Chromosomal Abnormalities in Genetics

Chromosomal Abnormalities

• Genetics is the study of heredity and its variations.
• A gene is the basic unit of inheritance, formed of DNA, and responsible for protein formation.
• A chromosome is a thread-like structure of DNA that carries genes for hereditary information.
• Each chromosome is formed of two chromatids.
• Each chromatid has a short arm (p arm) and a long arm (q arm).
• The two chromatids are attached at a point called the centromere.
• Kinetochores are points on the centromere, where spindle fibers attach.
• A diploid state signifies the presence of two copies of each chromosome.
• A haploid state signifies a single copy of each chromosome.
• Somatic cells have a diploid number of chromosomes.
• Gametes (ova and testes) have a haploid number of chromosomes.
• Telomeres are repetitive DNA sequences at each chromosome end, protecting from deterioration or fusion with adjacent chromosomes.

Types of Chromosomal Abnormalities

• Structural abnormalities:

  • Deletions (del): Loss of a chromosomal segment.
  • Ring chromosomes (r): The ends of chromosome arms fuse to form a ring.
  • Inversions (inv): A chromosomal segment breaks, reorients 180 degrees, and rejoins.
  • Translocation: Transfer of segments between chromosomes.
  • Isochromosomes: Abnormal transverse division of the centromere, separating the two arms instead of longitudinally dividing the two chromatids.

• Numerical abnormalities:

  • Polyploidy: Abnormal duplication of the haploid chromosome number (e.g., 3n, 4n).
  • Aneuploidy: Abnormal number of chromosomes.
    • Trisomy: Extra chromosome (e.g., Down syndrome - trisomy 21, Patau syndrome - trisomy 13, Edward syndrome - trisomy 18 ).
    • Monosomy: Missing chromosome.
      • Monosomy of autosomes is lethal.
    • Aneuploidy of sex chromosomes (e.g., Klinefelter syndrome - 47, XXY, Turner syndrome - 45, X).

Causes of Chromosomal Abnormalities

• Hereditary factors
• Advanced maternal age
• Infections during pregnancy (e.g., German measles).
• Exposure to radiation or autoimmune diseases during pregnancy.

Sex Chromatin (Barr Body)

• Sex chromatin is an inactive X chromosome in female cells during interphase.
• Stained with basic dyes (Feulgen positive).