Archive for the ‘Molecular Anthropology’ Category

How Much of the European Contribution to the African-American Genome Comes From Females?

January 10th, 2012

Some African Americans have few European DNA markers and some have many; but overall, about 17 percent of the collective African-American gene pool comes from Europe. What fraction of that 17 percent comes from European females? Can DNA tell? The short answer is “no”.

Three Generations of Autosomal Mapping — A Case Study

December 12th, 2011

An opportunity recently arose to collect autosomal admixture mapping data for three generations of one family, a family that has European, subsaharan African, and Asian admixture. The results are educational. They exemplify the heredity of ancestry-informative DNA markers. They show, on the one hand, that the transmission of ancestry-informative markers from one generation to the next is predictable. On the other hand, they also show that each transmission is random, so that predictions must rely on probabilities. Such blending of randomness with predictability is usually hard to explain. The following case history makes it clear.

Why Are Europeans White? (E1)

May 21st, 2009

Tells why northern Europeans are so oddly de-pigmented compared to everyone else on the globe. Session E1 of a series of topics on molecular anthropology included in my lectures on “The Study of Racialism.” The prior session, E5 discussed the migrations that carried our species around the globe in prehistoric times. This topic looks at later regional adaptations.

Can DNA Tell What “Race” You Are? (E10)

September 6th, 2008

Molecular anthropologists are often asked if DNA markers can tell what “race” you are. The short answer is “no.” Mitochondrial DNA and Y haplogroups can tell from which continent your matrilineal and patrilineal ancestors came. And if you live in the Americas, autosomal mapping can tell what fraction of your ancestors came from Africa as slaves, what fraction came from Europe as colonists, and what fraction were Native Americans. But no DNA can tell your “race.”

Sexual Selection and the Color Line

August 1st, 2007

Is the extraordinary paleness of the natives of the Baltic region caused by sexual selection? Are the different skin tones of men and women caused by sexual selection or are they a side-effect? Many women believe that a pale skin attracts men. Is this genetic? The answers are presented in seven topics: (1) Nature selects among competing alleles, not among species, nor individuals. (2) Selection can help an allele but hurt its host species. (3) Sexual selection happens when two alleles conspire. (4) The five hallmarks of sexual selection. (5) Is the Baltic paleness adaptation sexual selection? (6) Was skin-tone dimorphism caused by sexual selection? And (7) do current fashions prefer paler skin?

The Heredity of “Racial” Traits

December 15th, 2004

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 of 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 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.

The Rate of Black-to-White “Passing”

September 9th, 2004

This essay discusses the annual rate of Black-to-White endogamous-group switching by Americans in four topics. The first section, The Average Yearly Rate is Between 0.10 and 0.14 Percent, uses several independent methods to compute the rate of switching. It shows that all converge to the same narrow range of numbers. The Percentage Rate Has Remained Relatively Steady over the Years, demonstrates why we know that Black-to-White group switching has been steady and continuous over the centuries. Our DNA reveals that the current African admixture in White Americans was neither the result of a one-time event before intermarriage was outlawed in 1691, nor the result of intermarriage after Loving v. Virginia 1967 ruled anti-intermarriage laws unconstitutional. How Can so Many People Falsify Their Paper Trail and Cut all Family Ties, explains that, except for a brief period in U.S. history—the Jim Crow era—there has never been a need to deceive nor to cut family ties. For most of the past 300 years, endogamous-group switching was done openly, just as it is today. Finally, The Maroon Escape Hatch suggests how, even during Jim Crow, families could pass from Black to White through a two-step process that included an in-between stage.

The Perception of “Racial” Traits

July 19th, 2004

This essay reviews what is known about the culturally dependent perception of “racial” traits in four topics. Harry Hoetink’s Somatic Norm Image considers whether predictable differences in colonial histories determine how people see “racial” group membership. How U.S. Children Learn to See Two Endogamous Groups examines the stages through which children learn to identify and to articulate what their culture sees as “racial” traits. The Instinctive Need to See “Otherness” identifies the cognitive system, selected by adaptation to hunter-gatherer life over 200 millennia ago, that has been co-opted to identify someone as having “racially” different looks—an encounter that no Paleolithic hominid could ever have experienced. Finally, The Decline of the Bio-Race Concept offers a brief explanation of why the biological concept of “race” as applied to humans has been abandoned by the hard sciences.

Afro-European Genetic Admixture in the United States

June 8th, 2004

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.

The Paleo-Etiology of Human Skin Tone

December 15th, 2002

The growing consensus for an out-of-Africa scenario of modern human dispersal has produced a two-part puzzle of regional variation. Since the last glacial maximum, Europeans developed fairer complexions than any other group on earth, fairer even than others at the same or higher latitudes. Equatorial Native Americans, in contrast, failed to turn very dark over the same time period, despite living at the same latitudes as Africans and Melanesians. This essay suggests a two-part solution. Compared to other high-latitude dwellers, European diet was uniquely cereal-based and so deficient in vitamin D. Compared to other low-latitude dwellers, the ancestors of Native Americans had already lost the alleles necessary for dark brown skin before they crossed Beringia. This essay comprises two parts. Part 1 introduces the puzzle. Part 2 presents the solution.