Chapter 5: Genes and Human Diseases

Genes and Human Diseases systematically categorizes genetic anomalies into single-gene Mendelian conditions, complex multigenic traits, numerical and structural chromosomal aberrations, and somatically acquired mutations. A core focus is placed on the transmission patterns and pathophysiological mechanisms of monogenic disorders, explaining how specific genetic lesions—such as missense, nonsense, and frameshift mutations—disrupt structural proteins, cellular membrane receptors, and critical metabolic enzymes. Key clinical examples covered include connective tissue abnormalities like Marfan syndrome and Ehlers-Danlos syndrome, transport defects such as familial hypercholesterolemia, and profound inborn errors of metabolism, notably lysosomal storage diseases (including Tay-Sachs, Niemann-Pick, and Gaucher disease) and glycogen storage diseases (like von Gierke and Pompe disease). The text thoroughly navigates clinical cytogenetics, explaining the impact of chromosomal aneuploidies and structural rearrangements, highlighting autosomal conditions such as Trisomy 21 (Down syndrome) and the 22q11.2 deletion syndromes, alongside sex chromosome anomalies like Klinefelter and Turner syndromes. Furthermore, the chapter delves into nonclassical inheritance models, unraveling the complexities of trinucleotide-repeat expansions responsible for Fragile X syndrome, mitochondrial DNA mutations, and the epigenetic parent-of-origin effects governing genomic imprinting seen in Prader-Willi and Angelman syndromes. Finally, the text reviews the rapidly advancing landscape of molecular genetic diagnostics, explaining the clinical utility of polymerase chain reaction (PCR), fluorescence in situ hybridization (FISH), microarray technology, and next-generation sequencing (NGS), including targeted gene panels, whole-exome sequencing, and the emerging role of circulating tumor DNA (ctDNA) in liquid biopsies for precision oncology and personalized medicine.