Three bases are cited to define an individual chromosome morphologically. Variations in the chromosome numbers give rise to different abnormalities which are fully studied one by one. The type and extent of mosaicism and the clinical feature of individual cells depend on the constitution of zygote, the stage at which the error occurs and the relative viabilities of the daughter cells. Morphology of chromosomes appears to be abnormal due to translocation or deletion of a large portion of one of the chromosomes 22 on to the chromosome 13, and the error could be detected by precise measurement. It is also indicated that the recent advances in the study of human chromosomes provides a research tool on human development problems. In most cases Karyotype abnormalities lead to abnormal human sexual developments and chromosome changes account for human tumours.