Genes and Phenotypic Expression

Life begins with fertilization forming a zygote (fertilized egg).
The zygote repeatedly divides by mitosis to produce genetically identical stem cells in a blastocyst.
These stem cells differentiate into specialized cell types by transcribing particular genes to produce specific proteins, determining cell structure and function.

Genotype: An organism's combination of alleles (forms of a gene) coding for a trait, inherited one from each parent.
Phenotype: The observable physical, biochemical, or physiological trait, dependent on the genotype.
The environment can alter an organism's phenotype (e.g., hair color, red blood cell production) but environmental changes are typically not inherited and may not affect gene expression.

Gene expression is tightly regulated via cellular processes that switch genes 'on' or 'off'.
Transcription Factors: Proteins that bind to DNA to either activate/increase (activators) or slow/stop (repressors) the rate of gene transcription.
DNA Methylation: Addition of a methyl group ( $CH_3$ ) to cytosine bases in DNA. This typically inhibits RNA polymerase from binding to promoter regions, thereby preventing transcription. Methylation patterns can be inherited by daughter cells.
Histone Modification: Histone proteins package DNA into chromatin.
- Acetylation: Loosens chromatin, promoting gene expression by allowing RNA polymerase access.
- Deacetylation: Tightens chromatin, inhibiting transcription.
- Histone Methylation: Effect on gene expression varies depending on specific amino acids and number of methyl groups.
Translation Regulation: Translation factors coordinate polypeptide and protein synthesis by enhancing or inhibiting mRNA translation to control protein levels and conserve resources.

Epigenetics refers to heritable changes in gene expression without altering the underlying DNA sequence.
Epigenetic changes are influenced by environmental conditions (e.g., smoking, diet) and can lead to phenotypic differences in genetically identical individuals (e.g., identical twins).
Key mechanisms include DNA methylation and histone modifications.
Associated with various health conditions (e.g., cancer, cardiovascular diseases, obesity) where epigenetic alterations, such as methylation of tumor suppressor genes, impact gene activity.
Can have transgenerational effects, where environmental exposures impact the epigenome and health outcomes of subsequent generations (e.g., Dutch famine study).