Chapter 17: Large-Scale Chromosome Changes

17.1 Changes in Chromosome Number

Chromosome number changes can profoundly affect an organism's phenotype and overall health. This section discusses euploid and aneuploid conditions in detail.

• Euploid Organisms

  • Definition: Organisms with a whole number multiple of a haploid chromosome set.
  • Normal euploids:
  • 1 set (haploid) or 2 sets (diploid).
  • Aberrant euploids: More or less than the normal number of chromosome sets, including triploids (3n) and tetraploids (4n).

• Aneuploids

  • Definition: Organisms whose chromosome number differs from the wild type by part of a chromosome set.
  • Types:
  • Monoploids: One chromosome set in a normally diploid species (e.g., male bees).
  • Polyploids: More than two chromosome sets, common in plants and some animals. Common forms of polyploids include triploids (3n), which are often sterile, and tetraploids (4n), which can yield larger fruit sizes.

17.2 Changes in Chromosome Structure

Changes in chromosome structure often lead to genetic disorders or changes in physical traits. Understanding these changes is crucial for genetics and evolutionary biology.

• Types of Chromosome Mutations:
  • Unbalanced: Changes in DNA amount affecting phenotype.
  • Examples:
    • Deletion: Loss of a chromosome segment, leading to loss of gene function and phenotypic changes, such as Cri du Chat syndrome, characterized by a deletion on chromosome 5.
    • Duplication: Gain of an additional copy of a chromosome segment, which can lead to gene dosage imbalances. Example: The expansion of CAG repeats in the HTT gene causes Huntington's disease.
  • Balanced: Alterations in gene order without changing the amount of DNA.
  • Examples:
    • Inversion: The chromosome segment is reversed, which may disrupt gene function or regulatory sequences. This can cause issues during meiosis where homologous chromosomes may not pair correctly.
    • Translocation: Exchange of segments between nonhomologous chromosomes, potentially leading to cancers or genetic diseases. An example is the Philadelphia chromosome, which results from a translocation between chromosomes 9 and 22, associated with chronic myeloid leukemia.

17.3 Phenotypic Consequences of Chromosomal Changes

Chromosomal changes can impact not only individuals but also populations and species over time. Here are notable consequences:

• Chromosome Number Variations:
  • Can cause hybrid incompatibility between species, affecting reproduction. For example, Indian and Chinese muntjacs have different chromosome numbers, leading to infertility when hybrids are attempted. Such instances highlight the importance of chromosomal integrity in species definitions.
  • Can contribute to conditions like Down syndrome (trisomy 21) and Turner syndrome (monosomy X) in humans, which have characteristic phenotypes and health challenges.
• Inversions:
  • Facilitate adaptive changes, such as unique color patterns in mimicry, enhancing survival. For instance, inversions in some species of Drosophila may contribute to adaptations to specific ecological niches.
• Chromosomal Rearrangements:
  • May lead to cancer due to aberrant gene expression, for example, MYC oncogene translocations disrupt normal cell regulation and lead to uncontrolled growth, underpinning many types of cancers. These changes can result in the formation of fusion proteins that drive tumorigenesis.
  • Gene dosage imbalances from aneuploidy (monosomy and trisomy) can also profoundly affect health and development across various species, not just humans. Trisomy 18 and trisomy 13