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Comment by rtl49 | Hacker News Reader

The article doesn't answer the question in any satisfying way. A layman would probably believe that the environment in the uterus is the same for both twins, too. The "satisfying" answer is still quite boring: as with everything else, there are stochastic processes in play during embryonic development. For one, cytoplasm is divided asymmetrically, so the two embryos don't begin their separate journeys with a perfectly equal share of their progenitor's resources.

Also, this is a rather silly sentence:

Although very similar, the fingerprints of identical twins are nevertheless unique because they are not completely determined by genetics: they are an example of phenotype, which refers to the physical characteristics of an individual that are determined by the interaction of genes and the environment.

By this reasoning, the only thing completely determined by genetics is genes, since every manifestation of genotype could be called a phenotype.

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> By this reasoning, the only thing completely determined by genetics is genes, since every manifestation of genotype could be called a phenotype.

But this is absolutely correct, so the fact that the sentence leads to this correct conclusion doesn't make it silly. Genotype is just genes. The manifestations are all determined by the interaction of genes with the environment and the manifestations of observable traits together make up the phenotype.

Some aspects of this are practically tightly controlled by genes because the environments which allow an organism to develop at all tend to also be ones in which the genes involved are expressed in certain ways, and some are less tightly controlled by genes, because there is more practical variation in interaction with the environment. But these are differences of degree.

There's no substantive point of difference between my post and yours. I'm simply suggesting that perhaps the "big reveal" for this question is a bit trivial, even for the general public. Everyone knows that no matter a boy's genome, he isn't going to grow as tall as Yao Ming if he only gets 1200 calories per day in his diet.

That is the definition of "phenotype" as I understand it. Phenotype is the observed traits of an organism, while genotype is the genetic (discrete) code in the organism's DNA. Genotype obviously largely affects phenotype, but it doesn't entirely determine it.

>> By this reasoning, the only thing completely determined by genetics is genes, since every manifestation of genotype could be called a phenotype.

I would say half half. Obviously a lot comes from genes, but then environment takes its toll as well.

Robert Sapolsky makes an interesting point about the idea of "a" gene-environment interaction[0]: genes only exist in the context of an environment, and so "The problem with "a gene-environment interaction" is the same as asking what height has to do with the area of a rectangle, and being told that in this particular case, there is a height/length interaction."

His course on Human Behavioral Biology [1] is perhaps the single best science course I've ever encountered. If someone could only take one science course, I'd recommend this one.

[0] https://edge.org/response-detail/25536 [1] https://www.youtube.com/playlist?list=PLD7E21BF91F3F9683

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