(CNN) -

Robin Roberts' battle against myelodysplastic syndrome, or MDS, is just beginning. The "Good Morning America" anchor will undergo chemotherapy before having a bone marrow transplant later this year.

"Bone marrow donors are scarce and particularly for African-American women," Roberts wrote Monday. "I am very fortunate to have a sister who is an excellent match, and this greatly improves my chances for a cure."

More than 10,000 people in the United States are diagnosed with blood-related disorders every year, according to the National Marrow Donor Program. Often the best treatment is a bone marrow transplant. During the procedure, a donor's stem cells are directly transfused into the sick patient's bloodstream. The patient's new cells multiply over time to create healthy bone marrow.

Unfortunately, the chance of finding a match on the national registry is as low as 66 percent for African-Americans and other minorities, compared with 93 percent for Caucasians.

Be the Match, the national registry, has 10 million potential donors, but only 7 percent are African-American. While the percentage is comparable to the overall African-American population in the United States (which is 12 percent), the registry is meeting only about a third of the needs for African-American transplants, said Dr. Jeffrey Chell, CEO of the National Marrow Donor Program.

It's a disparity that's come up time and again. Last year, the death of Shannon Tavarez attracted attention because doctors were unable to find a bone marrow match for the young Broadway star, who had acute myeloid leukemia.

"It's absolutely critical to have more people on the registry," Chell said. "You're more likely to find a match with someone who shares your common ancestry or ethnicity."

Be the Match tests the immune system's genetic coding to determine bone marrow compatibility. The human immune system has evolved over thousands of years, which is why racial and ethic background is so important. For instance, European-Americans' ancestors may have survived the medieval plague, while African-Americans could have a natural immunity to malaria because of their ancestors' environmental pressures.

If a good match isn't found, the donor's immune system will attack the sick patient's "foreign" cells in a condition called graft-versus-host disease.

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Roberts found a match; others won't be as lucky

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