(a) What is organic evolution ? Highlight the mechanisms governing organic evolution. (5+10=15 marks) (b) Explain the phenomenon…

(a) Organic Evolution and its Mechanisms

Organic evolution is defined as the change in the heritable characteristics of biological populations over successive generations, a process Darwin termed "descent with modification." It is not merely a historical fact but an ongoing biological process driven by specific mechanisms. Microevolution refers to changes in allele frequencies within a population, while macroevolution encompasses the emergence of new species and higher taxonomic groups. The Hardy-Weinberg equilibrium serves as the null hypothesis for evolutionary change; any deviation from this equilibrium indicates that evolutionary forces are at work. The primary mechanisms governing these changes include mutation, which introduces new genetic variation; natural selection, which acts non-randomly on phenotypes to increase fitness; genetic drift, which causes random fluctuations in allele frequencies, particularly in small populations; gene flow, which introduces or removes alleles through migration; and non-random mating, which alters genotype frequencies without changing allele frequencies directly. Reproductive isolation then allows diverging populations to become separate species.

(b) Incomplete Dominance

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype, distinct from either homozygous parent. A classic example is found in Mirabilis jalapa (Four o’clock plant) or Antirrhinum majus (Snapdragon). When a red-flowered plant (RR) is crossed with a white-flowered plant (rr), the F1 generation produces pink-flowered plants (Rr). In this context, neither allele is recessive; rather, the heterozygote exhibits an intermediate phenotype due to the partial expression of the pigment-producing enzyme. Self-pollination of the F1 generation yields an F2 phenotypic ratio of 1 Red : 2 Pink : 1 White. This 1:2:1 ratio is the key distinction from complete dominance (3:1) and codominance, where both parental traits are expressed simultaneously and distinctly (e.g., blood type AB). The significance of incomplete dominance lies in its contribution to genetic variation. By producing intermediate phenotypes, it expands the phenotypic range within a population, providing more raw material for natural selection and enhancing evolutionary potential.

(c) Gene Editing

Gene editing refers to a group of technologies that allow scientists to make precise changes to the DNA of a cell. Unlike traditional transgenic approaches that insert foreign DNA, gene editing modifies the organism’s existing genome. The most prominent technology is CRISPR-Cas9, which utilizes a guide RNA (gRNA) to direct the Cas9 nuclease to a specific DNA sequence. The Cas9 protein cuts the DNA at a site adjacent to a Protospacer Adjacent Motif (PAM), triggering repair mechanisms that can be harnessed to knock out or edit genes. This represents a significant advancement over earlier techniques like Zinc Finger Nucleases (ZFNs) and Transcription Activator-Like Effector Nucleases (TALENs), which were more complex, expensive, and less efficient to design.

The applications of gene editing are vast. In agriculture, it is used to develop disease-resistant crops and improve nutritional content. For instance, researchers have used CRISPR to develop wheat varieties resistant to powdery mildew and to enhance the shelf life of tomatoes. In medicine, it holds promise for treating genetic disorders such as sickle cell anemia and beta-thalassemia by correcting or bypassing the defect in patient cells, and the first CRISPR therapy, Casgevy, was approved for these diseases in the UK and US in late 2023. In India, the MoEFCC in 2022 exempted SDN-1 and SDN-2 genome-edited plants free of foreign DNA from the biosafety rules for transgenic organisms under the Environment (Protection) Act, 1986, and in 2025 ICAR released the first genome-edited rice varieties, DRR Dhan 100 (Kamala) and Pusa DST Rice 1.

The advantages of gene editing for genetic engineering are profound. It offers high precision, allowing for single-base pair modifications. It is highly efficient and cost-effective compared to older methods, making it accessible to a wider range of laboratories. Furthermore, it enables multiplexing, where multiple genes can be edited simultaneously in a single experiment. However, the technology also raises ethical concerns, particularly regarding "off-target" effects where unintended mutations occur, and the moral implications of germline editing in humans.