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Article: "The Heredity of "Racial" Traits"

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  • multiracialbookclub
    SUMMARY: 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
    Message 1 of 2 , Oct 21, 2006
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      SUMMARY:

      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)


                             ARTICLE:

      "The Heredity of "Racial" Traits"


      [Introduction]

      "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.

      [Analysis]

      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
      INTERMEDIATE `AFRO-EUROPEAN
      ADMIXTURE' ARE NOT
      "FIRST-GENERATION" [TYPE OF]
      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-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 feature—skin 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
      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.

      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. ...


      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.

      http://staff.jccc.net/scorbett01/Physical/populations.htm
      http://nhs.needham.k12.ma.us/cur/Baker_00/2001_p2/baker_dc_ad_p.2/human_genome.htm 
      http://www.getcited.org/?PUB=103410785&showStat=Ratings

      RELATED LINK:
      http://groups.yahoo.com/group/MGM-Mixed/message/240

    • multiracialbookclub
      Related Links: http://groups.yahoo.com/group/MGM-Mixed/message/458
      Message 2 of 2 , Oct 21, 2006
      • 0 Attachment

        Related Links:

        http://groups.yahoo.com/group/MGM-Mixed/message/458
        http://groups.yahoo.com/group/MGM-Mixed/message/464

        SUMMARY:


        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)


                               ARTICLE:

        "The Heredity of "Racial" Traits"


        [Introduction]

        "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.

        [Analysis]

        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
        INTERMEDIATE `AFRO-EUROPEAN
        ADMIXTURE' ARE NOT
        "FIRST-GENERATION" [TYPE OF]
        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-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 feature—skin 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
        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.

        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. ...


        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.

        http://staff.jccc.net/scorbett01/Physical/populations.htm
        http://nhs.needham.k12.ma.us/cur/Baker_00/2001_p2/baker_dc_ad_p.2/human_genome.htm 
        http://www.getcited.org/?PUB=103410785&showStat=Ratings

        RELATED LINK:
        http://groups.yahoo.com/group/MGM-Mixed/message/240

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