The Heredity of “Racial” Traits
Essays on the Color Line and the One-Drop Rule

by Frank W Sweet
December 15, 2004

“Because they look different,” explained the State Park Service historian in charge of living history at Olustee, Florida. She was answering an ethnic acculturation question. Why has almost every non-White immigrant group in U.S. history—Irish, Jews, even Chinese in Jim Crow Mississippi—been embraced by America’s ever-expanding blanket of Whiteness and joined the mainstream via intermarriage within a few generations, while African-Americans remain non-White?1 The irony in the historian’s answer could be seen in the four children playing in the grass nearby—two apparently members of the Black endogamous group and two apparently White.
Every February, the Florida Park Service hosts a Civil War reenactment at Olustee. The real battle, fought in February 1864, comprised about five thousand Union attackers and the same number of Confederate defenders. The reenacted battle is choreographed to replicate actual events. Over a hundred Black re-enactors from Charleston play the role of the famed 54th Massachusetts. Thousands of other Civil War re-enactors from throughout the Southeast come to portray the other Union and Confederate regiments involved. The event attracts thousands of spectators. Some come for the day to cheer one side or the other, but most are families who bring their children to learn about a dramatic event in the state’s past.

In recent years, the local NAACP chapter has picketed the event as a painful reminder of slavery. In response, Black volunteers involved in the production have increased the number of living history presentations and dioramas by Black History groups, who lecture on the African-American heroes and heroines of abolition and the war. It is a family event for the living history interpreters as well, and the volunteers bring their own children, dressed out in the Alice-in-Wonderland childhood fashions of the 1860s.

One pair of living history presenters are of first-generation mixed heritage. He has one Black parent and one White parent, and so does she. Both display the in-between appearance of, say, Jennifer Beals or Gary Dourdan of the CSI TV show. Technically speaking, both are heterozygotic at each of the three-to-six genes for dermal melanization. But their four children could be used to exemplify Mendel’s wrinkled peas and smooth peas without the in-between peas. The two oldest kids are quite dark, taking after their Black grandparents (homozygotic for African skin tone). The two youngest are European-looking, taking after their White grandparents (homozygotic for European skin tone). The standing family joke (which outsiders endure stoically despite having heard it many times) is that the two older children used up all of the family melanin, leaving none for their younger siblings. The parents identify the family as part of the Black community. Nevertheless, the two European-looking children will probably switch to self-identifying as Hispanic or even White after high school. There is little to be gained in today’s America, for a young person who looks European, by voluntarily remaining a member of the Black endogamous group.

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This essay explains, in four topics, that much is known about the heredity of those physical features important to U.S. society in assigning someone to one side or the other of the endogamous color line. Three-to-Six Co-Dominant Skin Tone Genes discusses the genes that determine skin tone. Mendelian Inheritance explains that, on average, half of the children of admixed parents inherit a skin tone between those of their parents, one fourth come out darker than both parents, and one-fourth come out lighter than both. This means that any Afro-European admixed population will not blend homogeneously after many generations, but will continue to produce a few African-looking and European-looking individuals indefinitely. Appearance is not the Same Thing as Ancestry explains that, in admixed populations, even people who share identical ancestry may wind up with different Afro-European admixtures due to the random recombination of parental genes at each generation. This is why about five percent of the African-American population has no detectable African genetic admixture. Finally, Hardy-Weinberg Distribution shows how to compute the rate at which European-looking children are born into various Black communities in the United States, and the rate at which African-looking children are born into European-looking populations in other countries.

