Chapter 6: Chromosome Number & Structural Variation

Chromosome Number & Structural Variation delves into the specialized field of cytogenetics, which focuses on the analysis of chromosome number and structure within dividing cells, utilizing advanced techniques like Giemsa staining, quinacrine banding, and chromosome painting for high-resolution viewing. The creation of a karyotype—an organized chart of matched homologous chromosomes—allows researchers to identify numerical and structural abnormalities by examining features like size, shape, and unique banding patterns along the short (p) and long (q) arms of the chromosome. Numerical variations are categorized into euploidy (normal complete sets) and polyploidy (extra full sets), the latter being particularly common in agricultural species such as wheat, which is often a robust hexaploid. Polyploids derived from the hybridization of distinct species followed by chromosome doubling are termed allopolyploids and tend to be fertile, unlike sterile autopolyploids or triploids where meiotic segregation is irregular, producing unviable aneuploid gametes. A highly specialized form, the polytene chromosome found in Drosophila, is generated through endomitosis and is essential for detailed cytological study. The text extensively covers aneuploidy, characterized by a genetic imbalance due to the under- (hypoploid) or over-representation (hyperploid) of a single chromosome or segment. Significant human examples include various trisomies like Down syndrome (Trisomy 21), which is strongly linked to increased maternal age and meiotic nondisjunction, as well as Klinefelter syndrome (47, XXY), and the only viable human monosomy, Turner syndrome (45, X). Lastly, the chapter addresses structural rearrangements, including deletions (e.g., cri-du-chat syndrome) and duplications (e.g., Drosophila Bar eye), alongside major structural shifts: inversions (paracentric or pericentric), which force chromosomes to form meiotic loops, and translocations (interchanges between nonhomologous chromosomes), which lead to cruciform pairing and subsequent fertility reduction due to the production of unbalanced gametes, exemplified by Robertsonian translocations.