Global Study Examines Toll of Genetic Defects

The rate of defects per 1,000 births ranges from 82 in Sudan and 81 in Saudi Arabia at the high end, to 40 in France and 42 in Austria. The United States has the 20th-lowest rate, with 48.

Affluence is a big determinant of the risk. In low-income countries, the average rate is 64; in middle-income countries, 56; and in rich countries, 47.

Within those categories, however, there are widely divergent rates because of differences in culture and biology. For example, one reason that low-income Sudan and middle-income Saudi Arabia are at the top of the list is the high frequency of marriage between cousins.

Inbreeding produces a high rate of "recessive single-gene disorders," which arise when both the mother and father give their child the same rare gene. That is unlikely to happen unless the parents are related, or at least members of the same small ethnic group. Worldwide, the rate of such disorders is seven per 1,000 life births. In Sudan it is 26, and in Saudi Arabia it is 23.

Similarly, hemoglobin disorders such as sickle cell anemia and thalassemia are much more common in places where malaria was prevalent over the eons. The mutations that cause those ailments provide some protection against malaria when a person inherits only a single copy of the mutated gene, but inheriting two copies, one from each parent, results in disease.

Today, the rate of hemoglobin defects is 23 per 1,000 births in Benin in West Africa, a region of intense malaria transmission. The rate is a relatively high 2.3 per 1,000 in Greece, a country that has eliminated malaria but whose population still carries the protective thalassemia mutation at a high frequency.

The authors of the report hope it will spur countries to realize they can do something about birth defects. One of them, Arnold Christianson, said an effort is underway in his country, South Africa.

A recent survey there revealed that 70 percent of women older than 35 who go to clinics early in pregnancy are not asked their age or told they are at an increased risk of having a baby with Down syndrome, a chromosomal disorder that causes mental retardation.

Although most Down children have characteristic facial features, in a rural area outside Johannesburg, only 16 percent received diagnoses at birth, and only 50 percent by 6 months. And practitioners who did suspect the defect were often wrong. Of the babies sent for testing, 32 percent did not have the disorder, said Christianson, a geneticist at the University of Witwatersrand in Johannesburg.

With the help of a grant from the March of Dimes, he and his colleagues are developing a course to teach nurses and birth attendants the rudiments of medical genetics and how to examine newborns for common birth defects, including Down syndrome.