Three-to-Six Co-Dominant Skin Tone Genes
Some people erroneously assume that physical traits associated with the U.S. endogamous color line “blend” in some non-Mendelian way. They assume that children cannot come out looking more European than both parents nor more African than both. They assume that endogamous populations become ever more homogeneously blended with the passage of generations. And they assume that any given New World resident of mixed Afro-European appearance must descend from colonists who were themselves of one hundred percent European or one hundred percent African genetic admixture.2 All of these assumptions are mistaken.3

Parents of mixed intermediate Afro-European genetic admixture can, and often do, produce strongly European-looking or African-looking children. To be precise, there is a 1/2 probability that any given child of two genetically admixed parents will display color-line-related features midway between those of the parents, 1/4 probability that it will look more European than either parent, and 1/4 probability that it will look more African than either parent. Furthermore, most Americans of intermediate Afro-European admixture are not first-generation dual-heritage individuals. Instead, most spring from parents who are also of Afro-European genetic admixture. In fact, many of the New World’s alleles for European features came to this hemisphere within the bodies of African slaves, whose ancestors had mingled with Arabs, Berbers, and Mediterranean Europeans for centuries. And many of the alleles for African features came to the Americas within the bodies of European Mediterranean colonists, whose ancestors had mingled with Arabs, Berbers, and Africans for centuries. The Mediterranean has been a genetic mixing bowl for tens of thousands of years.

None of this has anything to do with “race,” as many use the term, since what non-scientists mean by “race” is hard to pin down. Americans tend to think of Africans, Europeans, and Asians as different “races.” But nobody else sees it this way. Japanese, Australian Aborigines, Tahitians, Malaysians, Pakistanis, Turks, and Israelis are all Asians, for example, but no one of them would consider themselves to be of the same “race” as any of the others. Even the obsolescent craniofacial anthropometry of the past does not match preconceptions of “race.” Carleton S. Coon, the greatest race-defining craniofacial anthropometrist of the twentieth century, whose definitions filled the U.S. textbooks of fifty years ago, considered neither Ethiopians nor Khoisan to be of the “negroid race.”4 Rather than “race,” this discussion is interested only in those physical traits that lead U.S. society to assign a person to one side or the other of the endogamous color line—to consider a person a suitable marriage partner for Whites or Blacks.

Understanding the heredity of physical traits associated with the endogamous color line can help us better to grasp how genes leaked through the barrier as much as they have (one third of White Americans having 2-20 percent African genetic admixture). But a difficulty in discussing heredity is the indeterminacy of just which features are associated with the color line. We know that people who “look African” are usually assigned to the Black endogamous group by U.S. society. But precisely what does it mean to say that someone “looks African”? As explained in the essay “The Perception of ‘Racial’ Traits,” the features associated with African ancestry depend upon which society is making the determination. As Harry Hoetink pointed out, the very same individual may be considered White in Puerto Rico, Coloured in Jamaica, and Negro in Georgia.5 Modern craniofacial anthropometrists (forensic anthropologists) give more importance to prognathism than to skin tone,6 and nineteenth-century Americans once emphasized foot shape.7 Consequently, the following discussion of heritability simplifies such traits to a single feature—skin tone.

Throughout the following discussion, three things should be kept in mind. First, many societies (Hindu India, for example) do not consider skin tone to be associated with any endogamous barrier. This discussion focuses on skin tone because most Americans consider it significant, hence the term “color line” and the group labels “Black” and “White” corresponding to brown versus pinkish beige skin tone. Second, melanization is mechanically complex. Some people are darker than others before tanning, some tan more easily, some tan more deeply, and some tans last longer than others. Despite its complexity, dermal melanization depends on just a few genes. Finally, the following discussion could be repeated for any feature that depends on a handful of additive genes, each with co-dominant alleles, such as hair curliness, nose width, lip thickness, prognathism, steatopygia, and the like. Hence, it applies to any of the physical traits that U.S. society associates with membership in the Black or White endogamous groups.

Alleles do not blend. They are not analog recordings. They are digitally encoded (the human genome contains about 750 megabytes of data).8 Because they are digitally encoded, alleles combine in simple, mathematically predictable ways. Since 1910, researchers have known that human skin pigmentation is polygenic, depending on just a few codominant additive genes of essentially two alleles each. We have known that skin tone is polygenic, rather than the result of one gene with many alleles, because breeding of palest with darkest yields a spectrum of offspring genotypes from the same parents, not just the four Mendelian ones. We have known that human pigmentation genes are additive and codominant because half the offspring of differently colored parents have a skin tone between that of their parents, no matter how similar the parents (one-fourth are outside each extreme of the parental span). We have known that at least three genes are involved because histograms of population skin reflectance yield continuous, not discrete, values.9

