Identical twins form when a single fertilised egg splits into two separate embryos early in development. Because they originate from the same cell, the embryos have virtually identical DNA.
This means they’ll be similar for characteristics that have a strong genetic underpinning, such as blood type and eye colour. From that moment on, however, they only become more different.
Although the twins may share the same womb, they experience it differently. Subtle kinks in the umbilical cord, for example, might skew the delivery of nutrients to one twin over the other.
This can lead to changes in the pattern of gene expression, which in turn can influence many features, such as growth, personality and susceptibility to disease.
Slight differences in pressure or position in the womb, meanwhile, mean that identical twins are born with different fingerprints. Although genetic factors control their basic shape, the amniotic environment sculpts their finer features.
After birth, further differences emerge. Random genetic mutations can crop up in one or other twin at any time, which helps to explain why identical twins can develop different diseases, such as cancer.
Serendipity also plays a role. One twin might catch a virus that triggers an autoimmune disease, while the other dodges the bullet.
In short, nature is important, but so is nurture. Plus, their environment only keeps becoming more different as time goes on.
Although identical twins may grow up in the same household, they may have different teachers, friends and role models. As they get older, they may go off to live in different places, with different levels of social support, healthcare or pollution.
All of these environmental factors feed back to, and interact with, their DNA, exacerbating differences and further shaping them into the unique individuals they inevitably are. So, although they’re called identical twins, they’re anything but.
This article is an answer to the question (asked by Chris Montgomery, via email) 'How identical are identical twins?'
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