A comment on the 'heredity' of "racial" traits ....
The 'heredity' of "racial" traits
THE HEREDITY OF "RACIAL" TRAITS:
"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 the mainstream within a few generations,
while African-Americans remain [outsiders]?
The irony in the historian's answer could be seen in the four
children playing in the grass nearbytwo apparently members
of the Black endogamous group and two apparently White.
Every February, the Florida Park Service
hosts a Civil War reenactment at Olustee .
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 .
* * * * ** * * * ** * * * ** * * * ** * * * ** * * * ** * * * ** * * * *
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 on 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.
The fact that 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 [so-called] 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.
---All of these assumptions are mistaken.
---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.
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."
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
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 .
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 IN MEAN
TO SAY THAT SOMEONE "LOOKS AFRICAN"?
As explained in the essay "The Perception of `Racial' Traits,"
THE FEATURES ASSOCIATED WITH
AFRICAN ANCESTRY DEPENDS 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.
Modern craniofacial anthropometrists (forensic anthropologists)
give more importance to prognathism than to skin tone,
and nineteenth-century Americans once emphasized foot shape.
Consequently, the following discussion of heritability
simplifies such traits to a single featureskin tone.
Throughout the following discussion,
three things should be kept in mind.
-- First, MANY SOCIETIES 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).
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.
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.
Some polymorphisms have been related to phenotype.
And gene-enzyme-protein reaction chains have been identified.
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.
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.
APPEARANCE IS NOT THE SAME THING AS ANCESTRY
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
THREE PERCENT OF BLACK AMERICANS HAVE NO
DETECTABLE AFRICAN GENETIC ADMIXTURE AT ALL.
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.