Where knowledge has improved over the past century has been in precisely how many genes are involved and their specific loci. As of 1998, five human pigmentation genes had been identified. Their symbols and genome loci are: “TYR” at 11q14-21 (chromosome eleven long arm, 14 to 21 centimorgans out), “TYRP1” at 9p23, “TYRP2” at 13q31-32, “P” at 15q11.2-12, and “MC1R” at 16q24.3.10 Subsequent work has identified five non-synonymous polymorphisms at the MC1R site.11 Some polymorphisms have been related to phenotype.12 And gene-enzyme-protein reaction chains have been identified.13

Much of the genetic mechanism remains to be unraveled but one finding is pertinent here. Skin color is determined by a (definite) minimum of three and a (probable) maximum of six additive genes, each with two co-dominant alleles. This means that skin-tone inheritance is predictable. Imagine a population composed of two same-sized groups. The first group comprises individuals who (like many sub-Saharan Africans) are homozygotic for dark alleles at all of the (three to six) dermal melanization loci. The other group comprises individuals who (like Nordic Europeans) are homozygotic for fair alleles at the same loci. Given random mating within the population composed of the two equal-sized groups, within a few generations the resultant population would fall into a Poisson skin-tone distribution.14

Mendelian Inheritance
In other words, if a large population (more than a few thousands individuals) were assembled out of equal numbers of the darkest and the fairest humans on earth, within a few generations, their descendants’ skin tone would fall into a normal (Gaussian bell-curve) distribution. The number of genes involved would not affect the form of the distribution. If skin tone were determined by only three genes, then the resultant population would fall into the seventh line of Pascal’s triangle with, on average, 1, 6, 15, 20, 15, 6, and 1 out of every 64 individuals having each skin-tone gradation, from the fairest to the darkest possible. If skin tone were set by six genes, then the descendants would fall into the thirteenth line of Pascal’s triangle with, on average, 1, 12, 66, 220, 495, 792, 924, 792, 495, 220, 66, 12, and 1 out of every 4096 individuals having every skin-tone gradation from the fairest to the darkest. Neither the shape, the height, nor the width of the consequent distribution would vary with number of genes. The number of genes involved would affect only the fineness of the skin-tone gradations.

The above explanation may seem trivial, but it is important to understanding U.S. color line permeability. It is important because exactly the same results would unfold if one were to start with a homogeneous population where every individual were heterozygotic at each locus. In other words, if you started with a population of first-generation admixture (each with a fair-skin allele from one parent, and a dark-skin allele from the other parent, at each of the three-to-six genes), then their descendants would fall into precisely the same pattern as above, with precisely the same numbers of individuals having every skin-tone gradation from the very fairest to the darkest possible.15

Puerto Rico Skin-Tone Histogram As a practical example, consider Puerto Rico. From the late eighteenth century until the U.S. conquest at the turn of the twentieth century, Spanish censuses of the island consistently reported that 45 percent of the population was White (predominantly European-looking) and 55 percent was Black (predominantly African-looking). Census after census, these numbers did not change more than three percentage points until the United States took possession of the island.16 Today, the native-born island population physically matches the theoretical Poisson distribution almost precisely. About one Puerto Rican in ten looks White to the typical American, about one in ten looks Black, and the rest look “Hispanic,” which in this context means “in between.”17

Appearance is not the Same Thing as Ancestry
To be precise, the above figure shows that a skin-tone histogram of Puerto Ricans has a single peak at the 50-50 mark with population fractions diminishing in both directions—dark and light.18 Nevertheless, judging by the Spanish census data and considering that the Johnson-Reed Act of 1924 cut off immigration, virtually every native-born Puerto Rican on the island today has equal numbers of African and European ancestors.19 This means that those few Puerto Ricans who are of predominantly European appearance as well as those who are of predominantly African appearance descend from the same population of ancestors. The figure above depicts a computer simulation of this result.20 This point bears repeating. Those few Puerto Ricans of European appearance whose families have been on the island since colonial tines have equal numbers of African and European ancestors, as do those Puerto Ricans of strongly African appearance. In short, skin tone is so ephemeral and so sensitive to a few genes, that it is nearly useless as an indicator of either Afro-European ancestry or Afro-European genetic admixture (which are themselves different things). As another example of this point, a recent admixture study conducted in Columbia, South Carolina, found that about five percent of Black Americans have no detectable African genetic admixture at all.21 Their family oral histories accurately trace their descent partly from African slaves. But over the course of many generations, even the negligible intermarriage rate between Whites and Blacks gradually eliminated the genetic markers of African origin from those few families by random chance.

