Chaos (or randomness) is a fundamental law of life. Ever try to organize/clean something only to have a young one (two-legged or four legged) come through and undo all your efforts? That’s chaos exerting its authority! We experience chaos all the time.
However, when it comes to understanding our physical traits, we tend to forget about chaos and assume physical traits are pretty much determined through either genetics, personal decisions or environmental exposure. Our undying urge to understand ourselves tends to ignore (intentionally or not) the central component of randomness.
So how does chaos help determine physical traits? One great example involves the X-chromosome. In some animal species, such as humans or cats, females receive two X-chromosomes from their parents, but they only need to use one. To achieve this, one of the X-chromosomes will randomly “inactivate” early on in development, though not from the very beginning.
Calico/tortoiseshell female cats show just how random this inactivation can be. The gene controlling whether a cat will have gold or black fur happens to be on the X-chromosome. As an example, a female cat received a version of the gene encoding for black fur from one parent and one encoding for gold fur from the other parent. All those lovely patches of black and gold on the cat (such as the one in the picture) are from the inactivation of the X-chromosome that harbors either the black or gold coloring. As pictured, the cat does not possess equal parts of gold and black, but a beautiful, chaotic mix.
Another strange mammal, the nine-banded armadillo, also serves as an example of chaos disrupting genetic determinism (i.e., genes determine everything). Every nine-banded armadillo born is a quadruplet, i.e., having three “genetically identical” siblings. Even though the identical quadruplets share the same genes, researchers find that they can be quite different (behaviorally and physically) (Ballouz et al., 2019).
In a bioRxiv preprint, Ballouz et al. describe how they tried to control for any environmental variation in armadillo quadruplets to figure out where all the differences were coming from. In short, they found that chaos-driven X-chromosome inactivation happens soon after the four embryos physically separate, and that differences between the four can be tied directly to ~150 instances of X-chromosome inactivation.
Although this work has yet to be fully published, it reminds us that life is far more than nature and nurture, and that we have to account in some way for the seemingly random hand of chaos.
Ballouz S, Pena MT, Knight FM, Adams LB and Gillis JA. The transcriptional legacy of developmental stochasticity. bioRixv. December 12, 2019; doi: http://dx.doi.org/10.1101/2019.12.11.873265.