Victor Jollos studied fruit flies and microorganisms in Europe and the US, and he introduced the concept of Dauermodifikationen in the early 1900s. The concept of Dauermodifikationen refers to environmentally-induced traits that are heritable for only a limited number of generations. Some scientists interpreted the results of Jollos's work on Paramecium and Drosophila as evidence for cytoplasmic inheritance. Jollos was forced to emigrate from Germany to the United States due to anti-semitic government policies in the early 1930s. Nevertheless, his work on Dauermodifikationen remained central to theoretical discourse among German zoologists concerning heredity, development, and evolution.

Eugenical Sterilization in the United States is a 1922 book in which author Harry H. Laughlin argues for the necessity of compulsory sterilization in the United States based on the principles of eugenics. The eugenics movement of the early twentieth century in the US focused on altering the genetic makeup of the US population by regulating immigration and sterilization, and by discouraging interracial procreation, then called miscegenation. Published in December 1922 by the Psychopathic Laboratory of the Municipal Court of Chicago in Chicago, Illinois, the book reports Laughlin's analysis of how states could benefit from sterilizing their mentally disabled residents, and it reprinted his model sterilization law, which he encouraged state governments to adopt. Laughlin's model sterilization law stressed the need for the sterilization of populations that Laughlin deemed inadequate for reasons ranging from physical appearance to socioeconomic status. The document influenced twentieth century legislation in the US about reproduction and compulsory sterilization.

At the turn of the twentieth century, William Bateson studied organismal variation and heredity of traits within the framework of evolutionary theory in England. Bateson applied Gregor Mendel's work to Charles Darwin's theory of evolution and coined the term genetics for a new biological discipline. By studying variation and advocating Mendelian genetics, Bateson furthered the field of genetics, encouraged the use of experimental methodology to study heredity, and contributed to later theories of genetic inheritance.

Edmund Beecher Wilson experimented with Amphioxus (Branchiostoma) embryos in 1892 to identify what caused their cells to differentiate into new types of cells during the process of development. Wilson shook apart the cells at early stages of embryonic development, and he observed the development of the isolated cells. He observed that in the normal development of Amphioxus, all three main types of symmetry, or cleavage patterns observed in embryos, could be found. Wilson proposed a hypothesis that reformed the Mosaic Theory associated with Wilhelm Roux in Germany. Wilson suggested that cells differentiated into other cells when influenced by physiological (dynamic) changes in the hereditary substance contained in cells, and not because of the qualitative division, or parcelling out, of the substance into daughter cells. Wilson published his results in August 1893.

The Cell in Development and Inheritance, by Edmund Beecher Wilson, provided a textbook introduction to cell biology for generations of biologists in the twentieth century. In his book, Wilson integrated information about development, inheritance, chromosomes, organelles, and the structure and functions of cells. First published in 1896, the book started with 371 pages, grew to 483 pages in the second edition that appeared in 1900, and expanded to 1,231 pages by the third and final edition in 1925. Wilson dedicated the book to the cell biologist Theodor Boveri, whose work established the roles of chromosomes in cell division. With its explanations and many illustrations and diagrams, The Cell in Development and Inheritance enabled embryologists to better understand development in terms of cell structure and function.

Petr Kropotkin proposed the theory of Pleistocene ice age, alternative theories of evolution based on embryology, and he advocated anarchist and communist social doctrines in Europe during the nineteenth and twentieth centuries. He traveled in eastern Siberia and Manchuria from 1863 until 1867, and his subsequent publications about that area's geography became authoritative until the middle of the twentieth century. Kropotkin argued that his geographic and geologic observations in Asia, Finland, Sweden, and Canada, supported the theory of Pleistocene continental glaciation, often called the ice age. He was one of the first to study the ancient geography and climate of the Quaternary period, which spans from 2.5 million years ago until the present. Around the turn of the nineteenth century, Kropotkin offered what he said were complementary amendments to Charles Darwin's 1859 theory of evolution by natural selection. Kropotkin employed a variety of arguments from natural history, embryology, and geography to support his theory of mutual aid, which he argued was a positive mechanistic addition to the theory of evolution.

August Friedrich Leopold Weismann studied how the traits of organisms developed and evolved in a variety of organisms, mostly insects and aquatic animals, in Germany in the late nineteenth and early twentieth centuries. Weismann proposed the theory of the continuity of germ-plasm, a theory of heredity. Weismann postulated that germ-plasm was the hereditary material in cells, and parents transmitted to their offspring only the germ-plasm present in germ-cells (sperm and egg cells) rather than somatic or body cells. Weismann also promoted Charles Darwin's 1859 theory of the evolution of species. Weismann argued that only changes to the germ cells, and not body cells, could be inherited, a theory that influenced theories of heredity throughout later centuries.

The American Eugenics Society (AES) was established in the US by Madison Grant, Harry H. Laughlin, Henry Crampton, Irving Fisher, and Henry F. Osborn in 1926 to promote eugenics education programs for the US public. The AES described eugenics as the study of improving the genetic composition of humans through controlled reproduction of different races and classes of people. The AES aided smaller eugenic efforts such as the Galton Society in New York, New York, and the Race Betterment Foundation in Battle Creek, Michigan, and it influenced eugenic policy set by the US Supreme Court in cases including Buck v. Bell (1927) and Skinner v. Oklahoma (1942). The AES was renamed the Society for the Study of Social Biology in 1972.

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.

Harry Hamilton Laughlin helped lead the eugenics movement in the United States during the early twentieth century. The US eugenics movement of the early twentieth century sought to reform the genetic composition of the United States population through sterilization and other restrictive reproductive measures. Laughlin worked as superintendent and assistant director of the Eugenics Research Office (ERO) at Cold Spring Harbor Laboratory in Cold Spring Harbor, New York, alongside director Charles Davenport. During Laughlin's career at the ERO, Laughlin studied human familial ancestry, called pedigrees, and in 1922 published the book Eugenical Sterilization in the United States, which influenced sterilization laws in multiple states. Laughlin's support of compulsory sterilization to control the reproductive capacity of entire populations influenced the history of eugenics and reproductive medicine.