What is perhaps one of the most widely fascinating parts about genetics is the way in which two people who are very closely related can look entirely different -- so much so that others can barely fathom a blood relationship between the two. Take, for example, 18-year-old twins Lucy and Maria Aylmer who recently made headlines because of their unusual appearance: Maria has darker skin, dark brown eyes, and black hair, while Lucy has pale skin, blue eyes, and red hair. While twins like this are certainly surprising, they are not abnormal in the genetic sense, simply unusual. Understanding these rare genetic occurrences can illuminate invaluable perspectives around race and its role in society. 
Much like the Aylmer twins, Triniti and Ghabriael Cunningham, now about 6 years old, were featured on an ABC News segment in 2011 because of their remarkably unusual appearance. The fraternal twins were delivered looking significantly different, each more like one parent than the other. Ghabriael, born with blond hair, a fair complexion, and blue eyes, takes after his white, blue-eyed mother, Khristi. Triniti, born with a significantly darker complexion and deep brown hair and eyes, much more closely resembles her father, Charles, who is black with brown eyes. 

The twins' parents, from Akron, Ohio, explain that their children's appearances have sparked some rather ignorant responses from strangers, but they don't let it bother them. Triniti and Ghabriael are quite the rarity in appearance, but according to genetics, they aren't much different than any other set of biracial siblings.  

Dr. Ronald Bachman, chief emeritus of the genetics department at Kaiser Permanente of California, explained to ABC News that the Cunningham twins' situation -- one twin appearing to be white while the other appears to be black -- is "no big deal" from his viewpoint. He said that many geneticists, including himself, "don't know a hell of a lot and don't pay much attention to skin color and eye color." So why is this?

It has to do with the number and type of genes that a child inherits from each parent. Because eye color and skin color are determined by multiple genes that operate under "incomplete dominance," these traits end up manifesting as a visual mix of both parents. The genes for these traits can come in several different forms, referred to as alleles, that arise by mutation but are found on the same place on a chromosome. Both skin and eye color are determined by multiple alleles, so there is a ton of room for variation. Mirror UK explained that a recessive "white skin gene" could be one of many alleles, maybe even as many as 20, so it could take countless generations of new genetic pairings before that recessive gene is paired with another recessive "white skin gene." 

Atavism, otherwise called an "evolutionary throwback," is the term that refers to the way in which people can pass on a trait for many generations without it being exhibited -- except for when it finally manifests in that one person whose appearance leaves everyone else in the family scratching their heads. Ghabriael Cunningham's skin tone and eye color, for example, are the result of atavism; his father was unknowingly carrying genes for white skin, blond hair, and blue eyes from some ancestor who also possessed those traits. If Ghabriael's skin tone and eyes had been different in color, his father may have never known that he carried the blond hair, blue eye, and white skin alleles. 

When you think back on the history of slavery in America, it makes a lot of sense that certain traits could lay dormant in a family line for countless generations. Look at the example of Michelle Obama's history as determined by genealogist Megan Smolenyak along with the New York Times

"In 1850, the elderly master of a South Carolina estate took pen in hand and painstakingly divided up his possessions. Among the spinning wheels, scythes, tablecloths and cattle that he bequeathed to his far-flung heirs was a 6-year-old slave girl valued soon afterward at $475.

"In his will, she is described simply as the 'negro girl Melvinia.' After his death, she was torn away from the people and places she knew and shipped to Georgia. While she was still a teenager, a white man would father her first-born son under circumstances lost in the passage of time ...

"Melvinia Shields, the enslaved and illiterate young girl, and the unknown white man who impregnated her are the great-great-great-grandparents of Michelle Obama, the first lady."  

As Jason A. Gillmer, a law professor at Texas Wesleyan University, told the New York Times, "No one should be surprised anymore to hear about the number of rapes and the amount of sexual exploitation that took place under slavery; it was an everyday experience." When you scrutinize American history, it's impossible not to see how people of different races, through their sexual relationships, have been combining diverse and complicated sets of genes since the foundation of this country. America has long-drawn immigrants to its shores from all over the world, and biracial babies have been born on American land since the Colonial settlers first mingled with American Indian tribes.

Sets of siblings like Triniti and Ghabriael and Lucy and Maria are proving an interesting point in a world that is still experiencing large amounts of racism, especially in America. That lesson can be painfully simple: Skin color has nothing to do with the quality of a person's character, it is simply an evolutionary adaptation that can vary greatly even amongst close family members.  

The genes that determine skin color control melanin levels, and melanin is nothing more than pigmentation that protects skin from harmful UV rays from the sun, as explained by the Smithsonian Institution. UV radiation levels increase as you move closer to the equator and decrease as you move away, hence why people at the poles of the Earth tend to be very pale, while people living right around the equator tend to be very dark. Skin color is a human adaptation to our environments, and it is certainly not set in stone across family lines. 

Once the realization sets in that skin color is the result of different environments creating different types of bodies, it can be a bit hard to wrap your head around why it has been the cause of people killing, enslaving, and discriminating against each other for so long. 

Triniti and Ghabriael's story is powerful because it dispels ignorance: it helps us see the arbitrary nature of using skin tone as a tool to judge others because skin tone itself can be quite arbitrary. 

In sharing the stories of siblings like the Cunninghams and Aylmers, countless people are being exposed to the idea that skin color is much more fluid across generations than most people would like to believe. These twins are telling a new story of how people who had previously thought they were "only black" or "only white" were wrong about their ancestral pasts, while simultaneously reminding the rest of us that we could find out the same thing one day. 

Most important, however, is the lesson coming from Khristi and Charles Cunningham, who pay very little attention to their twins' different skin tones. They acknowledge that they live in a world that tends to be more focused on skin color than not -- they told ABC News that they have experienced racism both at work and in their community -- but they see both of their children as perfect while choosing not to focus on the color of their twins' skin. 

Charles Cunningham told ABC News, "No one gets to say if they are black or white. It's not a choice, and to me they are perfect and will grow up to be loved. There's too much hate. People should be more worried about whether you're a Republican or a Democrat." In a world that can't seem to stop focusing on color, the Cunninghams have made a beautiful and powerful argument for a colorblind future.