When a new individual is formed, heredity and other factors may play a role in producing sexual malformations.' Within the past few years, there has been a considerable breakthrough in our knowledge of human genetics, and findings that are in many ways quite revolutionary have given us new and valuable insights in this area and have even changed some of our basic ideas. This body of knowledge, however, is continuing to expand, so that currently-held views must be regarded as only tentative at best and subject to further change as additional discoveries are made. The human body is composed of cells; these combine to form organs; these, in turn, combine to form systems, such as the reproductive, respiratory, and digestive systems; and these, then, combine to form the individual. Cells differ in structure and function, but all cells contain chromosomes. Thirty-three years after the problem of human chromosome number was considered to be solved-that is, forty-eight had generally been accepted as the number of chromosomes in every human cell, except the sperm and the ovum-investigators examining some human cells in I956 were surprised to find that the chromosome number 46 predominated,2 a finding that has since been substantiated by others.3 These forty-six human chromosomes exist in the