Chapter 5: Genetic and Congenital Disorders – Causes & Mechanisms

Genetic disorders are fundamentally categorized by changes in the DNA sequence (single-gene disorders) or chromosomal aberrations. Single-gene disorders follow Mendelian patterns of inheritance, including autosomal dominant disorders such as Marfan Syndrome and Neurofibromatosis (NF-1 and NF-2). In contrast, autosomal recessive disorders, which often involve loss-of-function mutations affecting enzymes (like PKU and Tay-Sachs disease), manifest only when an individual is homozygous for the mutant allele. X-linked disorders are typically recessive, affecting males who inherit the trait from an unaffected carrier mother. The chapter also explores chromosomal disorders (cytogenetics) resulting from structural changes or abnormal numbers (aneuploidy). Numeric disorders involving autosomes, most commonly Down Syndrome (Trisomy 21), typically arise from nondisjunction during meiosis. Numeric disorders of sex chromosomes, which are generally better tolerated, include Turner Syndrome (45,X/0) and Klinefelter Syndrome (47,XXY). Furthermore, the chapter addresses Mitochondrial gene disorders, which are exclusively inherited from the mother and primarily impact energy-dependent tissues like the neuromuscular system. Congenital defects also result from environmental influences, with the embryo being most susceptible during the period of vulnerability or organogenesis (days 15 to 60 post-conception). Teratogenic agents—including radiation, chemicals/drugs (like alcohol causing Fetal Alcohol Syndrome), and infectious agents (e.g., TORCH group)—can interfere with development. Finally, the text outlines diagnosis and counseling methods, emphasizing genetic assessment, noninvasive prenatal screening (ultrasonography, maternal serum markers like AFP and hCG), and invasive diagnostic procedures such as amniocentesis and chorionic villus sampling for definitive cytogenetic and DNA analysis.