The sickle cell gene carried by thousands of blacks comes in two forms that appear to have emerged separately in different parts of the world, a team of California scientist has discovered.

The form carried more commonly by American blacks comes from a relatively small area of West Africa that was handy to slavers, in what is now Ghana, Nigeria, Upper Volta, Benin and Togo.

The other form, which is found in black people of East and West Central Africa, evolved in a larger region covering East Africa, Saudi Arabia and India. These genes, scientists feel, probably cropped up independently in more than one place.

Sickle cell anemia distorts red blood cells and makes them clog blood vessels. The disease itself affects about 70,000 Americans, but eight American blacks in 100 carry the sickle cell gene or "sickle trait." If both parents are carriers, each child born to them has a 25 percent chance of having the disease, not just the trait.

So far no thoroughly effective treatment has been found, though in the past decade a concerted search for a cure has resulted in generally better treatment. Many victims die at an early age, and medical scientists, like the team at the University of California at San Francisco, are seeking every bit of knowledge available on the disease's history and characteristics.

Drs. Yuet Yai Kan and Andree Dozy of the California school's Howard Hughes Medical Institute examined the blood of American blacks, as well as that of East and West Africans, Saudi Arabians and three tribes in India. All of these peoples, as well as Mediterraneans such as Sicilians and Greeks, sometimes carry the sickle trait.

New methods of splitting and studying DNA -- deoxyribonucleic acid, every human being's basic genetic material -- helped the scientists identify two basic forms, essentially different sizes of the guilty gene.

The West African form apparently spread throughout North Africa to the Mediterranean. The other spread both westward in Africa and northward.

New genes -- mutants -- appear all the time among the body's plentiful supply. Most disappear because they confer no evolutionary advantage on the person who has them. Indeed, some are lethal.

Sickle genes apparently survived and spread because they gave the people who carried them an advantage against malaria. The sickle shape and strange chemistry of the red cells made it harder for malaria organisms to survive in the blood.

All this problem happened thousands of years ago. The usefulness of knowing it goes beyond mere understanding, Kan and Dozy explained in an article in this week's issue of the journal Science.

The same methods are being used at some medical centers to study fetal cells taken during amniocentesis -- sampling of the amniotic fluid during pregnancy. By this technique, originated by Kan, doctors can often tell with high accuracy whether a child of sickle-trait parents will be born with the actual disease.

The choice of whether to bear the child then is up to the parents, but in more cases than not, geneticists say, the method encourages parents who would not otherwise risk having childrend to have them successfully.

The new "molecular anthropology," say Kan and Dozy, should also lead to knowledge of the origin of other genes that cause inherited ills.