[caption id="" align="aligncenter" width="599" caption="you've probably seen me use this photo before, but they're just so cute!"][/caption]
Chong Go Sunim brought up another interesting point during our talks about Karma and DNA, specifically in identical twins. "One of the really interesting things that researchers have begun to accept is the idea that genes can also be turned on or off. So if the gene is turned off, it has no effect. And in the case of identical twins, the gene can be turned on in one twin and off in the other."
I found an interesting article online about discordant monozygotic twins (a pair of twins where one twin has a disorder and the other does not) that explained that in the single chromosome we receive from each of our parents, bits of DNA can either be missing or contain multiple copies of a particular bit. In most cases, this has no effect, but it is possible for this mutation to lead to health or developmental problems.
That explains the "what?" but it still leaves the "how?" and "why?" unanswered.
A predictable Buddhist answer might be, Karma...