Nowadays, people always associate twinning with in vitro fertilization (IVF). They may be right but there is actually more to twinning than IVF. Especially when it comes to monozygotic twins, commonly known as identical twins. Monozygotic twins develop when an embryo splits into two, resulting into two embryos. The reason for the splitting has always been a puzzle. This is the puzzle that researcher Bruno Reversade is trying to solve as he goes around the world investigating so-called twin villages, as reported in Nature News.

In the general population, the rate of having identical twins is 1 in every 250 to 300 births. In some remote villages in the world, the rate is as high 1 in 10 births. One of these villages is Mohammad Pur Umri in northeastern India, where about 10% of its 2000 inhabitants are monozygotic twins. A stranger who happens to inadvertently walk into this farming village would think he is seeing double. In another case, Reversade traveled to Jordan to interview and collect saliva samples (for DNA testing) from a family with 15 pairs of monozygotic twins. Then his next stop is Linha São Pedro, a Brazilian village predominantly inhabited by German immigrants, where again, 10% of births are twins.

Over the years, human biology experts cannot agree about the mechanisms of monozygotic twinning. Dizygotic twinning is easy enough to figure out because the two embryos come from two eggs which are separately fertilized by sperms. The reasons for multiple egg production have been linked to hormones, maternal age, and well, the number of embryos transferred during an IVF cycle. Monozygotic twinning however is more of a mystery.

Some experts say it’s something to do with genetics, e.g. “it runs in the family”. Other says it’s the environment.

There is also the question whether producing twins is an advantage at all. Some experts believe that at least 12% of natural conceptions can result in twin embryos. However, in most cases, one embryo doesn’t survive so that only about 2% those pregnancies successfully produce twins. The disappearing twin is a common phenomenon. However, there could possibly be even more cases where one twin is lost before it is even detected.

Why a lot of twin pregnancies only deliver one baby is theorized to be due to the fact that a woman’s body is designed to carry only one baby to full term. Having twins, therefore, may actually be an anomaly, a reproduction process gone awry. The disappearing twin is probably a mechanism to correct this. When uncorrected, the pregnancy can result in complications such as the high rate of congenital defects in twins, including conjoined (Siamese) twins. And what about those twins who survive and develop normally? Well, those theorists say it indicates “a healthy system that is able to support two embryos through implantation and pregnancy.” Hmmm… at least something that makes me, mother of monozygotic twins, feel better, for a change.

Whatever findings Reversade’s investigation of the twin villages may reveal will definitely interest me. They might even help me in answering one of the most common questions I hear: Which side of the family has twins? Yours or his?

Just when I thought I know everything there is to know about identical twins, comes this new revelation. It seems that identical twins, previously thought to have 100% genetic resemblance, are anything but identical.

A group of researchers from the US, the Netherlands, and Sweden studied the genetic make up of 19 pairs of identical twins. They found that there are subtle but clear differences between the DNA of these twin pairs. These variations may be due to differences that occur during genetic copying and to mutations. In most cases, these slight differences have no observable effect in the physical and developmental make up of the twins. In some cases, however, these variations can make a world of difference health-wise. And we are not talking only about simple ailments but also serious and debilitating diseases as Parkinson`s and cancer.

Doctors were always puzzled how one twin can develop a disorder or disease and the other not. They`ve always attributed these differences to environmental factors. In the case of my twin boys, F developed eczema and asthma while R didn`t. And I`ve always asked myself, where did I do wrong? Have I somehow neglected one and favor the other? After all, I, the mom, am responsible for a lot of these so-called environmental factors, from food, to beds, to toys and baby products. It`s a relief to know that genetics, something beyond my control, plays a role in all of these.

But now, I`m at loss. With this new discovery, the the term ?identical twins? has become a misnomer. Monozygotic would be a more appropriate term but who would understand that? The next time somebody asks me whether my boys are identical, how do I respond?

Source:

Bruder et al., 2008. Phenotypically Concordant and Discordant Monozygotic Twins Display Different DNA Copy- Number-Variation Profiles,

The American Journal of Human Genetics (2008), doi:10.1016/j.ajhg.2007.12.011.

