Post by Human on Feb 24, 2005 18:14:23 GMT -5
Genetic Admixture is not the Same as Appearance
About one-third of White Americans are of between two and twenty percent recent African genetic admixture, as measured by the ancestry-informative markers in their DNA.19 This comes to about 74 million Americans. And yet, day-to-day experience teaches that virtually all White Americans look, well, White. Some may look more Mediterranean and others may look more Nordic, but very few White Americans have a distinctively African appearance. How can one reconcile DNA measurements with common experience?
An anecdote may help illustrate the problem. Look again at the chart of Skin Tone as Function of Afro-European Admixture. Consider one of the graph’s outlier points—the “European American” individual plotted as having 23 percent African genetic admixture. Dr. Shriver, the project team leader, became curious about this individual for two reasons. First, the person’s African genetic admixture was unusually high for someone who self-identified as a member of the U.S. White endogamous group. Second, the sample had been taken from State College, Pennsylvania, the site of Dr. Shriver’s own campus.20 According to Dr. Shriver:
I had the result for two or three years before I even looked up the ID number of the person whom we tested. I looked at who it was and it was me! I checked myself and the rest of my relatives and tracked it through my family. I never considered that there were any African people in my family. There’s no real variation in my family. The admixture must have been pretty far back. It just so happens that we can detect it with the markers we have. My mom especially stood out as being surprised, maybe because I told her it was coming through her father. She still doesn’t believe it about her family! The part of Pennsylvania where my mother’s father came from is where the Underground Railroad ended. There are several towns right here in Southern Pennsylvania where there are very light-skinned African-American communities that are the remnants of the Underground Railroad.21
It seems that Dr. Shriver’s maternal grandfather moved from Pennsylvania to Iowa, then to California, leaving behind in the process most of his ties with his relatives.22 Dr. Shriver, it turns out, (see photograph above) is one of the 74 million White Americans with significant recent African genetic admixture.
In a coincidentally similar fashion, Dr. Rick Kittles, Shriver’s collaborator from Howard University in Washington, discovered that he carries the FY-null genetic marker at genome position 16q24.3. This marker is found in 998 out of every thousand Europeans but found in only one out of thousand Africans. Many of Dr. Kittles’s other ancestry-informative markers tell the same unexpected story. Dr. Kittles (see photograph above) is one of the many Black Americans with strong European genetic admixture. And yet, and there is no other way to say this, Dr. Shriver “looks White” and Dr. Kittles definitely “looks Black.” 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.23 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.
Blacks’ Skin Tone as Function of Admixture The deeper answer becomes evident if you plot each of the two U.S. endogamous groups from the skin tone diagram separately. The next graph, “Blacks’ Skin Tone as Function of Admixture,” shows just one portion of the population of the earlier chart—those who self-identified as members of the U.S. Black endogamous group. In this diagram you can see a definite positive correlation (R2=0.211) between skin tone and African admixture. Clearly, if you are considered a Black American, the more African admixture you have, the higher your melanin index. More importantly, the range of skin reflectance as measured by the melanin index of Black Americans is very large, spanning a vertical distance of fifty points from 30 to 80.
“Whites’ Skin Tone as Function of Admixture,” below, shows the other U.S. endogamous group from the earlier chart—those who self-identified 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, spanning 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.
Whites’ Skin Tone as Function of Admixture 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. 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 very bad, a few are very good, and most spread across the entire range of batting averages 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. Similarly, wild cows vary greatly in the amount of milk that they produce. The cows in a dairy farm produce more milk on average but, more importantly, their milk production varies very little among themselves (compared to wild cows). The reason? 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 of the two endogamous U.S. groups are not symmetrical because the selection process acts only upon the White group. As revealed in court records, discussed elsewhere, 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, thus joining the White endogamous group or a Black self-identity, thus joining the other group. 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.24
In conclusion, U.S. society has unwittingly applied selection pressure to the color line. 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 color. That skin-color does not vary with African genetic admixture among American Whites, despite their measureably recent African admixture, demonstrates and 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.
check the link below:
www.backintyme.com/Essay040608.htm
About one-third of White Americans are of between two and twenty percent recent African genetic admixture, as measured by the ancestry-informative markers in their DNA.19 This comes to about 74 million Americans. And yet, day-to-day experience teaches that virtually all White Americans look, well, White. Some may look more Mediterranean and others may look more Nordic, but very few White Americans have a distinctively African appearance. How can one reconcile DNA measurements with common experience?
