Chapter 12: The Chromosomal Basis of Inheritance

The chromosome theory of inheritance emerged from Thomas Hunt Morgan's groundbreaking experiments with fruit flies, particularly his observation that white eye color segregated with the X chromosome, providing concrete evidence that genes have chromosomal addresses. The chapter then explores sex-linked inheritance, examining how traits controlled by genes on sex chromosomes display distinctive patterns across populations. In mammals, the SRY gene on the Y chromosome initiates male development, while X-linked conditions such as color blindness, muscular dystrophy, and hemophilia disproportionately affect males because they possess only a single X chromosome. Females exhibiting X-linked traits demonstrate the phenomenon of X-inactivation, wherein one X chromosome becomes condensed into a Barr body in each cell, resulting in a cellular mosaic where different alleles are expressed in different tissues. The chapter then addresses linked genes, which occupy nearby positions on the same chromosome and consequently tend to be inherited together. Morgan's analysis of body color and wing morphology in flies revealed that although linked genes show parental phenotype predominance, crossing over during meiotic prophase generates recombinant gametes and offspring. This discovery enabled Alfred Sturtevant to develop the first genetic linkage maps by calculating recombination frequencies as indicators of physical distance between genes. The final section examines chromosomal abnormalities arising from meiotic errors and structural rearrangements. Aneuploidy, characterized by incorrect chromosome numbers such as monosomy or trisomy, produces conditions like Down syndrome when three copies of chromosome twenty-one are present. Polyploidy, involving extra complete chromosome sets, occurs more frequently in plants and influences evolutionary processes. Structural alterations including deletions, duplications, inversions, and translocations disrupt normal gene function and dosage, sometimes initiating malignant transformations as exemplified by the Philadelphia chromosome in chronic myelogenous leukemia. Together, these concepts explain both normal inheritance patterns and human genetic disease.