Sorry to get this late start.  Last Friday was my son’s birthday (more on that later), and life got unexpectedly complicated in the interim.  Well, let me rephrase that – it got more complicated.  More on that later, too.  Maybe.

Suz asks:

So cool! Okay – here’s what I want to know and I apologize if it’s a silly question as I never paid attention in biology. I have “identical” twins, but the question is whether there is any genetic difference between them. I read about how there can be slightly differing conditions in their amniotic fluid which could slightly alter their cells (okay – this is a bad explanation), but I was just wondering if they were 100% genetically similar and, if not, why not.

All right.  This is a very cool genetics lesson.  Because, even though I finally got my PhD in Biochemistry (for the sole reason that it tends to be more lucrative), I am basically a geneticist at heart.   How nature creates almost infinite variety with a very small set of building blocks is something that is so elegantly beautiful that it never ceases to amaze me.

The short answer is this – identical twins are, in essence, naturally occuring clones.  They are the result of an embryo, in a very early stage of development, collapsing and separating and then repairing into two individual embryos.  But they started out as one being. 

So, the short answer is – Identical twins are the closest thing to identical as any two beings can be to one another.  From a genetic standpoint, anyway.  But genetics are not the sum total of what we are.

Genetics are not 100% deterministic.  We are not the sum total of our genes; we are the sum total of our genes plus the way that outside factors act on our genes.  We are a product of action and reaction, of the constant interplay between our predisposition and our environments from the very moment that we are conceived.   But let’s set that aside for a moment and concentrate on genetics.  We will come back to environment later.

Rather than repeat an entire lecture on DNA replication, I have provided you a link to a pretty good explanation of the topic.  And just because it’s neat, here is an animated molecular model of what DNA replication would look like, if we actually had microscopes that could see that small.  Cool, but I digress.  The point is, while the mechanisms that allow replication have an enormous level of fidelity, if you consider the millions and billions of pairs of genetic code that are reproduced in the world every minute, they are not perfect.  Mistakes get made.  And when you are talking about something as complex as a whole person, well, chances aren’t bad that two cells of YOU are not identical.  Any two of your own cells may differ by one or two mutations.  So, when I say that identical twins are about as identical as two beings can get, that’s where the perspective comes from.  You aren’t even IDENTICAL to yourself.  But any small changes, with few exceptions, are generally insignificant when viewed in the context of the complete organism.

So, now we have established that identical in genetics basically means very-very-very close to identical.  And we have one more step to go before we even get to environment.  We have epigenetics.  Genetics is all about the code we were born with.  But not necessarily about how that code will be expressed.  Part of what determines that expression is that environment-thing I keep putting off to the end.  But, when we have tried to separate what of “us” results from the simple inheritance of a gene, and what of “us” is molded by our environment, we have tended to see a bit of a maddening gap between them.  We can’t always get them to add up to 100%.  And some of that gap is accounted for by modifications that are made to our genes, that are NOT directly inherited, but that change their expression.  One of the most fascinating examples is genomic imprinting. 

Basically, it doesn’t only matter that your parents gave you a gene.  It matters which parent gave you the gene.  Some genes are only expressed when they come from the mother, some are only expressed when they come from the father.  As this pattern of inheritance in identical twins is the same, genomic imprinting has no net effect, however, since your twins are girls, another factor comes into play.  In girls, although we inherit two copies of the X chromosome, we only actually use one of them.  The other simply “shuts off”.  This happens more or less at random, long before birth.  Which means one of your girls may actually be “using” the X chromosome she got from you, while the other is “using” the one that she got from her father.  This inactivation isn’t perfect, and even on the “dormant” chromosome, every woman has a different pattern of residual activity, but it also may account for differences in even genetically identical twins.  To add even more variety – which X is “dormant” may not even be the same in all the cells of a single person.  At the time the X shuts down, we are beings of about 32 cells. Not all those 32 cells may shut down the same X.  So patches of us express mom’s X, and patches of us express dad’s X.  We are literal mosaics of gene expression (think calico cat).  In a very real way, men, having only one X to express, really ARE simpler beings than women.  And these are only two examples of how epigenetics can radically alter how we use the inhertance that we received at conception.  There are a myriad of other modifications made to our genes by the sun, substances in the water we drink and air we breathe, that accumulated tiny changes to our genes that change the way they behave, not just in response to the influence of the environment at the time, but permanently.

And now we get down to the “other” e-word.  Environment.  The educated layman tends to think of biological questions in terms of the classic nature vs. nuture dichotomy, but I think this gives the implication that we have more control over our environment than is reflected in life.  Your environment starts acting on you from the very moment that the egg and the sperm combined to start the journey of your life.  In fact, even before, as environmental factors mutated genes on the developing egg, and pre-sorted the available sperm.  Your destiny was more than genetics even before you were born.

We are only beginning to fully appreciate the affect that our prenatal environment has on who we become as a individual.  Subtle changes in amniotic fluid affect, not the makeup of our genes (as your question suggests), but how they are expressed.  And in an organism where a single cell can go on to produce an entire heart, those effects can be amplified in profound and unexpected ways.  The environment we find ourselves surrounded with in our very earliest days can have the most lasting impact on how we grow, simply because there is so little of us that has to grow up to be so very much.

Fraternal twins are the product of two completely different conceptions, and they each have their own “mini-environment” inside individual amniotic sacs with their own individual placentas.  Since your prenatal environment is the result of a complex biological interplay between mother and child, no two “wombs”, or even “rooms in the womb” are alike, even between children of the same mother.  And there is an interplay between the twins themselves.   Females born of sister/brother fraternal pairs tend to have more masculine behavoral characteristics, and this has been attributed to the influence of her brother’s testosterone in the womb.  Although identical twins are free from this effect (by their nature, they share a gender), they are not free from the complexities of prenatal interaction.  Identical twins may or may not share the same amniotic sac and placenta, which may affect their degree of “sameness”.

The thing that attracts our curiosity to identical twins is obvious.  There is an attraction to seeing life in duplicate and we marvel in how “alike” they are.  But the thing that is probably even more amazing is, given the fact that they started life’s journey as a single being, how different they can become.