Genes and Genealogies

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Genes and Genealogies




When the “one drop rule” was implemented in the United States during the period of slavery to designate all people with African ancestry as “Negroes,” the question of descent often became a matter of life or death. People with even one African ancestor and no documents of “manumission,” which freed an ancestor from slavery, could be returned to slave status, frequently by means of a public auction. Before the Civil War, free blacks were in constant danger of being kidnapped, deprived of their documents, carried to another city, and sold back into slavery. On one notorious occasion, Black Seminoles on their way to Indian Territory under protection of a treaty were lined up in New Orleans and inspected, with darker-skinned people sold into slavery and the profits going to the U.S. government. Even after the war, the “one drop rule” was directed against people of color—blacks, Native Americans, and Asians—most notably in the 1924 Virginia Racial Integrity Act, the most vicious of the “Jim Crow” laws.

Another aspect of this genealogical approach to ancestry is represented in the idea of blood fraction. As the African slave trade developed in the seventeenth century, there was increased mating between black and white, and one’s worth as a slave depended on the fraction of white as opposed to black ancestry. A vocabulary was developed to describe ancestry in a genealogical manner. One could be half white (mulatto), one-fourth white (quadroon), or one-eighth white (octoroon), and other terms were invented to describe a nearly white appearance such as “High Yellow,” a designation applied especially to the mistresses of white aristocrats in southern cities, including the Texas woman memorialized in the song “The Yellow Rose of Texas.” One’s role in slave society, then, depended largely on color, so that lighter-colored slaves were expected to perform household work and skilled trades, while darker-colored slaves were field hands and manual laborers who performed more exhausting and dangerous work.

The genealogical idiom, then, incorporates at least two mistaken ideas: (1) that each person is equally descended from each parent and (2) that a person can retain an indelible marker of racial origin, characterized in this context as “one drop of blood.” This idiom becomes even more complicated, and more mistaken, in the calculation of “blood quantum” among Native Americans.


Genealogy has been the most traditional method of describing ancestry, with people recounting the identity of their parents, their parents’ parents, and so on back in time as far as they could go. Before writing was invented, about 3000 BCE, people had to rely on memory to recall their ancestors, often appointing special persons to remember everyone’s ancestry. Native Hawaiian society, for example, is notable for maintaining a class of historians who could recite genealogies reaching hundreds of years into the past. Many societies in Africa and South America supported similar specialists. Without some special effort of this kind, “genealogical amnesia” usually sets in, so that most people even now cannot recall the names of all eight of their great-grandparents without help—and those ancestors are only three generations back.

Within “clan” or “unilineal” societies, which preceded complex societies around the world and are still maintained by hundreds of “tribal” or small-scale societies, these genealogies have been crucial in determining access to property, political office, and religiousroles. Insome “unilineal” societies, it was necessary to remember only the male or the female line, since property or privileges only passed through one line or the other. But in “bilateral” societies such as existed in Europe, tracing ancestry through both males and females was more common, and much more difficult, since in every ascending generation there were twice as many ancestors to remember. This takes the form of a geometric progression so that in a bilateral society, after five generations there are 2 + 4 + 8 + 16 + 32 = 62 ancestors in the family tree to be remembered. But after 300 years, or twelve generations, a person has an estimated 8,190 ancestors. Inverting the genealogy, and estimating that each person has two children, a single person emigrating to America in 1700, black or white, would have had an estimated 8,190 descendants by the year 2000.

While many people’s ancestors have been completely forgotten by history, the descendants of celebrated people often have their names recorded, or they constitute special clubs or societies of some sort, providing some notion of how extensive a person’s genealogy might be. For example, the names of Pocahontas’s descendants, since about 1614, are presently exhibited in three official volumes, comprising more than 30,000 people. At a more serious, religious level, the descendants of the Prophet Muhammad are said to constitute more than 30 million people, often designated by distinctive dress or title, although these figures are disputed among the various sects of Islam. King Edward of England is said to be the ancestor of 80 percent of living English people, while 90 percent of the people who lived before 1100 are said to be the ancestors of everyone now living, while the other 10 percent have died without issue (Olson 2002).

