Domen
Misico dux Vandalorum
'One drop rule', 'passing' and all that.
If someone has just "one drop" (and not much more) of "Black genes", then you won't even be able to tell this (even though light skin - just like blond hair or red hair - is a recessive trait, IIRC, and therefore mixed Mulatto children of Black-White couples more frequently tend to be darker rather than whiter).
In NYC people who are "Non-Latino Whites" in censuses have on average 10% of Sub-Saharan African Y-DNA Chromosomes (see the study I linked).
This is probably due to large scale of interracial marriages in NYC, but it might be also due to some "rather Black" people declaring Whiteness in censuses.
In Brazil there is also a mix of races, and on a much larger scale than in the USA. All three groups - Whites, Blacks and Pardos (Browns) - have been proven to have various degrees of both European, Sub-Saharan African and Amerindian genetic ancestries. Statistically these groups differ on proportions - people who describe themselves as Whites have (on average) larger % of European genes than people who describe themselves as Blacks. Pardos tend to be the most mixed group, with most similar levels of European, Amerindian and Black ancestries. I actually already wrote about this some time ago in another thread:
http://en.wikipedia.org/wiki/Pardo#Ancestry
http://en.wikipedia.org/wiki/Pardo#History
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3040205/
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Mexico is also very mixed (most of the population are Mestizos) and groups are statistically different only in terms of proportions.
Nevertheless, it was not an obstacle for Mexican government to introduce a "racially-based" caste system:
The Caste War of Yucatán (1847–1901), during which Mayans created an independent state, which existed for some 50 years:
http://en.wikipedia.org/wiki/Caste_War_of_Yucatán
Background
In Spanish colonial times, Yucatán (like most of New Spain) was under a legal caste system, with peninsulares (officials born in Spain) at the top, the criollos of Spanish descent in the next level, followed by the mestizo population, then the descendants of the natives who had collaborated with the Spanish conquest of Yucatán, and at the bottom were the other native indios.