The most common type of numerical chromosomal abnormality is aneuploidy, which involves an abnormal number of chromosomes due to either an extra or missing chromosome. 

Types of structural abnormalities include: 

• Duplications:presence of extra copies of chromosome segments
• Deletions:loss of chromosome segments
• Translocations:exchange of genetic material between non-homologous chromosomes
• Inversions: reversal of chromosome segments
• Isochromosomes:chromosomes with identical arms due to unequal splitting
• Ring chromosomes:formation of circular chromosomes due to deletions and reconnections