The first child was probably conceived in Paris while Jefferson was minister to France. Then in 1998, Dr. Eugene Foster, a retired pathologist in Charlottesville, Virginia decided to seek conclusive evidence about the relationship between Jefferson and Hemings. He knew that scientists could determine relationships between people by looking at their DNA, the genetic material passed from parents to offspring. He had also learned that scientists could trace a lineage of men by looking at their Y chromosomes. The X and Y chromosomes are known as the sex chromosomes.
Females have two X chromosomes.
Males have one X and one Y. Y chromosomes are useful for determining the male lineage in a family because they are always passed from father to son. Unlike other chromosomes, the Y chromosome changes very little from generation to generation. For example, male descendants of Thomas Jefferson would have Y chromosomes almost identical to that of the third president of the United States. Scientists look for specific sites on the Y chromosome that tend to vary among unrelated men and compare these sites to determine whether the men are related to a common male ancestor. To determine whether Sally Hemings' descendants are related to those of Thomas Jefferson, Dr. Foster first had to find living male descendants of both Hemings and Jefferson.