Identical genes but non-identical phenotypes

DNA doesn't determine phenotype or organismal traits

1. At cellular level

https://medcraveonline.com/MOJCSR/epigenetic-regulation-mechanisms-in-stem-cell-differentiation.html
 

Excerpt: "Stem cells give rise to almost 200 different cell types that are present in a mammalian organism. In spite of the fact that these distinct cell types have different functions and morphologies, they descend from a common ancestor cell and essentially share the same DNA. The underlying cause for the rise of specific cell types is not genetic differences; it is mostly due to how the genetic information is interpreted. The epigenome, which consists of several degrees of regulatory mechanisms, dictates the gene expression profile of a certain type of cell. Eukaryotic DNA is packaged into chromatin, folded and compacted which in turn affects its functionality as certain regions of DNA will not be accessible whereas some other regions will be easier to access for effect or proteins and modifiers to bind. The chromatin dynamics are modulated by several machineries including but not limited to histone PTMs and their variants, DNA methylation and RNA interference, which have crucial roles in regulating stem cell differentiation, cell fate determination and lineage specification."

2. At tissue level

https://www.colorado.edu/today/2020/07/07/how-does-stem-cell-know-what-become-study-shows-rna-plays-key-role

Excerpts: "Look deep inside our cells, and you’ll find that each has an identical genome –a complete set of genes that provides the instructions for our cells’ form and function.

But if each blueprint is identical, why does an eye cell look and act differently than a skin cell or brain cell? How does a stem cell – the raw material with which our organ and tissue cells are made – know what to become?
 

“All genes are not expressed all the time in all cells. Instead, each tissue type has its own epigenetic program that determines which genes get turned on or off at any moment,” said co-senior author Thomas Cech, a Nobel laureate and distinguished professor of biochemistry. “We determined in great detail that RNA is a master regulator of this epigenetic silencing and that in the absence of RNA, this system cannot work. It is critical for life.”"

3. At body plan level

https://ussromantics.com/2020/01/03/epigenetics-and-imprinting-2-identical-genes-and-non-identical-phenotypes/

Excerpt: "it’s obvious that genetics isn’t the whole story of our inheritance and development because it doesn’t begin to explain how, from one fertilised egg – the union of, or pairing of, two sets of chromosomes – we get, via divisions upon divisions upon divisions, a complex being with brain cells, blood cells, skin cells, liver cells and so forth, all with identical DNA. It also doesn’t explain how a maggot becomes a fly with the same set of genes (or a caterpillar becomes a butterfly, to be a little more uplifting). These transformations, which maintain genetic inheritance while involving massive change, must be instigated and shaped by something over and above genetics but intimately related to it – hence epigenetics. Other examples include whether a crocodile hatchling will turn out male or female – determined epigenetically via the temperature during development, rather than genetically via the Y chromosome in mammals.
 

So, to add to the description I gave last time, the histone proteins that the DNA wraps itself round come in batches or clusters of eight. The DNA wraps around one cluster, then another, and so on with millions of these histone clusters (which have much-studied ‘tails’ sticking out of them). And I should also remind myself that our DNA comes in a four-letter code strung together, out of which is constructed 3 billion or so letters."

My comment: With the same set of DNA there could be a huge number of different phenotypes or outcomes. DNA doesn't determine phenotype or organismal traits or characteristics. It's all epigenetics. DNA does nothing without epigenetic programming. In other words, without epigenetic information profiles the DNA is just passive information.