Chromosomal Basis of Inherited Disorders

Overview

• Inherited disorders can arise from changes in chromosome number or structure.
• Chromosomal abnormalities can be caused by errors in meiosis or mitosis.
• Such disorders are often identified through karyotyping, where chromosomes are visually inspected for anomalies.

Types of Chromosomal Abnormalities

Numerical Abnormalities

• Aneuploidy: Presence of an abnormal number of chromosomes.
  • Trisomy: Having an extra chromosome (e.g., Trisomy 21 - Down Syndrome).
  • Monosomy: Missing a chromosome (e.g., Turner syndrome, where one X chromosome is missing).

Structural Abnormalities

• Deletions: A part of a chromosome is missing.
• Duplications: A segment of a chromosome is duplicated.
• Inversions: A chromosome segment breaks off, flips around, and reattaches.
• Translocations: A segment of one chromosome is transferred to another chromosome.
  • Reciprocal translocation: Exchange of segments between two chromosomes.
  • Robertsonian translocation: Entire chromosome attaches to another at the centromere.

Causes of Chromosomal Abnormalities

• Errors during meiosis, particularly during anaphase, where non-disjunction can occur, leading to an imbalance in the number of chromosomes.
• Mitotic errors can also lead to mosaicism, where different cells in the same individual have different chromosome numbers.

Diagnosis and Detection

• Karyotyping: A laboratory technique used to visualize chromosomes.
• Prenatal testing: Includes procedures like amniocentesis and chorionic villus sampling to detect chromosomal abnormalities in the fetus.

Implications of Chromosomal Abnormalities

• Often result in developmental delays, physical abnormalities, and can affect the lifespan.
• Some conditions, like Down syndrome, are associated with specific physical features and health issues.

Genetic Counseling

• Essential for families with a history of chromosomal abnormalities.
• Provides information on the risk of recurrence and implications for future pregnancies.