Hardy-Weinberg Distribution
Once a population’s genotype stabilizes into a Poisson bell curve, it will remain in that configuration as long as the conditions of Hardy-Weinberg equilibrium hold and mating is random. Hardy-Weinberg equilibrium is not discussed at depth in this essay.22 It suffices to point out that where a closed population is large enough to avoid loss or fixing of alleles by mere chance (termed drift), and where no selection mechanism operates on the genes of interest (affecting either survival, reproduction, or mate selection), then the percentages of alleles (fifty-fifty in the above example) within a population will remain unchanged indefinitely.23

In the context of this discussion, absence of mate selection implies random mating with regards to skin tone, which may seem inapplicable to this study. After all, in contrast to Puerto Rico, the defining feature of the U.S. color line is that it marks an endogamous barrier. But the point here is that the U.S. Black community can itself be defined as a closed population precisely because it is endogamous. Whether mate-selection within this group depends on skin tone is an interesting question. Many anthropologists of the 1920s and 1930s reported detecting a Black male mate-selection preference for fair women. Melville J. Herskovitz, Gunnar Myrdal, Edward Reuter, Ralph Linton, Gustavas Steward, and Robert E. Park referred to it.24 But these finding were eventually overturned because they were based on skin-tone differences between husbands and wives. As it turns out, studies of siblings show that females typically display less melanization than males,25 just as children display less melanization than adults.26 It is simply one of the neotenous features associated with human sexual dimorphism.27 In any event, it is now accepted that, within the U.S. Black community, mating has been essentially random for skin tone.

Gullah Skin-Tone Histogram How Many White Children are Born Into Black Families?
The occurrence frequency of European admixture (implying alleles for fair skin tone) among members of the U.S. Black community varies by region, from around 3 percent among the Geechee/Gullah people of the Sea Islands of South Carolina, Georgia, and Florida, to about 20 percent among the African-Americans of Philadelphia and Boston.28 Given these numbers, it is a straightforward task to compute the rate at which any given African-American population will produce members who are as fair as Europeans.29

Black Philadelphia Skin-Tone Histogram Various sources agree that, on average, people with 12 percent or less African admixture appear White to the average American and those with up to 25 percent look ambiguous (with a Mediterranean skin tone).30 Given this finding, random chance will produce one such Philadelphian out of every five hundred born to parents who are members of the Black community (rare, but measurable), and one such Gullah out of every 1.4 billion (virtually impossible).31 (In the accompanying graphs, only the two leftmost bars depict the numbers of people with European skin tone.)

How Many Black Children are Born into White Families?

White America Skin-Tone Histogram The mirror-image situation is also easily computed. African admixture averages about 0.7 percent among White Americans, about 8 percent in Portugal, and about 5 percent in Spain, based upon recent genetic admixture studies as well as upon accepted estimates of sixteenth-century sub-Saharan immigration to Iberia.32 Hence, typical White Americans cannot produce children with unambiguously dark skin but it occurs among Spaniards at the rate of about one in two hundred.33

Iberia Skin-Tone Histogram This explains why modern-day Chileans and Argentineans, as well as Spaniards, look White to Americans, despite having recently absorbed large African populations.34 Alleles for African appearance are so thinly distributed throughout these predominantly European populations that the genes seldom happen to match up in any one individual.

