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Afro-Euro DNA Admixture in the U.S. Loading...
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Sea level rose when the ice melted 16 KYA (16 thousand years ago), genetically isolating the New World from the old. For thousands of years, plagues and diseases then swept the Old World forcing its inhabitants to develop immunities.
Native Americans never developed similar immunities. Within two generations of Columbus's landing, 95 percent of the Native American population was wiped out by Old World diseases to which Europeans, Asians, and Africans were immune.
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Over the next few centuries, the population of the New World was replenished by millions of people from the Old, carried across the Atlantic in sailing ships.
Eleven million came from Africa. Most of these were taken to the New World against their will as slaves, although some were indentured.
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Three million people came from Europe. Most came freely but then became indentured forced laborers upon arrival. Some became slaves.
(The difference between indenture and slavery was that the former form of forced labor was for seven years and not hereditary, while the latter was life-long and hereditary.)
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There are thousands of genetically identifiable populations on earth. Scientists no longer call them many "races," partly because there are so many of them: dozens in Europe and hundreds in Africa, for example.
There are three ways of using DNA to study the history of human populations: mitochondrial or mtDNA, Y chromosome or Y DNA, and autosomal or admixture DNA.
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Mitchondrial markers (mtDNA haplogroups) were discovered first and have been well studied. Everyone carries them. They descend through the maternal line, passing from mother to daughter. Men get theirs from their mothers but cannot pass them on.
They remind you of the clan system of the Native American tribes of the U.S. southeast, so mtDNA haplogroups are often called "clans." The map shows prehistoric migrations of mothers.
Over tens of thousands of years, carried the different mtDNA markers around the globe. Here are three examples: If your mtDNA is "H" then you descend from mammoth hunters who moved into Europe about 45 millennia ago.
If your mtDNA is "B," then you either descend from Native Americans or from their ancestors who remained in Asia about 22 millennia ago. If your mtDNA is L3, then you descend from east Africans.
But mtDNA tells little about your own makeup. Ten generations ago (around the year 1800) you had a thousand ancestors, only one of whom (your mother's, mother's, mother's .... mother) had that particular mtDNA.
Your other 999 ancestors might all have had totally different ancestry. For example, my own mtDNA is "A" (Native American) and yet I am northern European in appearance and my family has no tradition of having any Native American ancestry at all.
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Y chromsome markers (Y haplogroups) are more recent but are also well understood. Only males carry them. They descend through the paternal line, passing from father to son. It reminds you of the way surnames work in the Western world.
So Y haplogroups are often used by genealogists pursuing surname ancestry. The map shows prehistoric migrations of fathers who, over the past tens of thousands of years, carried the different Y markers around the globe.
If your Y is "R1b" then your ancestors arrived in Europe before the ice ages. If your Y is "Q," then you either descend from Native Americans or from their ancestors who remained in Asia. If your Y is ExE3b, then you descend from west Africans. 
Again, only one man, out of your thousand ancestors from 1800, carried your Y haplogroup. The other 999 might have had totally different ancestry. For example, my own Y is R1* (from Cameroon in west Africa).
And yet, as already mentioned, I have a northern European appearance and no credible family tradition of sub-Saharan ancestry. Hence, the "race" that most people see in me matches neither my mtDNA nor my Y. 
To historians of the New World, of the thousands of human populations on earth, three large groups are of special interest:
(1) Those populations who were already in the New World when Europeans and Africans arrived. (2) Those who came from Europe. (3) Those who came from Africa.
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Autosomal markers are different from mtDNA or Y haplogroups in that they measure the average continent-of-ancestry admixture in your entire DNA. The technique of autosomal mapping is more recent that either mtDNA or Y, and it is constantly being refined. It was originally devised for New World inhabitants, to measure what fraction of their ancestry came from each of the three demographic sources: Africa, Europe, and Native America.
On the other hand, autosomal markers do not go back before colonial times, and cannot make distinctions between tribes or clans, but they can tell how many of your ancestors came from each of the three colonial sources.
