The research presented in this dissertation consists of four papers that revolve around the structure of human chromosomes and their relationship to birth defects. A new technique is described to produce spiralization of human metaphase chromosomes. The important feature is heat followed by trypsin treatment. By varying conditions, it is possible to produce bands, spirals and intermediate states. An investigation of human metaphase chromosomes reveals identical lateral bands in sister chromatids when stained with Quinacrine mustard or Giemsa-trypsin. A hybrid of these two methods produces banding patterns which are different in sister chromatids yet may be repeated in homologous chromatids. A case study is presented in which a 3l-year old white female with a history of ovarian dysfunction and infertility delivered a male infant with trisomy 13. Her cultured leucocytes were mosaic for trisomy X. The natures of trisomy X and trisomy 13 are discussed with particular emphasis on the genetic transmission. In another case study of a family, it is found that some individuals who completely lack dermal ridges are mosaic for an extra X chromosome with deletions in both arms. A mechanism is proposed to account for the extra chromosome. / Ph. D.