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Medical researchers, doctors and census-takers may lump Latinos or Hispanics into one group, but a giant study of Mexican genetics shows there’s really no such thing.

Mexicans themselves show such extreme diversity that someone from the dry north is genetically as different from someone from the Yucatan as a European is from an East Asian.

The findings, published in the journal Science, are important for medical research and healthcare, the researchers said.

"In this study, we realized that for disease classification it also matters what type of Native American ancestry you have,” said Carlos Bustamante, a professor of genetics at Stanford University.

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“In terms of genetics, it's the difference between a neighborhood and a precise street address."

Before the Spanish conquerers arrived in the 16th century, Mexico was inhabited by Native Americans who had carved out their own, often isolated, kingdoms. These genetic differences show up even today, the team at Stanford, the University of California, San Francisco and the Mexican National Institute of Genomic Medicine found.

"Mexico harbors one of the largest amounts of pre-Columbian genetic diversity in the Americas," said Stanford’s Dr. Andres Moreno-Estrada. "For the first time, we've mapped this diversity to a very fine geographic scale, and shown that it has a notable physiological impact on an important clinical trait: lung function."

They compared their genetic map to tests of lung function as measured by the volume of air a person can expel in one second or FEV1. They found a 7 percent difference in baseline FEV1 as they moved from populations in the western state of Sonora to Yucatan in the east.

"We were really fascinated by these results because we had expected that 500 years of population movements, immigration and mixing would have swamped the signal of pre-Columbian population structure," said Bustamante.

The differences hold even though most Mexicans are mestizos. “Today, the majority of Mexicans are admixed and can trace their ancestry back not only to indigenous groups but also to Europe and Africa,” the researchers wrote.

It will be important as medicine becomes increasingly tailored based on an individual’s genes. Their specific origin — not just a broad racial group — may provide crucial pointers.

"We can't just clump everyone together and call them European Americans or Mexican Americans,” added Dr. Esteban Burchard of UCSF.