For example, my own autosomal markers show 75 percent European, 13 percent subsaharan African, and 12 percent Native American (a typical Puerto Rican.)
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The two scientists who pioneered autosomal DNA mapping are Mark Shriver of Penn State University (on left) and Rick Kittles now at the University of Chicago (on right). (Dr. Kittles was with Howard University at the time of the original research.)
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This is one of the scatter diagrams that they made after measuring admixture in several hundred subjects. Each dot represents an individual. It is plotted left-to-right depending on Euro-Afro admixture.
People of 100 percent Euro DNA are on the left edge, 100 percent Afro are on the right, and everyone else is proportionally in between.
This particular chart was meant to see if admixture matched each person's skin tone, so each dot was plotted higher (darker skin) or lower (paler).
The trend is upwards and to the right. This shows that, although you can see hundreds of exceptions on the chart, in general the more African DNA you have, the darker your skin tone.
What is of interest to the historian is that each subject was also asked his/her "racial" self-identity. Those who considered themselves White were plotted with tiny diamond shapes. Self-identified African Americans were plotted as tiny circles.
With that in mind, you can see that self-identified White Americans range from zero to 25 percent African admixture inherited from slave ancestors. African Americans span the entire range. And there is a lot of overlap in the 15-25 Afro percent range.
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This graph is from a different study. It also matches the ancestral Afro-Euro DNA admixture of several hundred individuals with their ethnic self-identity. Dots are plotted 100 Euro at the top, 100 Afro at the bottom in four groups.
From left to right, the groups are White Americans, African Americans, inhabitants of Zaire (formerly Congo), and inhabitants of Nigeria.
As in the prior chart, there is a lot of overlap between White and Black Americans in the range of zero to 30 percent Afro admixture. Some White Americans have over twenty percent African genetic admixture and some Black Americans have little or none.
VITAL POINT #2. The Black and White groups are not symmetrical. The mean African admixture among White Americans is about 0.7 percent African and 99.3 percent European admixture.
Only about one-third of White Americans have detectable Afro admixture (averaging about 2.1 percent), but this varies regionally from nil (in the north) to 5 percent (in the Cumberland plateau).
VITAL POINT #3. Virtually all Black Americans have some European admixture (averaging about 83 percent African and 17 percent European).
Again this varies regionally from 25 percent Euro in Philadelphia to nil among the Geechee/Gullah people of the Sea Islands of South Carolina and Georgia.
The United States is the only nation in the western hemisphere with two clusters--two genetically distinct populations.
Every other country in the hemisphere has a single large scattering of dots centered on the admixture ratio that matches the colonial populations of Europeans and Africans:
Argentina's cluster would be at the top of the chart, Haiti's at the bottom, and Puerto Rico's in the middle.
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There is sometimes a mismatch between a person's autosomal DNA markers and his or her external appearance.
Only about a dozen genes determine visible traits (skin tone, hair texture, features), compared to 275 (at last count) markers that track ancestral colonial source populations.
Since heredity at each generation is random between parents, it is not unknown for people to inherit Euro appearance while carrying Afro markers or vice-versa.
For example, the man on the left has about eleven percent Afro DNA and the man on the right has nearly 50 percent Euro. Why is there such a discrepancy between measured genetic admixture and physical appearance?
There is an immediate answer to this question, and a deeper answer. The immediate answer is that many different invisible genes identify continent of ancestry.
As of the summer of 2004, the private DNA lab DNAPrint Genomics, Inc. uses up to 175 single nucleotide polymorphisms (markers) in order to analyze a client's ancestral continents of origin.
On the other hand only a handful of genes encode for the few superficial, externally visible features (skin color, hair curliness, etc.) that Americans see as "racially" significant. Parental genes are randomly recombined with each passing generation.
It can happen, through sheer chance, that an individual (like Dr. Shriver) can inherit many invisible African DNA markers, but few or none of the handful of alleles that encode for "racial" appearance.
