Cytologic observations have greatly contributed to our understanding of the modes of transmission, expression, and action of genetic determinants in tissues and organisms. While the elucidation of chromosome migration and distribution during gametogenesis (i.e., the segregation and independent assortment of chromosomes in the germ cells) provided a physical basis and a mechanism for the implementation of Mendelian principles, cytogenetics did not illuminate our practical knowledge of human biology until it was applied to eukaryotic somatic cells with ease, speed, and accuracy. As a result of these achievements of the late 1950s, the study of the chromosome complement is now routinely available. Karyotypic analysis established that genetic imbalances such as trisomies and deletions account for one-fifth or more of all spontaneous abortions and for one-fifth of all serious but viable birth defects, most of them with impaired mental development. Most chromosome errors are the result of a gametal or zygotic mishap and can be detected by fetal cell sampling through amniocentesis. In recent years more refined techniques (chromosome banding methods) have further increased the level of resolution at which anomalies can be detected. These techniques, along with the development of methods to hybridize somatic cells, have greatly assisted in the assignment of particular genes to particular chromosomes.