The U.S. Supreme Court ruling gutting race-conscious admissions last month placed a spotlight on legacy preferences in higher education, a practice that mostly helps white and wealthy students.

That focus intensified on Tuesday with the news that the Education Department had opened a civil rights investigation into Harvard University’s legacy admissions policy. That inquiry will examine allegations by three liberal groups that Harvard’s practice of showing preference for the relatives of alumni and donors discriminates against Black, Hispanic and Asian applicants in favor of white and wealthy students who are less qualified.

Here’s a short guide to understanding legacy admissions.

What are legacy admissions?

Many selective colleges give a boost during the admissions process to the children or grandchildren of alumni, making them more likely to gain admission.

The exact number of colleges that use legacy preferences is unknown, but a survey by Inside Higher Ed in 2018 found that 42 percent of private universities — including most of the nation’s elite institutions — and 6 percent of public colleges used the strategy.

Critics have said for years that the century-old practice perpetuates privilege, and a handful of colleges, including Amherst and Johns Hopkins, have recently stopped using the preferences.

Others, including the University of California system, the University of Georgia and Texas A&M University, ended the practice after they were pressured by lawsuits and ballot initiatives to stop using affirmative action, according to a Century Foundation analysis.

Why do colleges use them?

Colleges say that legacy preferences help create an intergenerational community on campuses and grease the wheels for donations, which can be used for financial aid. Some college leaders have said that legacy preferences play a small role in admissions decisions and that the students who are admitted under the system are highly qualified.

A Harvard committee expressed worry in 2018 that no longer considering whether an applicant’s parent had attended Harvard could lead to diminished donations from alumni — financial support that helps “the diversity and excellence of the College’s student body.”

Jeremiah Quinlan, the dean of undergraduate admissions at Yale, said at a legislative hearing last year that a state ban on legacy admissions would undermine academic freedom and that he was skeptical that such a move would have a “material effect” on the number of low-income students or those from underrepresented groups.

“The process for selecting students for admission, together with the processes for hiring faculty and deciding which courses to offer, defines a campus community and culture,” Mr. Quinlan said in written testimony.

How have colleges and political leaders reacted to the recent developments?

The Supreme Court ruling put a renewed emphasis on fairness in college admissions. The high court voted 6 to 3 to reject affirmative action programs at Harvard and the University of North Carolina, a decision expected to lower the number of Black students at selective college campuses.

Since the ruling, a handful of colleges have dropped legacy preferences, including the University of Minnesota and Wesleyan University. In an interview, Michael S. Roth, Wesleyan’s president, said legacy status had played a “negligible role” in admissions but that it was becoming a distraction and “a sign of unfairness to the outside world.”

The opposition to legacy preferences is bipartisan. President Biden; Representative Alexandria Ocasio-Cortez, Democrat of New York; and Senator Tim Scott, Republican of South Carolina, have all spoken out against the practice.

In a concurring opinion in the ruling on race-conscious admissions, Justice Neil M. Gorsuch criticized preferences for the children of donors and alumni, saying they “undoubtedly benefit white and wealthy applicants the most.”

Even before the Supreme Court ruling, the practice was not popular with the public. A Pew Research Center survey last year found that 75 percent of those asked believed legacy status should not be a factor in college admissions.

What’s next?

Legacy preferences face an uncertain future on campuses.

After the Supreme Court decision, Mr. Biden said he would ask the Education Department to examine “practices like legacy admissions and other systems that expand privilege instead of opportunity.”

In the Harvard investigation, the Education Department’s Office of Civil Rights has a powerful enforcement authority that could eventually lead to a settlement with Harvard or set off a lengthy legal battle.

Nicole Rura, a spokeswoman for Harvard, said in a statement that the university was already reviewing the way it admitted students to ensure that it was in compliance with the law after the court’s decision and would continue to “strengthen our ability to attract and support a diverse intellectual community.”

Reporting was contributed by Anemona Hartocollis, Stephanie Saul and Michael D. Shear.