Chapter 16: Applications of Molecular Genetics

The chapter explains how DNA diagnostic tests, often relying on PCR and gel electrophoresis to detect unstable trinucleotide repeats (as seen in HD) or specific deletions (like the ΔF508 mutation in CF), are crucial tools in genetic counseling. A major focus is Human Gene Therapy, detailing its somatic-cell approach, which involves introducing functional wild-type genes via vectors (often modified viruses) to treat diseases such as Leber’s congenital amaurosis type II blindness. Ethical complexities and safety concerns regarding vector integration, highlighted by the development of leukemia in some X-linked SCID patients after retroviral treatment, underscore the strict criteria required for human gene therapy approval. Beyond medicine, the text explores the industrial use of genetically engineered bacteria for producing valuable eukaryotic proteins, like human growth hormone (hGH) and insulin, and the creation of Transgenic Animals and Plants using techniques such as microinjection, embryonic stem cell technology, and Agrobacterium tumefaciens-mediated T-DNA transfer. A pivotal section introduces Reverse Genetics, which uses known gene sequences to disrupt function through methods like knockout mutations (via homologous recombination in mice), T-DNA insertions in plants, and the gene-silencing mechanism of RNA interference (RNAi). Finally, the cutting-edge field of Genome Engineering is thoroughly detailed, focusing on the bacterial CRISPR/Cas9 system, which employs a guide RNA (sgRNA) to target the Cas9 endonuclease to specific DNA sequences, enabling precise cleavage and subsequent repair (often by error-prone NHEJ) to create targeted indel mutations, deletions, or gene replacements. The chapter concludes by demonstrating the forensic power of DNA Profiling (or fingerprinting), which relies on analyzing highly polymorphic short tandem repeats (STRs) using the CODIS panel to establish identity in paternity and criminal cases with extremely high statistical certainty.