Doctors say the first application of a technique for isoalting human genes allowed them to diagnose a blood disease in a fetus and pointed the way to what may be a method for determining a host of other health problems in unborn babies.

The doctors said the technique permitted them to isolate a single gene among the thousands in a single human cell, and demonstrated the presence of a type of generally mild thalassemia, a blood disease, in the fetus of a Turkish woman.

But, they said, the method also can be used to detect the presence of a fatal form of thalassemia, and may in a few years be adapted to finding other, more common birth defects such as cystic fibrosis and sickle cell anemia.

The work, reported to be the first such isolation of a human gene, was done by a group of doctors and scientists headed by Dr. Stuart Orkin of Harvard and reported in this week's New England Journal of Medicine.

The report is the first example of using some of the tools of gene-cutting - called recombinant DNA technology - to find and understand hereditary defects while a child is still in the womb.

The technique was developed by scientists from Harvard, Yale and Haceteppe University in Turkey.

The team of doctors photographed the gene that directs the production of hemoglobin, which carries oxygen from the lungs to the blood and carbon dioxide back to the lungs. A lack of hemoglobin is called anemia.

They found that in two rare forms of anemia - which occur in people from Mediterranean and southeast Asian countries - the gene containing hemoglobin instructions is missing all or part of its DNA is a long molecule that has four main chemicals.

The only way previously to tell if an unborn baby had anemia was to take a sample of its blood, a risky procedure for the fetus, and look for the hemoglobin itself. The new technique allows doctors to take cells from the amniotic fluid surrounding the fetus in the womb and look for the gene containing hemoglobin instructions.

"Since DNA is such a tremandously long molecule, it's so big you can't work with it," Orkin said. "So you have to cut it up first."

Once the hemoglobin material was isolated in the DNA molecule, the scientists photographed it by attaching a piece of radioactive DNA exploded like tiny flashbulbs and the image was captured on a piece of X-ray film.

The picture that came out was a row of fuzzy - looking black bands. Babies with the thalassemia diseasewere missing one or more bands.

The doctors said that while two types of anemia are detectable by the method - alpha - thalassemia and betadelta thalassemia - the most common from of the disease in this country, beta - thalassemia or Cooley's anemia, cannot be detected by looking at the DNA.

Alpha - Thalassemia is fatal at birth. The other two types are milder but often require regular blood transfusions to make up for the lack of hemoglobin.