1402Article: "The Heredity of "Racial" Traits"
- Oct 21, 2006
A Person's 'Phenotype' (i.e. Appearance)
Does Not Always Have to Reflect
Their Full-'Genotype' (i.e. Lineage)
The article below clearly proves the point that ...
it is not essential for a multiracial person's
`Phenotype' (physical appearance) to always
reflect their `Genotype' (ancestral lineage)
"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 been embraced by America's ever-expanding
blanket of ... 'the mainstream' ... within a few generations,
while [the] African-American [Ethnic group] remain [outsiders]?
The irony in the historian's [rather illogical]
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...
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"-[categorized] 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 [socio-politically] "identify"
the family as part of the "black" community.
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.
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' [Ethnic] population
[not to be mistaken for the `Black-American'
"Racial" population] has no detectable
African genetic admixture [at all].
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
outlooking 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
ADMIXTURE' ARE NOT
"FIRST-GENERATION" [TYPE OF]
INSTEAD, MOST SPRING
FROM PARENTS WHO ARE
ALSO OF AFRO-EUROPEAN
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-Lline'
canhelp 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-Lline" and the group labels "black" and 'White'
corresponding to Brown versus Pinkish-Beige skin tone.
-- Second, MELANIZATION IS
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,
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 [ALTHOUGH
SOME SOURCES SAY IT'S 4%] OF [SO-CALLED'
"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.
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