Chapter 5: Chromosomal Basis of Mendelism

The foundational principle that abstract Mendelian genetic factors are localized on visible cellular structures, specifically chromosomes, emerged from the collaboration between the fields of genetics and cytology, particularly through the pioneering work of Thomas Hunt Morgan and Edmund Beecher Wilson. Chromosomes, first observed in the nineteenth century, are fundamental structures, characteristic of all cell nuclei, appearing as tightly organized cylinders during cell division. Most somatic cells are diploid (2n), containing two of each chromosome, while gametes are haploid (n). The behavior of homologous chromosomes during meiosis provides the chromosomal basis for Mendel's principles: the Principle of Segregation is explained by the separation of homologous chromosomes during Anaphase I, while the Principle of Independent Assortment is explained by the random alignment of non-homologous chromosome pairs during Metaphase I. Morgan used the fruit fly, Drosophila melanogaster, to demonstrate that the gene for eye color was X-linked, leading to different phenotypic ratios in males and females (who are hemizygous for X-linked traits). Definitive proof for the Chromosome Theory of Heredity was provided by Morgan’s student, Calvin Bridges, who used the rare event of X chromosome nondisjunction (failure to separate during meiosis) to correlate abnormal chromosome sets (like XXY females and XO sterile males) with exceptional inheritance patterns. The determination of sex varies across species; in humans, the presence of the Y chromosome is dominant for maleness, driven by the SRY gene, which produces the testis-determining factor (TDF), leading to testosterone production and male development. Conversely, in Drosophila, sex is determined by the ratio of X chromosomes to sets of autosomes (X:A ratio), where the Y chromosome only governs male fertility. In organisms where one sex has a single X chromosome (like mammals and Drosophila), dosage compensation mechanisms equalize gene expression; mammals achieve this via the random inactivation of one X chromosome in females, which condenses into a Barr body, resulting in mosaicism (e.g., tortoiseshell cats), while male Drosophila achieve this through hyperactivation of their single X chromosome. X-linked traits in humans, such as hemophilia and color blindness, are consequently much more prevalent in males.