== C3-99 1.50 3.00


The Heredity of "Racial" Traits. Loading...


== C3-01 1.84 2.45 (Byrd)


Some people assume that the physical traits associated with the U.S. 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 over generations. All three assumptions are mistaken.


== C3-02 1.73 2.60 (Kitt)


It is true that the child of a Nordic-looking parent and a Nigerian-looking spouse will be of in-between appearance. But a child of two such in-between-looking individuals can appear anywhere along the range, from one extreme to the other.

=T 5.0

Parents of mixed intermediate Afro-European genetic admixture often produce White-looking or Black-looking children.


== C3-03 2.51 1.79 (Becker)


VITAL POINT #1. One-fourth, one-half, one-fourth.


A child of mixed parents has a 50-50 chance of showing color-line-related features midway between those of the parents, a 1/4 chance that it will look more European than either parent, and a 1/4 chance that it will look more African than either parent.


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 (one third of White Americans having 2-20 percent African genetic admixture).


== C3-04 1.91 2.35 (mixed family 1)


A problem in discussing heredity is knowing just which features are associated with the color line. People who look Black are usually assigned to the Black endogamous group by U.S. society. Those who look White can often choose their ethnicity.


=T 40.0

But precisely what does it mean to say that someone "looks Black"? The very same individual may be considered White in Puerto Rico, Coloured in Jamaica, and Negro in Georgia.


== C3-05 1.81 2.48 (mixed family 2)


Craniofacial anthropometrists (forensic anthropologists) today give more importance to prognathism than to other traits, and nineteenth-century Americans emphasized foot shape. So this discussion of heredity looks at a single feature: skin tone.


Keep three things in mind. First, many societies (Hindu India, for example) do not associate skin tone with any endogamous barrier.


== C3-06 1.84 2.45 (Rowell)


We focus on skin tone because it is important to most Americans, hence the term "color line" and the group labels "Black" and "White" corresponding to brown versus pinkish beige skin tone.


Second, skin tone 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.


=T 20.0

== C3-07 1.50 3.00 (Jones)


Finally, the following applies to any feature that depends on a handful of co-dominant additive genes, such as hair curliness, nose width, lip thickness, prognathism, steatopygia, and the like.


It applies to all of the physical traits that Americans see as "looking White" or "looking Black." Not just skin tone.


== C3-08 2.19 2.05 (O'Brien)


Alleles do not blend. They are digitally encoded, not analog. (The human genome contains about 750 megabytes of data). Because they are digitally encoded, alleles combine in simple, mathematically predictable ways.


Imagine that skin tone depended on just one gene with two possible values (rather than the actual three-to-six genes).


== C3-09 1.50 3.00 (JEJones)


Someone with BB would look Nigerian. WW would look Norwegian. And BW (or WB) would look Arabic or Puerto Rican. Now imagine that a BW father and a BW mother have a child. What are the chances that the child will be BB, BW, or WW?


Work it out. It is the same as flipping two coins and getting two heads, one of each, or two tails.


== C3-10 1.71 2.63 (twins)


One fourth, one half, one fourth. The photo shows twin girls. One is lighter than both parents and the other is darker. The chances of such siblings happening to any mixed couple are 1/4 X 1/4 or 1/16.


(Incredibly, hundreds of genetically illiterate newspapers reported this photo as a "one in a million" chance. As every Puerto Rican and Brazilian family well knows, it happens all the time.)


== C3-11 1.68 2.68 (Summer)


One fourth, one half, one fourth. It makes no difference how many additive co-dominant genes affect skin tone. There are at least three and probably no more than six. But the number of genes affects only the smoothness of the probability distribution.


The probability does not depend on number of genes, nor even on the species of organism. It is characteristic of all sexual reproduction with digital encoding. One fourth, one-half, one-fourth.


== C3-12 1.61 2.80 (Puerto Rico)


Now consider entire populations. What are mere random-chance probabilities for individual families are predictable distributions when you look at populations.


In the 18h and 19th centuries, Spanish censuses of Puerto Rico reported that half of the population was White and half Black. Today, the native-born island population physically matches the theoretical Poisson distribution almost precisely.


About one Puerto Rican in ten looks White to most Americans, about one in ten looks Black, and the rest look in between. The island's skin-tone histogram has a single peak at the 50-50 mark with population fractions diminishing towards both extremes.


== C3-13 1.63 2.75 (Philadelphia)


How many White-looking children are born into Black families? It varies by region. On average, people with 12 percent or less African admixture look White to the most Americans and those with up to 25 percent look mixed (Hispanic or Mediterranean).


Assuming that skin tone is set by 4 co-dominant additive genes (3-6 in actuality), anyone with 6 or more Euro haplotypes out of the 8 looks White.


The mean Euro admixture in Philadelphia's African-American community is about 20 percent. As shown by the two leftmost bars on the chart, this predicts about one White-looking child out of every 500 [0.2^^6 x 28 permutations].


Of course, counting how many children are born with European appearance measures opportunity, not action. There is no reason to think that all or even most such Americans actually cross the color line and re-designate themselves White as adults


== C3-14 1.63 2.75 (Geechee-Gullah)


On the other hand, the mean Euro admixture among the Geechee-Gullah community of the Sea Islands is only about 3 percent. As show by the two leftmost bars, the result is nil. It cannot happen in the absence of out-marriage [0.03^^6 x 28 permutations].


== C3-15 1.63 2.76 (Average American)


How many Black-looking children are born into White families? It varies by region. Following the same assumptions, anyone with 3 or more Afro haplotypes out of the 8 looks Black.


The mean Afro admixture among White Americans is about 0.7 percent. As shown by the chart, the result is nil. It cannot happen in the absence of intermarriage [0.007^^3 x 56 permutations].


== C3-16 1.64 2.74 (Iberia)


On the other hand, the mean Afro admixture among Argentineans and southwestern Iberians is about 5 percent. As show by the chart, this predicts about one Black-looking child out of every 200 [0.05^^3 x 56 permutations].


For more on this topic, see http://backintyme.com/essays/?p=11


== C3-99 1.50 3.00


The End