If mutations of genetic material are large enough to be seen under the light microscope, they are called chromosomal aberrations.
They can be divided into structural and numerical abnormalities.
The smallest visible alteration to a chromosome that is visible is approximately four million base pairs.
Chromosomal disorders are very common, affecting 7.5% of all conceptions (1), yet, due to spontaneous miscarriage, their livebirth frequency is only 0.6%. Hence, amongst spontaneous early miscarriages, 60% have a chromosomal abnormality, usually that of trisomy, 45,X, or triploidy (1).
Disorders of this kind may result from germ cell mutations in the parent that have been passed on to sex chromosomes or autosomes in the affected individual. Alternatively, chromosomal aberrations may arise out of somatic mutation in the generation affected.
Generally, autosomal chromosomal disruptions are more severe than sex chromosome abnormalities. Similarly, deletions are more deliterious than duplications.
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