Chapter 2: Genes and Genetic Diseases

Genes and Genetic Diseases begins with the structural basis of heredity, describing nucleotide composition, complementary base pairing, and the double helix architecture, progressing to how DNA sequences direct protein synthesis through transcription and translation. The chapter explains the roles of messenger RNA, transfer RNA, and ribosomal RNA in converting genetic code into functional proteins, including the processes of gene splicing that remove introns and join exons. DNA replication mechanisms and the proofreading functions of DNA polymerase are presented alongside the types and consequences of mutations, ranging from silent changes to frameshift alterations that disrupt protein function. The discussion then shifts to chromosomal organization and numerical abnormalities, distinguishing between diploid and haploid cells while introducing clinical examples such as Down syndrome resulting from trisomy, Turner syndrome from monosomy, and Klinefelter syndrome from an extra sex chromosome. Structural chromosomal variations including deletions, duplications, inversions, and translocations are examined through specific disorders like cri du chat syndrome and fragile X syndrome. The chapter presents classical Mendelian inheritance patterns, explaining how alleles segregate during reproduction and how dominance relationships determine phenotypic expression. Autosomal dominant conditions such as Huntington disease and neurofibromatosis are contrasted with autosomal recessive disorders like cystic fibrosis and Tay-Sachs disease, while X-linked inheritance patterns account for the disproportionate expression in males. Key concepts including penetrance, expressivity, and recurrence risk are developed to predict disease likelihood in families. The Lyon hypothesis and X inactivation explain dosage compensation mechanisms that equalize sex chromosome gene expression between males and females. Sex determination pathways and rare exceptions involving the SRY gene are discussed alongside epigenetic mechanisms such as DNA methylation and genomic imprinting, which demonstrate how parent-of-origin influences disease expression in conditions like Prader-Willi and Angelman syndromes. The chapter concludes by introducing multifactorial inheritance, where polygenic contributions interact with environmental factors to influence complex traits such as heart disease, diabetes, and neural tube defects, establishing the framework for understanding genetic variation in human disease.