Loading ...
Sorry, an error occurred while loading the content.

(Article) “The Heredity of “Racial” Traits”

Expand Messages
  • multiracialbookclub
    SUMMARY: PHENOTYPE DOES NOT HAVE TO REFLECT GENOTYPE The article below clearly proves the point that ... it is not essential for a multiracial person s
    Message 1 of 1 , Sep 5, 2005
    • 0 Attachment
      SUMMARY: PHENOTYPE DOES NOT HAVE TO REFLECT GENOTYPE

      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 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 [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[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'
      [Ethic] population [not to be mistaken for the
      `Black-American' "racial" 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 "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"
      [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 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 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 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.

      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 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.]]]

      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=3410785&showStat==Ratings

      RELATED LINK:
      http://groups.yahoo.com/group/Generation-Mixed/message/171
    Your message has been successfully submitted and would be delivered to recipients shortly.