’s an ancestral affiliation that fades over time. That is why immigrants from Latin America commonly describe their identity with reference to their Hispanic origin, while a majority of their grandchildren call themselves simply … American.

What is true of Hispanics is true of other racial/ethnic groups. A study published last month by the National Bureau of Economic Research finds that when respondents are asked to specify their race, 96 percent immigrants from Asia self-identify as Asian. But that rate falls to 79 percent for their children, and only 58 percent for their grandchildren.

In the ways that matter most — how they label themselves, whom they marry — tens of millions of Americans have no use for racial pigeonholes. To turn those pigeonholes into litmus tests for employment or voting rights or college admissions is, as Thurgood Marshall said of Jim Crow, “irrational, irrelevant, [and] odious to our way of life.”

In a nation that aspires to meritocracy, quotas of any kind are an embarrassment. Racial quotas should be unthinkable. Our DNA has always known that race is only a delusion. When will our law and public policy catch up?

Jeff Jacoby is a columnist for The Boston Globe.