This 2015 Harvard fruit fly study found:

“Genetically identical individuals display variability in their behaviors even when reared in essentially identical environments.

Individual flies exhibit significant bias in their left vs. right locomotor choices during exploratory locomotion.”

Here’s an example of why population statistics such as in GWAS didn’t necessarily apply to an individual:

“The probability of turning right averaged across all individuals within each line was statistically indistinguishable from 50%. However, an individual fly’s probability of turning right often diverged markedly from the population average.

For example, nearly one quarter (23.5%) of CS [Canton-S] flies turned right greater than 70% of the time or less than 30% of the time. This distribution would be unlikely indeed if all flies were choosing to turn right with identical probabilities.”

The researchers noted other species with similar findings:

“Individuals can develop idiosyncratic behaviors, morphology, and gene expression profiles. For example, stochastic DNA methylation may contribute to phenotypic variation that is uncorrelated to genetic variation.”

This study should inform other studies such as the Separating genetic from environmental factors when assessing educational achievement, to the degree its findings apply to humans.

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As the findings applied to neurological areas:

“The magnitude of locomotor handedness is under the control of neurons within a brain region implicated in motor planning and execution.”

I was surprised that the study’s news coverage included this opinion:

“They are suggesting that variation [read: individuality] itself might be a genetic trait.”

The researchers stated their case in the companion study Changing an individual’s future behavior even before they’re born.

http://www.pnas.org/content/112/21/6700.full “Neuronal control of locomotor handedness in Drosophila”