Chromosomal abnormalities can either be numeral or structural. 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. Aneuploidy often results in trisomy rather than monosomy. Down syndrome is a well-known example of chromosomal aneuploidy, specifically trisomy 21. Another chromosomal abnormality is Turner's syndrome, which arises from the absence of an X chromosome (meaning only 45 chromosomes are present). 

Structural abnormalities refer to changes in the arrangement of one or more chromosomes within the genome, leading to abnormal lengths of the chromosome arms. These alterations often arise from unequal exchanges between chromosomes or incorrect repairs of chromosome breaks. Examples 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