Chapter 15: The Chromosomal Basis of Inheritance

The segregation of homologous chromosome pairs and their independent assortment during meiosis directly account for the segregation and independent assortment of alleles observed in Mendelian crosses, providing the cellular mechanism underlying classical genetics. The chapter then examines sex-linked inheritance, showing how genes located on sex chromosomes, particularly the X chromosome, produce characteristic inheritance patterns that deviate from typical Mendelian ratios, with males expressing recessive X-linked alleles in their phenotype due to their hemizygous condition. Landmark experiments by Thomas Hunt Morgan using Drosophila fruit flies revealed that genes are physically located on chromosomes and established the concept of genetic linkage—the tendency of genes on the same chromosome to be inherited together rather than assorting independently. These discoveries led to the development of gene mapping techniques, which use recombination frequencies during crossing over to determine the relative distances between linked genes on chromosomes, producing linear genetic maps that predict offspring ratios in testcrosses. The chapter also addresses chromosomal abnormalities arising from errors in meiosis and mitosis, including nondisjunction events that produce aneuploid cells with abnormal chromosome numbers, exemplified by conditions like Down syndrome resulting from trisomy. Beyond numerical changes, the chapter discusses structural chromosomal rearrangements including deletions that remove chromosome segments, duplications that repeat regions, inversions that reverse segment orientation, and translocations that move segments between chromosomes—alterations that can disrupt normal gene expression and significantly affect phenotype. By integrating observations of chromosome behavior with inheritance patterns, the chapter reveals how chromosomes function as the physical carriers of genes, explaining both the reliability of inheritance and the generation of genetic variation that provides raw material for evolution and the diversity observed in natural populations.