An anecdote may help illustrate the problem. Look again at the chart of Skin Tone as Function of Afro-European Admixture. Consider one of the graph’s outlier points—the “European American” individual plotted as having 23 percent African genetic admixture. Dr. Shriver, the project team leader, became curious about this individual for two reasons. First, the person’s African genetic admixture was unusually high for someone who self-identified as a member of the U.S. White endogamous group. Second, the sample had been taken from State College, Pennsylvania, the site of Dr. Shriver’s own campus.20 According to Dr. Shriver:
I had the result for two or three years before I even looked up the ID number of the person whom we tested. I looked at who it was and it was me! I checked myself and the rest of my relatives and tracked it through my family. I never considered that there were any African people in my family. There’s no real variation in my family. The admixture must have been pretty far back. It just so happens that we can detect it with the markers we have. My mom especially stood out as being surprised, maybe because I told her it was coming through her father. She still doesn’t believe it about her family! The part of Pennsylvania where my mother’s father came from is where the Underground Railroad ended. There are several towns right here in Southern Pennsylvania where there are very light-skinned African-American communities that are the remnants of the Underground Railroad.21
It seems that Dr. Shriver’s maternal grandfather moved from Pennsylvania to Iowa, then to California, leaving behind in the process most of his ties with his relatives.22 Dr. Shriver, it turns out, (see photograph above) is one of the 74 million White Americans with significant recent African genetic admixture.
In a coincidentally similar fashion, Dr. Rick Kittles, Shriver’s collaborator from Howard University in Washington, discovered that he carries the FY-null genetic marker at genome position 16q24.3. This marker is found in 998 out of every thousand Europeans but found in only one out of thousand Africans. Many of Dr. Kittles’s other ancestry-informative markers tell the same unexpected story. Dr. Kittles (see photograph above) is one of the many Black Americans with strong European genetic admixture. And yet, and there is no other way to say this, Dr. Shriver “looks White” and Dr. Kittles definitely “looks Black.” 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.23 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.
Blacks’ Skin Tone as Function of Admixture The deeper answer becomes evident if you plot each of the two U.S. endogamous groups from the skin tone diagram separately. The next graph, “Blacks’ Skin Tone as Function of Admixture,” shows just one portion of the population of the earlier chart—those who self-identified as members of the U.S. Black endogamous group. In this diagram you can see a definite positive correlation (R2=0.211) between skin tone and African admixture. Clearly, if you are considered a Black American, the more African admixture you have, the higher your melanin index. More importantly, the range of skin reflectance as measured by the melanin index of Black Americans is very large, spanning a vertical distance of fifty points from 30 to 80.
“Whites’ Skin Tone as Function of Admixture,” below, shows the other U.S. endogamous group from the earlier chart—those who self-identified 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, spanning 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.
Whites’ Skin Tone as Function of Admixture 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. 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 very bad, a few are very good, and most spread across the entire range of batting averages 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. Similarly, wild cows vary greatly in the amount of milk that they produce. The cows in a dairy farm produce more milk on average but, more importantly, their milk production varies very little among themselves (compared to wild cows). The reason? 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 of the two endogamous U.S. groups are not symmetrical because the selection process acts only upon the White group. As revealed in court records, discussed elsewhere, 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, thus joining the White endogamous group or a Black self-identity, thus joining the other group. 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.24
In conclusion, U.S. society has unwittingly applied selection pressure to the color line. 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 color. That skin-color does not vary with African genetic admixture among American Whites, despite their measureably recent African admixture, demonstrates and 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.
check the link below:
www.backintyme.com/Essay040608.htm