As explained above, the same can be said for any trait associated with the U.S. endogamous color line. In short, even without taking into account a steady inflow of genes for European appearance, the Black community has always produced a steady and predictable stream of European-looking individuals whose immediate parentage is socially Black. They owe their European phenotype to the random matching up of genes from distant European ancestors. Of course, counting how many people are born with European appearance measures opportunity, not action. There is no reason to think that all or even most such Americans actually cross the color line and designate themselves White.35

* * * * *

This essay described the heredity of skin tone, the most important of the physical features used by U.S. society in assigning someone to one side or the other of the endogamous color line. It listed the genes that determine skin tone. It explained that any Afro-European admixed population (such as the African-American community) will never blend homogeneously, but will continue to produce a few European-looking individuals indefinitely. It showed that even people who share identical ancestry may wind up with different Afro-European admixtures due to the random recombination of parental genes at each generation. Finally, it demonstrated how to compute the rate at which European-looking children are born into the African-American community.

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Frank W. Sweet was accepted to Ph.D. candidacy in history with a minor in molecular anthropology at the University of Florida in 2003 and has completed all but his dissertation defense. He earned an M.A. in History from American Military University in 2001. He is the author of several historical booklets, trade paperbacks, and published historical essays. He was a member of the editorial board of the magazine Interracial Voice, is a regular lecturer and panelist at historical and genealogical conferences, and moderates an online discussion group on the history of U.S. racialism (the “race” notion) sponsored by Backintyme Publishing. To discuss this item with the author, visit the “Meet the Authors” forum of the OneDropRule discussion group. To send email, click here.

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1. Noel Ignatiev, How the Irish Became White (New York: Routledge, 1995); Karen Brodkin, How Jews Became White Folks and What That Says About Race in America (New Brunswick NJ: Rutgers University, 1998); James W. Loewen, The Mississippi Chinese: Between Black and White (Cambridge MA: Harvard University, 1971).

2. See, for example, Ira Berlin, Slaves Without Masters: The Free Negro in the Antebellum South (New York: New Free Press, 1974), 108-9; Gwendolyn Midlo Hall, Africans in Colonial Louisiana: The Development of Afro-Creole Culture in the Eighteenth Century (Baton Rouge: Louisiana State University, 1992) as quoted in Virginia Meacham Gould, “The Free Creoles of Color of the Antebellum Gulf Ports of Mobile and Pensacola: A Struggle for the Middle Ground,” in Creoles of Color of the Gulf South, ed. James H. Dormon (Knoxville: University of Tennessee, 1996), 28-50, 32; Jane G. Landers, ed. Against the Odds: Free Blacks in the Slave Societies of the Americas (London: Frank Cass, 1996), 86; Frederick P. Bowser, “The Free Person of Color in Mexico City and Lima: Manumission and Opportunity, 1580-1650,” in Race and Slavery in the Western Hemisphere: Quantitative Studies, ed. Stanley L. Engerman, et al. (Princeton: Princeton University, 1975), 331-361, 333.

3. See, for example, the extended critique of Ira Berlin, Slaves Without Masters: The Free Negro in the Antebellum South (New York: New Free Press, 1974) in Gary B. Mills, The Forgotten People: Cane River’s Creoles of Color (Baton Rouge: Louisiana State University, 1977), xv-xvi.

4. See Carleton Stevens Coon, The Origin of Races (New York: Knopf, 1962).

5. Harry Hoetink, Caribbean Race Relations: A Study of Two Variants (London: Oxford University, 1971), xii.

6. George W. Gill, “Does Race Exist?: A Proponent’s Perspective,” [web page] (Nova, 2000), available from http://www.pbs.org/wgbh/nova/first/gill.html.

7. 23 Ark. 50.

8. Three billion nucleotides, each carrying two bits of information, divided by eight bits per byte.

9. The two most accessible sources of historical surveys and synopses of current understanding are Curt Stern, Principles of Human Genetics, 3d ed. (San Francisco: W. H. Freeman, 1973), 443-65 and L. L. Cavalli-Sforza and W. F. Bodmer, The Genetics of Human Populations (Mineola NY: Dover, 1999), 527-31.

10. Richard A. Sturm, Neil F. Box, and Michele Ramsay, “Human Pigmentation Genetics: The Difference is Only Skin Deep,” BioEssays 20 (1998): 712-21.

