Genetic Testing: What Will It Tell Me About My Fraternal Twins?

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!

Newborn twins.

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?

10 thoughts on “Genetic Testing: What Will It Tell Me About My Fraternal Twins?

  1. jwo79

    It’s fascinating isn’t it?

    We were told that our twins were non-identical when I was pregnant and as there is a history of twins in the family (my Dad is a twin) we went along with it.

    When the girls were born they looked sufficiently different to confirm the fraternal twin ‘diagnosis’. However, it got harder to tell them apart through the first year and we did the DNA test when they were 13 months old. Every single marker matched up and they are 99.96% identical

    (They still look different to me though – I don’t care what anyone says!)

    1. Christina Baglivi Tinglof

      Your situation is pretty typical. From what I’ve read, MZ twins often don’t look much alike when they’re born but slowly become more similar as they grow up. And it’s just the opposite for DZ twins–they look a lot alike at birth (I know my DZ twins did) and then slowly begin to look dissimilar as they grow. It’s just the opposite of what you’d think it would be!

  2. Nanner

    I have a friendly wager to propose. I’ll bet you a guest blog-entry that your boys are identical! I say this as a mom to 2 sets of identical twins.
    First of all, unless you’ve had the DNA test done in the past, they have different blood types, or they are different sexes, there isn’t much justification for considering them fraternal, especially if people have had a hard time telling them apart! I can’t tell you how many nurses and even doctors sometimes considered my di/di twins fraternal. It is an assumption in the medical field – sometimes they know better but are too lazy or time-pressed to explain it to us M.O.M’s.

    My oldest looked different enough at first, mostly in head and face shape. By a year the there was mass confusion and the DNA test ocnfirmed our suspicion. At age 5, I can readily tell them apart, but others cannot until they see them quite often. Thier voices are different. One is left-handed, the other right-handed and they have opposite cross bites. But no identical moles!
    the younger set looked indistinguishable from day 1. Except for hte shape inside their ears. They have opposite cowlicks and one is showing tendancy towards her left hand already. They got some of their teeth on the same day. To be honest, I haven’t been able to bring my self to spend the several hundred bucks to get them tested, at this point, it seems wasted!
    Yet they are both monozygotic twins.

    So what do you say? If I am right, you post as a guest at tandemtwinning, and if I am wrong, name your price…

    1. Christina Baglivi Tinglof

      Funny! You’re on! It will be awhile, mind you, because I, too, don’t want to spend the big bucks. (And Christmas is coming, can you believe it?) When the New Year comes, I have every intention of having them tested. But I do truly know the answer–DZ. No one gets them mixed up even though they do have some similarities. I do agree, however, that many DZ twins are actually MZ. Just this weekend, I was speaking at a twins’ conference and I asked a woman if her kids were MZ or DZ. She said she didn’t know. I asked her what SHE thinks as studies have shown repeatedly that a mother’s best-guess is more than 95 percent accurate! A few months back, I was in the audience where several twin experts spoke. One of the moderators said he always thought his twins were DZ until he mentioned to a twin researcher that “everyone gets them mixed up.” The researcher said, “Get them tested. I bet they’re MZ.” He did. His grown daughters learned for the first time that they were in fact MZ! Fascinating!

  3. Nanner

    Good to know us mothers have a ‘motherly instinct’ after all! It may even be difficult though to verify exactly ‘what’ your twins are from the genetic testing – if the results are stated as mine were, “There is a 99.99999% probability of twin zygosity.” I called to verify my interpretation. But what if it is only 96%? Or 90%? I think there has been a tolerance set for what is considered MZ and what is not, and it has changed over the years.

    Just have to add…sorry for the poor spelling in my previous comments! I’m sure it was late at night – that’s about the only time I have to be online!

  4. Pingback: One is Not Like the Other: Differences in Identical Twins « Blog About Twins

  5. Elizabeth

    Hi, I was wondering what happened with the test. I didn’t see a follow up post.
    Also, I hadn’t heard of half – identical twins before. How fascinating!

    1. Melissa Hopkins

      First, siblings AVERAGE 50% overlap in DNA. They can have anywhere from zero to 100% overlap in their DNA, though of course, statistically speaking the vast majority end up between 40-60% overlap in DNA.

      That being said, half-twinning is possible exactly as described where an egg splits in half and then gets fertilized by two different sperm, though this is incredibly rare, but has been documented. I say this as a biochemistry and molecular biology major.

      One neat thing to consider though, is if you have identical twins, their children may be cousins but genetically speaking they will be half-siblings :)

  6. Pingback: Why Do So Many Identical Twins Think They’re Fraternal? | Blog About Twins

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