A commenter below sayeth:
I remember reading somewhere that a child can't be darker than the darker of his two parents and that in such instances the biological father is not the putative father. I have no idea if that's true or not.
This seems like a common sense assertion, but as I noted this is not really strictly correct in an apodictic sense. That is, just because you have a very dark skinned parent and a light skinned parent, it does not necessarily follow that the range of the offspring shall be bounded by the values of the parents. To some extent I can see how this makes sense; I believe it piggy-backs upon the blending intuitions about genetics which emerge out of innate folk biology, when in the case of complexion its discrete character is now well attested. But, it also bespeaks a specific empirical naivete, as anyone who is of South Asian or Brazilian background might attest to cases where offspring may express phenotypes outside of the ranges of both their parents. Consider height, one certainly knows cases where parents have offspring outside of the ranges of the heights of the parents! But the point I'm trying to make can be illustrated by concretely graphically.
Consider two parents, one of European ancestry, and one of West African ancestry. As it happens between population variation in skin color is controlled by & large by about 6 genes. Many of these genes, e.g., SLC24A5, exhibit a disjoint frequency between the two populations. For example, on the aforementioned locus almost all Europeans carry the derived variant which is correlated with lighter complexion, while almost all Africans carry the ancestral variant which is correlated with darker complexion. The alleles seems to exhibit a rough codominance, so one can model them as additive an independent. In other words, heterozygotes (those who carry a "European" and "African" variant) exhibit a phenotype between the two parents. If you assume that a European (the lightest human population) and Africa (one of the darkest human populations) are disjoint for all the loci which control between population variation, then you have a case where all offspring are heterozygote:
| European Parent | Gene 1 | Gene 2 | Gene 3 | Gene 4 | Gene 5 | Gene 6 | |
| African Parent | LL | LL | LL | LL | LL | LL | |
| Gene 1 | DD | LD | LD | LD | LD | LD | LD |
| Gene 2 | DD | LD | LD | LD | LD | LD | LD |
| Gene 3 | DD | LD | LD | LD | LD | LD | LD |
| Gene 4 | DD | LD | LD | LD | LD | LD | LD |
| Gene 5 | DD | LD | LD | LD | LD | LD | LD |
| Gene 6 | DD | LD | LD | LD | LD | LD | LD |
The European parent contributes a "light" variant at each locus, while the African parent contributes a "dark" variant at each locus. The offspring, the hybrid, has a dark and light variant at each locus, resulting in a complexion between the two parents. But what if two hybrids mate? Here the wisdom of farmers comes into play: hybrids do not "breed" true.
This matrix illustrates some of the combinations:
| Mixed Parent | Gene 1 | Gene 2 | Gene 3 | Gene 4 | Gene 5 | Gene 6 | |
| Mixed Parent | LD | LD | LD | LD | LD | LD | |
| Gene 1 | LD | LD | LD | LD | LD | LD | LD |
| Gene 2 | LD | LD | LD | LD | LD | LD | DD |
| Gene 3 | LD | LD | LD | LD | LD | DD | DD |
| Gene 4 | LD | LD | LD | DD | DD | DD | DD |
| Gene 5 | LD | LD | DD | DD | DD | DD | DD |
| Gene 6 | DD | LD | DD | DD | DD | DD | DD |
(there are more matrix combinations than this, and this is really just a binomial distribution, so the expectation is obviously [by definition] more frequent than the edges of the distribution)
As you can see some of the offspring exhibit the same complexion as the hybrid offspring. But, some of the offspring exhibit more extreme phenotypes than either parent. This is because heterozygotes produce homozygote offspring, as well as heterozygote offspring. This basic monogenic insight can be expanded to multiple loci, and the variance we see around the expectation is rather easy to model. Most offspring of brown-skinned parents will be brown-skinned, but some will be much lighter, and some much darker, than either parent.
The comment above is a clear manifestation of the human intuition that genetics is analog, and that it operates by blending the essences of the two parents. If variance is constrained by parental variance, then naturally over time the variance would diminish to the point where no extant variance would remain for evolution to operative upon via natural selection. This was a major problem with Charles Darwin's original model, and before the insight of Mendelianism and discrete inheritance it had to be explained away via hand waving. No need today. We know the genetic architecture of many traits, including skin color, and the shape of human variation is easy to comprehend.
Addendum: I'm ignoring environmental and epistatic components of variation, which might produce a different distribution than the one above, at least on the margins. Additionally, in terms of ascertaining paternity the real genetic architecture matters. The hybrids I used to illustrate why variance is maintained are "best case scenarios." There are genetic architectures where individuals could express the same phenotype in complexion as hybrids, but result in far less variance. I will leave that up to the readers to expand upon in the comments. Finally, the illustration above shows why racial admixture will not lead to the uniform browning of the human race!
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I am Brazilian, and my skin is darker than the skin of my both parents. In this country we are all "classified" as white. However, I think that mediterranean caucasian would describe our ethnicity and background better than simply white, since our natural tan is anything but that pinkish white so commom in the Northern European. Latino, as usually it is defined by the populations in what was Spanish America includes people of a very mixed combination of Iberican, Native American and African heritage, which is not the case for us.
After trying to show my own backgroud, I would like to confirm your information and say that I have always had the darker complexion of all my cousins. My parents' skin is has a lighter shade than mine. Many other people in a country of mixed heritage have this same darker phenotype than their own parents. Just in case who reads has any doubts, I am that guy who people cannot tell if he looks more alike to his father or to his mother.
But what I'd really like to say is that, in a country where interracial marriage has been commom for centuries, it is not unusual to know so-called white couples who have had a very dark-skinned child, who we should suppose is theirs. In this situation, there are still social issues that lead the parents to declare the child's ethnicity as "white", but this is another story. And the story happens in the other way back, too: it is even amusing to see two black parents having a child of very light complexion.
I knew a guy in college who was paler than I was, although he did have dark wavy hair and brown eyes. He carried around an album of pictures of him with his family to prove he was black. Most white people would not accept that he was black, and very few blacks would accept him as black.
6EQUJ5,
That's strange. I had a biracial friend who carried around a picture of her mother to prove she was black. She had wavy brown hair, light skin, blue eyes, and freckles.
Razib,
This answers a question I've been wondering about for some time. My half-sister, who has much more white blood than I do and two light-skinned parents, is dark-skinned like our grandfather. People have often joked about where she "came from" as if our mother was sleeping with the mailman or something. Mom always said that it just skipped a generation. I guess she was right. That doesn't stop me from being bitter about being born pale (and completely incapable of tanning) rather than darker like my father.
This misconception can also be proven false with only one gene, as long as there is incomplete dominance. The mating of two heterozygous individuals will result in 50% of the offspring (on average) having a trait "intensity" that is outside of the parental range.
It seems to me that this kind of variation is common. I had a school friend who could pass as Indian but claimed to have no non-white in his background. I have also met a Native Canadian couple who had a blonde, blue-eyed child and had an African colleague who looked to be of mixed background. In these cases, they knew of no European ancestors. Of course, something that happened four or five generations ago could have been forgotten by now.
I am reminded of a story told by a black colleague who married the blonde daughter of a local farming family. He said that when his wife was expecting their first, her little nephews and nieces became increasingly excited until the day after the birth when they saw their new cousin. They came out really disappointed saying 'It's just brown all over.' They expected the baby to be black and white in patches like their cattle.