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Gene Regulation and Interactions

1. Learning Objectives

• Define pleiotropy: The phenomenon where one gene influences multiple traits.

  • Phenotypic Outcomes: Understanding how changes in toolkit genes and regulatory elements can lead to adaptive variations in organisms.

    • Example: Changes in pelvic spines in stickleback fish and the yellow gene in fruit flies.

• Define epistasis: Interaction between genes in which the presence of one gene can mask or modify the expression of another.

  • Significance: Essential for understanding the genotype-phenotype relationship, including compatibility in nuclear and mitochondrial proteins.

2. Darwin's Insight

• Quote by Charles Darwin: "Natura non facit saltum" (Nature does not take leaps). This implies evolutionary changes are gradual and incremental.

3. Omnigenic Traits

• Gene Complexity:

  • Monogenic: Simple traits controlled by one gene.

  • Polygenic: Traits influenced by several genes.

  • Omnigenic: Proposed model that all genes contribute, even if only a few are essential.

• Example: Eye color is influenced by hundreds of genes; research reveals 50 genetic loci associated with it.

• Pleiotropy: Most traits are likely influenced by multiple segments of DNA, contributing to widespread traits. - Example: DSCAM gene's involvement in multiple biological processes.

4. Gene Regulation Principles

• Regulatory Elements: Changes in these areas can have extensive effects, showcasing low expression or high expression under different conditions.

  • Example: High vs low expression of the yellow gene in insects can influence wing patterns.

5. Embryonic Polarity and Hox Genes

• Embryo Orientation:

  • Eggs show polarized states before fertilization affecting developmental signaling.

  • Hox Genes: Responsible for the regulation of body plans and complex trait expression by controlling transcription factors that influence numerous gene expressions.

6. Gene Regulation Mechanisms

• Francois Jacob & Jacques Monod: Early pioneers of gene regulatory systems, particularly with bacteria (lac operon), laying foundations for modern genetics.

  • Cell Differentiation: Same genome can give rise to various cell types (muscle, blood, nerve, etc.) through differential gene expression.

• Promoter and Enhancers:

  • Promoter: Located upstream, binding to transcription factors for initiating transcription.

  • Enhancer: Can be distant from the gene, altering the transcriptional behavior when activated, through DNA bending for interaction.

7. Evolutionary Changes

• Evolutionary Patterns in Species:

  • Sticklebacks:

    • The Pitx1 gene drives the formation of pelvic spines; its expression determines spine presence or absence due to changes in enhancer elements.

    • Adaptive context: Spine loss in shallow waters due to lower predation impacts.

• Convergent Evolution: Multiple instances of spine loss through similar genetic modifications across different species and locations.

8. Mitochondrial and Nuclear Genomes

• Compatibility Testing: Research using copepods shows that crosses from differing populations can lead to mitochondrial incompatibility, affecting fitness.

  • Two genomes in a cell work together; mitochondrial and nuclear genomes can influence each other in protein complex formations.

  • Cytochrome Oxidase: Example of proteins requiring contributions from both mitochondrial and nuclear genes.

9. Conclusion and Future Directions

• Regulatory Changes: Impacts of mutations in regulatory regions on gene expression patterns can lead to significant phenotypic variation.

• Upcoming topics relate back to the early genetic principles established by R.C. Punnett, exploring the mathematical underpinnings of natural selection and evolutionary change.