Considerable interest in the ancestor tables of presidents has developed in the past few years (note the several articles on this subject in The American Genealogist) and with the recent publication of Ancestors of American Presidents (1989) by Gary Boyd Roberts, now in its first revised edition, published in December 1989 by Carl Boyer 3rd of Santa Clarita, California. A comparative study of the inbreeding of our presidents could be interesting, since inbreeding is something we can calculate from these ancestor tables.
Briefly one measures inbreeding with an inbreeding coefficient, a fraction which ranges from 0.0 (zero) to 1.0 (one). A person’s inbreeding coefficient is a fraction which estimates the probability that he or she received the same allele for a given gene through both parents, because the allele was derived from their common ancestm. Put another way, an inbreeding coefficient is that fraction of a person’s genes where identical contributions have come from both parents because of their descent from a common ancestor or ancestors. An inbreeding coefficient of 0 (zero) means that a person is not inbred at all, and an inbreeding coefficient of 1 (one) means that a person is completely inbred. But humans would rarely approach complete inbreeding. No doubt the average inbreeding in the U.S. is well below the 0.00006 which would result from a sixth-cousin marriage. Examples of high inbreeding in humans would be 0.25 from the union of a brother and sister or from a parent and child.
Inbreeding, then, depends on the relationship of a person’s parents. So, if a mother is inbred, but is not related to the father, there is then no inbreeding in the child, and vice versa, because the parents are not related. For example, President Kennedy’s mother Rose Elizabeth (Fitzgerald) Kennedy was a child of a second-cousin marriage (D.K. Goodwin, The Fitzgeralds and the Kennedys , pp. 88-89, 827). Mrs. Kennedy’s inbreeding coefficient is 0.0156, but her son President Kennedy was not inbred because we know of no relationship between his parents.
Of the presidents, from the published ancestries I have seen, President Carter could be slightly inbred through a possible relationship of Eliza Pratt (maternal 3rd-great-grandmother) to James Pratt (paternal 3rd-great-grandfather). If these Pratts were as close as brother and sister, President Carter’s inbreeding coefficient would be 0.0009, produced from a union of fourth cousins. According to Roberts (op. cit., p. 91), the parentage of Eliza is unknown.
I had thought President Taft might be our most inbred president. Burke’s Presidential Families of the United States of America, 2nd edition (1981) states (p. 426) that the parents of President Taft (Alphonso Taft and his wife Louisa Maria Torrey) were fourth cousins once removed. This relationship is shown (p. 429) to be through their common descent from Robert1 Taft and his wife Sarah. This information, and the more extensive chart of President Taft’s ancestors in the appendix, is apparently based largely on Clarence Almon Torrey’s outline of Taft’s ancestry in The American Genealogist 22(1945):205-210. Torrey’s outline, if one takes the time to work out an ancestor table, shows that Alphonso and Louisa Taft have several more relationships, although not as close as through Robert Taft and his wife. These relationships are through:
1. Samuel2 Hayward and Mehitable Thompson (4th cousins twice removed and 5th cousins once removed)2. John1 Hill and Frances (5th cousins once removed and 6th cousins once removed)3. John2 Thompson and Sarah (5th cousins twice removed, and again 5th cousins twice removed; 6th cousins twice removed, and again 6th cousins once removed).
All connections are important if we really want to assess the genetic relationship of the couple. These additional connections would seem to make Alphonso and Louisa very closely related, but actually they only double their genes in common over that from their descent from Robert1 and Sarah Taft. This doubling of their genetic relationship, however, is not sensed in the terminology we use to define cousinship. The doubling of relationship changes Alphonso and Louisa from fourth cousins once removed to a relationship a little closer than fourth cousins. This is still a far closer relationship than that of an average married couple in the United States. For those interested in presidential trivia, I believe President Taft was, physically, the heaviest Chief Executive. The relationship between his parents makes him one of the most inbred as well, with an inbreeding coefficient of 0.0012.
But then I recalled that President John Adams and his wife Abigail Smith were related, thus making their son President John Quincy Adams somewhat inbred. Burke’s Presidential Families of the U.S.A. (2nd edition, 1981) shows (p. 69) a third-cousin relationship between John and Abigail (Smith) Adams, making the inbreeding of their son John Quincy Adams equal to 0.0039, a little over three times Mr. Taft’s coefficient.
The highest inbreeding, however, is that of President William Henry Harrison whose paternal great-grandmother was Elizabeth Burwell (ca. 1678-I 734), sister of a maternal great-grandmother, Joanna Burwell (ca. 1674-1727) (see Gary Boyd Roberts, Ancestors of American Presidents, preliminary edition, revised 119891, p. 13). The inbreeding coefficient of children of this second-cousin marriage is 0.0156 -- four times that of John Quincy Adams. A further trivium is that this President Harrison had the shortest time in office of any president: one month.
In summary, the highest inbreeding among our presidents is
A further note: We all can find in our ancestry examples of first- or second-cousin marriages, so this amount of inbreeding is not at all uncommon. But in these three cases, the inbreeding is more than most of us have. Inbreeding can have harmful effects, but in the actual cases of these presidents it was still of such a low value as probably to have had no effects at all. So the genetic effects of these inbreeding coefficients could be considered “academic” and thus truly an item of trivia.
References for further reading on the subject of inbreeding:
L. L. Cavalli-Sforza and W.F. Bodmer, The Genetics of Human Populations (San Francisco: W. H. Freeman, 1971).J.F. Crow, Genetics Notes, 7th ed. (Minneapolis: Burgess, 1976).D.S. Falconer, Introduction to Quantitative Genetics, 2nd ed. (New York: Longman, 1983).Thomas H. Roderick, Ph. D. is a geneticist at The Jackson Laboratory in Bar Harbor, Maine. Interested readers may correspond with him at 4 Seely Road, Bar Harbor, ME 04609.