11. B.K. Rana and others, “High Polymorphism at the Human Melanocortin 1 Receptor Locus,” Genetics 151, no. 4 (1999): 1547-48.

12. R.M. Harding and others, “Evidence for Variable Selective Pressures at MC1R,” Journal of Human Genetics 66, no. 4 (2000): 1351.

13. P.A. Kanetsky and others, “A Polymorphism in the Agouti Signaling Protein Gene is Associated with Human Pigmentation,” American Journal of Human Genetics 70 (2002): 770-775.

14. C. Stern, “Model Estimates of the Frequency of White and Near-White Segregants in the American Negro,” Acta Genetica 4 (1953): 281-298, 445-52.

15. See, for example, A.K. Kalla, “Inheritance of Skin Colour in Man,” Anthropologist Special Volume (1968): 158-68; G.A. Harrison and J.J.T. Owen, “Studies on the Inheritance of Human Skin Colour,” Ann. Human Genetics 28 (1964): 27-37; C. Stern, “Model Estimates of the Frequency of White and Near-White Segregants in the American Negro,” Acta Genetica 4 (1953): 281-298.

16. For 1792, see Francisco Morales Padron, “La Vida Cotidiana en una Hacienda de Esclavos,” Revista del Instituto de Cultura Puertorriquena 4, no. 10 (1961): 23-33, 25. For a census-by-census table of subsequent years, see Frederick P. Bowser, “Colonial Spanish America,” in Neither Slave Nor Free: The Freedmen of African Descent in the Slave Societies of the New World, ed. David W. Cohen and Jack P. Greene (Baltimore: Johns Hopkins University, 1972), 19-58, 38.

17. Since the American occupation, the federal census in Puerto Rico has conformed to the U.S. viewpoint. Through 1950, all Puerto Ricans were census-defined as “colored” as per the enumerators’ instructions. Since 1960, the census has been self-administered, and about forty percent of Puerto Ricans have checked off “White,” while another fifty percent have checked off “other” and written in various terms (Hispanic, human, none, Puerto Rican, Latino, and the like), which the census bureau encodes as “White.” Only one Puerto Rican in ten checks off “Black.”

18. For a similar discussion of Puerto Rican admixture, rather than skin tone, see the essay “Afro-European Genetic Admixture in the United States.”

19. Clara E. Rodriguez, “Challenging Racial Hegemony: Puerto Ricans in the United States,” in Race, ed. Steven Gregory and Roger Sanjek (New Brunswick NJ: Rutgers University, 1994), 131-45, 137.

20. The program (a MS-Excel macro) can be downloaded from http://backintyme.com/admixture/genetics.xls.

21. E.J. Parra and others, “Ancestral Proportions and Admixture Dynamics in Geographically Defined African Americans Living in South Carolina,” American Journal of Physical Anthropology 114 (2001): 18-29. This study was also mentioned in the essay, “Afro-European Genetic Admixture in the United States.”

22. For more discussion of Hardy-Weinberg equilibrium, see the essay “The Rate of Black-to-White ‘Passing’.”

23. An accessible introduction to Hardy-Weinberg equilibrium can be found in Steve Jones, Robert Martin, and David Pilbeam, eds. The Cambridge Encyclopedia of Human Evolution (Cambridge UK: Cambridge University, 1992), 290.

24. A useful survey of this topic can be found in Joel Williamson, New People: Miscegenation and Mulattoes in the United States (New York: Free Press, 1980), 118-21.

25. E. Rebato and others, “Sibling Correlations of Skin Pigmentation During Growth,” Human Biology 71, no. 2 (1999): 277-293.

26. J.H. Relethford, F.C. Lees, and P.J. Bayard, “Sex and Age Variation in the Skin Color of Irish Children,” Current Anthropology 26, no. 3 (1985): 386-397.

27. Otherwise important distinctions among neoteny, paedomorphosis, and postdisplacement are irrelevant to the point being made.