Alternatively, a person (like Dr. Kittles) can inherit those few alleles that encode for visible "racial" appearance but otherwise inherit the invisible but ancestrally informative European admixture markers.
The deeper answer becomes evident if you plot each of the two U.S. endogamous groups from the skin tone diagram separately.
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This graph shows just one portion of the population of the earlier chart--those who self-identified as members of the U.S. Black endogamous group (African Americans). You can see a positive correlation (R2=0.211) between skin tone and African admixture.
On average, if you are considered an African American, the more African admixture you have, the higher your melanin index. There are many exceptions, of course (like Dr. Kittles, for instance), but on average this is the case.
What is more important is that the range of skin reflectance as measured by the melanin index of African Americans is very large, spanning a vertical distance of fifty points from 30 to 80. African Americans span a huge range of skin tones.
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This graph shows the other U.S. endogamous group from the earlier chart--those who self-identify as White. In this diagram, you can see that there is no correlation, either way (R2=0.001), between skin tone and African admixture.
Clearly, if you are considered a White American, your melanin index is unrelated to your degree of African admixture. More importantly, the range of skin reflectance as measured by the melanin index of White Americans is very narrow.
It spans a vertical distance of only twenty points from 20 to 40. This is less than half of the range of skin-tone variation found among Black Americans.
The combination of narrow phenotype variation (skin tone) along with a wide range of inter-population (Afro-European) admixture variation among White Americans shows that a selection process has taken place.
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Few human populations display such a clear mark of selection. Narrow phenotype variation alone does not necessarily indicate selection. Northern Europeans display little skin tone variation, but they lack a wide range of African admixture.
Broad genotype variation alone does not necessarily indicate selection. Puerto Ricans average 50-50 Afro-European admixture, but they also display a wide range of skin tones.
The late Stephen J. Gould, Harvard biology professor and columnist for Natural History magazine, used to explain this principle with a baseball analogy.
Plot a scatter diagram of the batting averages of a thousand amateur or minor-league players and you will find a very large range of variation. A few such players are bad, a few are good, but most spread across the entire range in-between.
Now plot the batting averages of professional athletes in the major leagues. All are very good indeed. More importantly, the range of batting averages among them is tiny.
The difference between an outstanding star of the game and a rookie is a matter of mere hundredths of a percentage point. The reason, of course, is because you cannot get into the majors unless you are very good at it.
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Similarly, wild cows vary greatly in the amount of milk that they produce. Cows in a dairy farm produce more milk on average but also their milk production varies very little among themselves. Those cows who do not make the cut become hamburger.
And so, why do few if any White Americans display a strongly African appearance (have a high melanin index) despite having detectable African admixture?
Because those Americans who "look Black" are assigned involuntarily to the Black endogamous group, whatever their genetic admixture. The scatter diagrams are not symmetrical because the selection process acts only upon the White group.
As revealed in court records, discussed in other topics, a person of mixed ancestry who "looks European" (like Dr. Shriver or his maternal grandfather) in practice has the option of either adopting a White self-identity or a Black self-identity.
But a person of mixed ancestry who "looks African" lacks such a choice. U.S. society assigns such a person to membership in the Black endogamous group, like it or not. U.S. society has unwittingly applied selection pressure to the color line.
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The only American families accepted into the White endogamous group have been those whose African admixture just happened not to include the half-dozen alleles for dark skin (or the other physical traits associated with "race").
Since those particular alleles were sifted out of the portion of the White population that originated in biracial families, the relative percentage of the remaining, invisible, African alleles in this population cannot affect skin tone.
That skin tone does not vary with African genetic admixture among American Whites, despite their recent African admixture, confirms that physical appearance has been an important endogamous group membership criterion throughout U.S. history.
It has resulted in genetic selection of the White U.S. population for a European "racial" appearance, regardless of their underlying continent-of-ancestry admixture ratio.
For more on this topic, visit http://backintyme.com/essays/?p=5
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The End