When my fraternal twins were born, many people including my husband, had a hard time telling them apart. It didn’t help that my sons were born bald, had the same coloring both in complexion and eye color, and weighed within six ounces of each other. As time passed, however, each boy developed his own distinct look. Although one grew curly hair, the other wavy, many people continued to ask the obligatory question, “Are they identical?” Thinking back to those early days, I never thought they looked alike but then again, I’m their mother. Yet now even when I look back at a photograph from that first year, I sometimes have trouble telling them apart!
As parents of twins, we always hear the importance of having our multiples tested to confirm their zygosity (the genetic relationship of a zygote). Although I’ve never questioned my sons’ zygosity (believe me, they’re fraternal), I have wondered, however, just exactly how genetically related they are. If fraternal twins are basically “siblings born on the same day,” and share 50 percent of their DNA at most, that leaves a lot of wiggle room. In theory, that would mean that fraternal twins can conceivably share only 25, 30, or even 40 percent of their DNA.
Most at-home twin zygosity tests analyze eight genetic markers. If twin siblings match all eight, they’re considered identical or monozygotic. Any less, fraternal or dizygotic. But what about twins who share less than four markers? Although still considered fraternal, are they less alike than fraternal twins who share more than four genetic markers? And what about those fraternal twins who share five or six markers? Are they considered polarbody twins or “half-identical,” a third twin-type theory where one egg splits and then is fertilized by two separate sperm?
I bring all this up because as my twins are getting older, I’m noticing that they’re beginning to once again physically resemble each other. Not a lot, mind you, but more than they have a few years ago. This all came to a head as I was cutting one twin’s hair the other day. I noticed he had a mole on the right side of his scalp just above his ear. Then it hit me. His twin also has a mole right above his ear but his is on the left side of his head. That alone isn’t enough to alert the media but it does add to a growing list of “mirror-image” similarities between the two. One twin is right handed; the other left. They have opposite hair whorls. They even share opposite dental patterns, namely an overlapping incisor. Although only identical twins can be mirror twins, it brings me back to my original question: How genetically similar are my fraternal twins?
So I’ve decided to conduct a little experiment: I’m going to get them tested to see how many genetic markers match up. And just to add a control group, I’ll include their younger brother to the mix, too. After all, wouldn’t it be interesting to see if the youngest, single-born brother had more DNA in common with just one twin than the two twins share with each other? Would that have an impact on their bond as brothers? I’m guessing not much but it sure would make the singleton feel a little special and a bit more included in the tribe.
Stay tuned for results.
What about you? Have you ever considered having your fraternal twins genetically tested?