Comprehensive Study Guide on Genetic Expression and Cellular Differentiation

Total Genetic Presence: Every single cell in the human body contains the entirety of an individual’s genetic code.
Chromosomal Integrity: Within these cells, there is a complete copy of all chromosomes, which house the genes.
Inclusion of Pathological Cells: This rule of universal genetic presence applies even to cancer cells. * Abnormalities in Cancer: While cancer cells contain the copies of all chromosomes, they are characterized by the fact that the copying process has gone "a bit wrong."

Selective Expression: Despite every cell possessing the full genome, only a "very small selection" of genes are actively being expressed in any given cell at any specific time.
Defining the Phenotype: The phenotype and the specific behavior of a cell are determined solely by the particular collection of genes that are being expressed.
Mechanism of Differentiation: The process of selecting which genes to express allows the body to create diverse tissue types from the same underlying genetic template.

The Liver Cell: A liver cell contains the exact same DNA as every other cell in the body, but it expresses a subset of genes that allow it to perform its specific "job" as a liver cell.
The Retina Cell: Cells located in the back of the retina, which are responsible for receiving light signals, also contain the same DNA as the liver cell.
Comparative Analysis: The fundamental difference between a cell receiving light in the eye and a cell performing metabolic functions in the liver is not their DNA, but the specific collection of genes that are being expressed in each cell type.