Part A: Limitations of Pedigree Analysis
Pedigree analysis is a diagrammatic method used to track the inheritance of traits through generations of a family. It is a valuable tool in genetic counseling and research, but it also has several limitations:
1. Incomplete or Inaccurate Family History
Sometimes, individuals may not have full information about their family’s health history, or may unknowingly provide incorrect data.
2. Incomplete Penetrance and Variable Expressivity
A gene may not always produce symptoms (incomplete penetrance) or may vary in severity (variable expressivity), making it hard to draw clear conclusions.
3. Small Family Size
In smaller families, the inheritance patterns may not be statistically significant, reducing the reliability of conclusions drawn.
4. New Mutations
Spontaneous or de novo mutations can appear in an individual without being present in previous generations, confusing the inheritance pattern.
5. Genetic Heterogeneity
Similar traits may arise from mutations in different genes, which can make it difficult to identify a single pattern.
Part B: Structural Chromosomal Aberrations
Structural chromosomal aberrations are changes in the physical structure of chromosomes, often caused by breakage and abnormal rearrangement of chromosomal segments. These include:
1. Deletion
A portion of a chromosome is lost or deleted. This leads to loss of genetic material. Example: Cri-du-chat syndrome (deletion on chromosome 5).
2. Duplication
A section of the chromosome is duplicated, resulting in extra copies of certain genes. This can lead to gene dosage imbalances.
3. Inversion
A chromosome segment breaks off, flips around, and reattaches in the reverse direction. It can be:
- Paracentric: Does not include the centromere
- Pericentric: Includes the centromere
4. Translocation
Segments from two non-homologous chromosomes are exchanged. There are two types:
- Reciprocal translocation: Mutual exchange of segments
- Robertsonian translocation: Fusion of two acrocentric chromosomes
5. Ring Chromosomes
Ends of a chromosome are deleted and the remaining arms fuse to form a ring. This can disrupt gene expression and lead to developmental issues.
Conclusion
While pedigree analysis is useful, it has its limitations due to genetic complexity and external factors. Structural chromosomal aberrations can cause a wide range of genetic disorders and are key areas of study in cytogenetics.