In the large genealogical picture, then, everyone is descended from everyone in the past thousand years, and applying the “one drop rule” to the human species means that everyone belongs to all races. Everyone has at least one ancestor among every “race” that has ever existed. If all humans are not literally brothers and sisters to one another, they are at least cousins—at least fortieth cousins to be exact. The genealogical method, then, does not provide a very good tool for differentiating among human beings or for tracing their migrations and histories.


The discovery of genes in the early twentieth century dramatically contradicted these notions of fractional ancestry and what was called at the time “descent by blood.” But to consider the advantages of genetic theory, we need to begin with a consideration of the difference between genes and alleles, a distinction that is frequently lost in popular and journalistic descriptions of modern genetics. Simply put, a gene is a location on a chromosome whose DNA transcribes the sequence of amino acids to build proteins that may, for example, relate to eye shape or color. An allele is one of the alternative sequences of DNA that then codes for a different color. That is, to simplify an example, the gene is for eye color, but the allele might be for black, brown, green, or blue eyes. Gene refers to the location; allele refers to the color. One cannot have a gene for blue eyes, but one might have an allele for blue eyes.

All humans have the same set of chromosomes and genes. Surprisingly, most of the alleles (the form of the genes) are also identical. However, each person is slightly unique in his or her alleles. Cavalli-Sforza, Menozzi, and Piazza’s (1994) massive survey of research in human genetics shows the worldwide distribution of alleles as known to him and his associates in 1994. Many more examples have been examined since then, but the general conclusion is the same—that all human societies on every continent comprise essentially the same genes, although the alleles of a gene might have a slightly different frequency in different populations.

From the fractional, genealogical perspective, biological information is in different packages, corresponding to one’s ancestors. If one is one-fourth from a grandmother, then one should have one-fourth of her biological characteristics. But genetic studies show that this is not the way it works. Although the genes are in different “packages,” the packages are chromosomes, each of which contains a string of thousands of genes. All of the genes on one chromosome are inherited together from parent to child, but the number of packages inherited from a single ancestor can be highly variant, beginning in the second generation. In the first generation a child inherits twenty-three chromosomes from each parent. But in the next generation it is possible, although only slightly, that a grandchild could inherit no chromosomes at all from a grandparent, receiving all its chromosomes from the other three grandparents. Although the genealogical method would designate all four grandparents as providing one-quarter ancestry, in fact one grandparent could provide from zero to twenty-three chromosomes. That is, a grandparent could provide from none to half the genetic material represented in a grandchild.

Concerning the “one drop rule,” a particular person’s genetic contribution to a lineage might be wiped out after only two generations, in defiance of the assertion that a particular ancestor has a permanent biological effect on all descendants. In sum, the one drop rule and the notion of fractional ancestry were both invented for social, economic, and political purposes and are only approximately related to human biology.

SEE ALSO Blood Quantum.


Brown, Stuart E., Lorraine F. Myers, and Eileen M. Chappel. 2003. Pocahontas’ Descendants. Baltimore, MD: Genealogical Publishing Co.

Cavalli-Sforza, L. L., Paolo Menozzi, and Alberto Piazza. 1994. The History and Geography of Human Genes. Princeton, NJ: Princeton University Press.

Malik, Ghulam. 1996. Muhammad: An Islamic Perspective. Lanham, MD: University Press of America.

Olson, Steve. 2002. “The Royal We.” Atlantic Monthly, May: 62–64. Available from

Porter, Kenneth W. 1996. The Black Seminoles: History of a Freedom-Seeking People, ed. Alcione M. Amos and Thomas P. Senter. Gainesville: University Press of Florida.

John H. Moore