28. C.L. Pfaff and others, “Using Estimates of Individual Admixture to Study the Genetics of Phenotypic Traits: Skin Pigmentation in African Americans,” American Journal of Human Genetics 69, no. 4 (2001): 410; E.J. Parra and others, “Ancestral Proportions and Admixture Dynamics in Geographically Defined African Americans Living in South Carolina,” American Journal of Physical Anthropology 114 (2001): 18-29; Esteban J. Parra and others, “Estimating African American Admixture Proportions by Use of Population-Specific Alleles,” American Journal of Human Genetics 63 (1998): 1839-51.

29. Again, this is easiest done by simulation rather than analytically, using something like the program at http://backintyme.com/admixture /genetics.xls.

30. Caroline Bond Day and Earnest Albert Hooton, A Study of Some Negro-White Families in the United States (Cambridge MA: Harvard University, 1932), 10; Melville J. Herskovits, The Anthropometry of the American Negro (New York: Columbia University, 1930); G.A. Harrison and J.J.T. Owen, “Studies on the Inheritance of Human Skin Colour,” Ann. Human Genetics 28 (1964): 27-37; A.K. Kalla, “Inheritance of Skin Colour in Man,” Anthropologist Special Volume (1968): 158-68; C. Stern, “Model Estimates of the Frequency of White and Near-White Segregants in the American Negro,” Acta Genetica 4 (1953): 281-298.

31. Assuming four genes (eight co-dominant haplotypes): For Philadelphia, if the probability of any haplotype’s being European is 0.20, then the probability of six out of the eight haplotypes being European (giving a European appearance to the individual) would be (0.20)^6 x 28 = 1.8 x (10)^-3. For the Sea Islands, if the probability of any haplotype’s being European is 0.03, then the probability of six out of the eight aligning to yield a European phenotype would be (0.03)^6 x 28 = 2.04 x (10)^-8. Where 28 is the number of two-in-eight combinations n!/j!(n-j)!.

32. For the U.S. rate, see Mark D. Shriver and others, “Skin Pigmentation, Biogeographical Ancestry, and Admixture Mapping,” Human Genetics 112 (2003): 387-99, Table 2. For immigration history, see: University of Chicago, ed. The New Encyclopaedia Britannica, 15 ed. (Chicago: William Benton, 1974), 15:859; Hugh Thomas, The Slave Trade: The Story of the Atlantic Slave Trade: 1440-1870 (New York: Simon and Schuster, 1997), 22-4, 48-86, 804; Lerone Bennett Jr., Before the Mayflower: A History of Black America, 6th rev. ed. (New York: Penguin, 1993), 32; Frank Tannenbaum, Slave and Citizen, the Negro in the Americas (Boston: Beacon Press, 1946), 14-15, 44; Leslie B. Rout, The African Experience in Spanish America, 1502 to the Present Day (Cambridge UK: Cambridge University, 1976), 9, 18-20. For DNA admixture studies, see: Martin Richards and others, “Extensive Female-Mediated Gene Flow from Sub-Saharan Africa into Near Eastern Arab Populations,” American Journal of Human Genetics 72 (2003): 1058-64; H.B. Corte-Real and others, “Genetic Diversity in the Iberian Peninsula Determined from Mitochondrial Sequence Analysis,” Annals of Human Genetics 60, no. 4 (1996): 331-50.

33. Assuming four genes (eight co-dominant haplotypes): For Spain, if the probability of any haplotype’s being African is 0.05, then the probability of three out of the eight haplotypes being African (giving an African appearance to the individual) would be (0.05)^3 x 56 = .007. For White Americans, if the probability of any haplotype’s being African is 0.007, then the probability of three of the eight aligning to yield an African phenotype would be (0.007)^3 x 56 = 1.92 x (10)^-5. Where 56 is the number of three-in-eight combinations n!/j!(n-j)!.

34. George Reid Andrews, The Afro-Argentines of Buenos Aires, 1800-1900 (Madison: University of Wisconsin, 1980).

35. See the essay “The Rate of Black-to-White 'Passing’” for an estimate of Black-to-White gene flow across the U.S. color line in order compute just how many individuals have made this choice over the years.

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