The way I understand it, identical twins have the exact same DNA. So why are some slightly differently sized and why do they have variations in their faces, to the point where you can usually differentiate between them? While nothing is identical all the way down to the molecular level, I’m curious at to why they can vary as much as they do.
My first thought on this is their position in the womb could be different, which could lead to one getting slightly different nutrition versus the other, and perhaps one being more or less affected by things pushing on them and by gravity to different body parts. If we were able to account for all the pre-birth differences in the womb, would we expect them to be exactly the same?
I read an explanation that, to me, made a lot of sense:
DNA is just the recipe. And just like a recipe, the quality of ingredients and the temperature and time in the oven all contribute greatly to the final product. So yeah, differences in nutrition, stress, and gestation time can result in some fairly big differences between twins.
There are a lot of misapprehensions about genetics among the general among the general public, but the key one is that the genome has absolute control over development. In fact, individual genes are just algorithms for building proteins, and while the genome has information that controls the pace of morphological development that influences everything from how many fingers you have and your genitalia to skin color and whether or not your eyes have an epicanthic fold, all of your development is also very much controlled by the epigenome, which in a narrow sense is the histone structure of the chromosome that contains, protects, and gives or restricts access to specific parts of the genome, but in a more holistic sense are actually all of the information—environmental, nutritional, cross-generational, et cetera—that controls the pace and degree of gene expression.
It is not widely understood that key aspects in what the public general thinks of as racial or genetic physical characteristics, such as skin color, eye shape, hair type, et cetera, can all be controlled at the epigenetic level such that you do occasionally have a ‘Black’ child conceived between two ‘Black’ parents with stereotypical ‘Caucasian’ skin tone and even features. It is well understood that certain medical treatments or conditions can cause eye color, which is controlled by the concentration of melanin pigment in the iris, to change radically due to epigenetic factors, and the same is true to differing degrees with other physical characteristics. (I put ‘Black’ and ‘Caucasian’ in air quotes because from a genetic standpoint these classifications are so broad as to be essentially meaningless even though there are genetic markers that are clearly associated with ‘Black’ and ‘Caucasian’ peoples.)
Monozygotic (‘Identical’) twins are not actually identical because each is separate organism with a unique developmental history even if they experience the same environment and eat the same diet. There can certainly be influences within the womb that affect development even if the twins share a placenta just because of random chance, although the differences are typically very subtle and only apparent to close family members. Post-birth, twins may have differential nutrition (one demands and receives more milk than the other), different parental attention, et cetera.
I know 2 pairs of identical twins. The first pair matched so close they both had moles on their upper lips in the same spot. One died from uterine cancer at 25, the other has lived in fear the same will happen to her. She had made it to 48 with no signs of cancer.
I’ve known the other pair for only about 10 years. They have worn different hair styles making it easy to tell the difference. One now has a tattoo making it even easier to tell them apart. Even today, their children still get confused on occasions at family gatherings.
Heritability is the proportion of the variation in the trait that is explained by genetics. In a highly heritable trait, such as height, about 80% of the difference in height between people is explained by differences in genes.
Identical twins have the same genes (excluding any random mutations, which usually are too unimportant to make a difference). So, identical twins tend to have very similar heights, because they share all of their genes.
But, height is not completely determined by genetics, so differences in the environment might cause differences in height. For example, perhaps one member of the twin pair sleeps on the top bunk, and the higher altitude has less oxygen, so the twin doesn’t grow quite as tall.
Even highly heritable traits are never going to be 100% heritable, therefore identical twins can have differences on traits for which there is variation.
So, as @Chronos says, “genetics isn’t everything.”
I’m sorry, but this doesn’t make sense. The difference in oxygen between the top and bottom bunk of bunk beds would be too small to measure if it even exists, so I don’t think that would explain a difference in height in a set of twins. Try again.
There were twins down the street when I was a kid. They were so identical, we always called each of them “Twin,” not their names. As a young adult, one of them cut off most of one thumb in a carpentry accident, so then we could tell them apart.
Then maybe it has to do with one of the beds being on a ley line, and the other being perpendicular to the ley line. It’s just a silly example, don’t worry about it.
Unless the twins’ parents are systematically feeding one more than the other, then height differences could just be some random difference in production of growth hormones. But then what causes production of growth hormone to be different in a pair of identical twins?
Height is pretty boring, because the answer is it’s mostly genetic, with other random things playing a small part.
For many, many traits the heritability is in the 40-60% range. That leaves lots of room for environmental things to make members of a twin pair different from each other. Sometimes there can be pretty fundamental differences, like having different teachers in school, attending different schools, working at different places, and living in different cities. Other times it might be minor differences that nobody even notices, but they add up to make readily apparent differences between the twins.
The expression of the genome in humans is not completely set at fertilization. Females are XX, right? Well, both X chromosomes are needed early in development, but one is shut down after a short time, leaving the other to carry out the functions of the X chromosome in the cell. These are duplicated in mitosis but aren’t active in the cells’ function. They appear as Barr bodies in the cheek cell smears used for sex tests for athletes. Which X shuts down can affect the phenotypes of the twins. My point here is while the twins have the same DNA, there are processes that can cause differences in which genes are expressed.
This is a very interesting example illustrating how a trait can be genetic, but not heritable. There is very little variation in the number of fingers people have. Sure, some people are polydactyl, but it is very rare, so doesn’t contribute much to the variance.
Anyway, most people have 10 fingers, so there isn’t any heritability, because heritability is the proportion of variance, we’re all the same, so there is almost no variance. However, when a person has fewer than 10 fingers, it is usually due to the environment, such as a carpentry accident.
Therefore, most of the variation in finger count is explained by the environment.
Yes, the age of affordable sequencing lets us look at all of these things. As far as we know right now, on average the differences are so small as to not make much of a difference. Kind of like taking a 20 mile trip, and arguing over if you’ll use more gas parking on the left or right side of the driveway at the end.
Where it would be extremely fascinating is if unique mutations can be identified which do make the twins discordant on some trait.
