What makes identical twins different?

Twins are fascinating, and identical twins even more so. Even though they have almost the same DNA (not quite exact), they’re quite different people, especially as they get older. My little sisters are identical twins, and that statement couldn’t be more true of them: E tends to be an anxious ball of knots, O tends to float calmly through life; E likes to party, O likes tranquil nights in; E prefers blue clothing, O prefers orange. While I do think that some of this differentiation is an intentional result of frustration that they’re often viewed as a single entity, “the twins,” I also know that other differences are natural. A common topic that twins are often studied to investigate is whether nature (genes) or nurture (environment) has a greater role in determining who we become. For many traits, researchers even quantify the percentage of variation in the phenotype that arises from each factor independently and from an interaction between the two.

The twins in my life, who have inspired my curiosity time and time again
The twins in my life, who have inspired my curiosity time and time again

A study of genetically identical mice summarized here found that identical mice who were placed in enriched environments (containing multiple levels, lots of tubes, multiple water spots, nesting boxes, and such) became quite different, and their individuality increased over time. Instead of attributing such differences to genetics or environment alone, the researchers argued that the key factor in determining the mice traits was the how they experienced the environment they were in, the context of their experiences in a certain environment.

The researchers hypothesized that epigenetics may be an explanation for the variability in identical twins. Epigenetic changes are alterations in the genome that aren’t caused by changes in the DNA sequence. In many cases, some aspect of an organism’s environment causes certain genes to become “turned on,” or activated, so that two individuals with the same nucleotide sequence actually exhibit different phenotypes. Epigenetic changes show that examining a DNA sequence alone won’t reveal everything about an individual. Instead, the context provided by environment will cause some genes to be silenced while others are activated.

For me, the message to take away from this study is one that manifests often. When we try to isolate causes and effects, thereby omitting the larger context, our conclusions are too simplistic and under-informed. But when we do look at phenomena in the true context in which they occur (or as much of that context as we can), we arrive at the most comprehensive understanding possible.

2 thoughts on “What makes identical twins different?

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