My wife and I did the whole in-vitro thing. In fact we did it four times. Three full rounds, with one round using our cryogenically frozen kidlets left over.

First round failed, second round failed, third round failed, and the fourth? Yeah, bupkis.

And then, we conceived without any intervention at all, except for perhaps a couple bottles of Cabernet Sauvignon, thus solidifying the question about whether or not God has a sense of humor.

Anyway, given our experience with in-vitro, when I read the news about Christa and Joseph MacKinnon, I spit my Americano half way across the room.

Feel free to follow the link, but here?s the summary:

Christa and Joseph MacKinnon went the in-vitro fertilization route to try and have their second child. They were cautious enough to put only two fertilized embryos in Christa?s uterus in order to minimize the likelihood of multiples. The most they could possibly get would be two ? but the odds were stacked against that outcome, statistically.

Well, they didn?t get two. They didn?t even get one.

They got four.

How is that possible, you ask?

According to the article, it was a 70-million to one chance, but BOTH embryos split, thus creating two sets of identical twins. This may solidify that the “big guy” upstairs has a particularly biting sense of humor.

Congratulations to the MacKinnon?s. You might want to write to Ty Pennington over at ABC about his Extreme Makeover, Home Edition…

This is the second most common question I encounter as I go through life as a mom of identical twins. And in case of telling identical twins apart, mommy really knows best. Ask my husband. Ask Grandma.

I guess it helped that F was much smaller than his bro R at birth. By the time he caught up, I?ve gotten the knack of telling who?s who. For some parents, it isn?t that easy. I remember stories of parents who had to go back to the hospital to have their babies` identities rechecked.

Which brings us back to the question: How can one tell identical twins apart?
Well, they do have identical genetic make up (except in very rare cases of chromosomal aberration*) so that they are identical physically as well. But they differ in the following:

• Fingerprints, footprints, and palm lines. Interestingly enough, these markings on the body seemed to be determined by something other than genes. This also brings up the fact that when it comes to forensics, physical fingerprints are still more reliable than DNA fingerprints!

• Susceptibility/resistance to certain diseases. Despite identical genetic make up, identical twins can still differ in physiology and how their bodies react to certain diseases. My son F is quite susceptible to respiratory tract infections. He also had eczema as a baby and contracted chicken pox despite inoculations. My son R, on the other hand, is quite robust but had blocked tear ducts as a toddler. They say that identical twins have about 60% health-related resemblance due to differences in gene expression.

• Mirror twins. About 25% of identical twins are ?mirror? twins. Thy have similar but opposite physical characteristics. One may left-handed and the other right-handed. The hair whorls may go in opposite directions. This has something to do with the division of embryo into 2 separate embryos.

Mind you, these are the scientific ways of telling them apart. Now, for more practical tips from an experienced mother of twins, identical twins will differ in the following easily observable features:

• Belly buttons. Even the best doctor on earth can never ensure that he cuts the umbilical cord in exactly the same way to produce identical belly buttons.

• Hair and hair whorls. My boys have the same hair color but one has 1 hair whorl, the other one has 2. The way their hair is distributed around the whorls is also different.

• Birthmarks and moles. As my babies grew older, I noticed differences in skin marks and moles.

• Scars. Despite inoculations, my son F had chicken pox and has a couple of chicken pox scars on his face. Faint scars, but I know where to look at. His brother didn?t get chicken pox and has therefore no pox marks. I`m pretty sure other scars, especially knee scars, will follow.

• Teeth. Although they were teething almost at the same time, my boys` baby teeth aren?t identical at all. R has one incisor tip broken off during a fall.

As the kids grow older, you, as a mom can differentiate them more and more ? from their voices, from the way the smile or turn their head, from their personalities. By the 3rd month, I could tell who?s awake and crying even though I?m in the next room ? a feat my husband only managed 6 months later.

To sum it all up, one mom once told a pediatrician who labeled her babies as monozygotic or identical ?My twins aren?t identical at all. I can easily tell them apart!?

*see Anna`s comment in this post.(18 Jan 2008).