Theophilus Shickel Painter studied the structure and function of chromosomes in the US during in the early to mid-twentieth century. Painter worked at the University of Texas at Austin in Austin, Texas. In the 1920s and 1930s, Painter studied the chromosomes of the salivary gland giant chromosomes of the fruit fly (Drosophila melanogaster), with Hermann J. Muller. Muller and Painter studied the ability of X-rays to cause changes in the chromosomes of fruit flies. Painter also studied chromosomes in mammals. He investigated the development of the male gamete, a process called spermatogenesis, in several invertebrates and vertebrates, including mammals. In addition, Painter studied the role the Y-chromosome plays in the determination and development of the male embryo. Painter's research concluded that egg cells fertilized by sperm cell bearing an X-chromosome resulted in a female embryo, whereas egg cells fertilized by a sperm cell carrying a Y-chromosome resulted in a male embryo. Painter's work with chromosomes helped other researchers determine that X- and Y-chromosomes are responsible for sex determination.
Carl Gottfried Hartman researched the reproductive physiology of opossums and rhesus monkeys. He was the first to extensively study the embryology and physiology of reproduction in opossums when little was known about this mammal. Hartman worked in Texas where opossums, the only marsupial that lives in North America, were abundant. The female opossum delivers her fetal opossums in her pouch, where one can easily observe their development. After studying opossums for thirteen years, Hartman investigated the reproductive physiology of rhesus monkeys, also known as macaques. This research led to the discovery of when ovulation occurs, as well as its relation to the human menstrual cycle. Later research on scientific methods of birth control relied heavily on Hartman 's discoveries about primate and human reproduction.