Mary Frances Lyon studied gene expression and developed the theory of X-chromosome inactivation, also called Lyonization, during the twentieth century in the United Kingdom. The Lyonization hypothesis proposes that, even though females have two X-chromosomes and males have only one, one X-chromosome in females is always randomly inactivated, which causes males and females to have the same level of X-chromosome gene expression. Prior to Lyon’s hypothesis, scientists understood that there must be a biological way to compensate for the difference in X-chromosomes in males and females, but they did not know the exact mechanism. To investigate the topic, Lyon studied coat color in mice, a trait influenced by genes on the X-chromosome. Her resulting hypothesis highlighted X-chromosome inactivation as a mechanism for controlling gene expression in females without altering their DNA sequence. Through her research, Lyon aided scientists in understanding X-linked disorders, which laid the foundation for the development of gene therapies designed to treat X-linked disorders that affect hundreds of thousands of people globally.

Mary Dinsmore Salter Ainsworth conducted research in the United States, the United Kingdom, and Uganda, in the mid-twentieth century, on how emotional attachment between mothers and infants impacts development. Together with researcher John Bowlby, she developed the Ainsworth-Bowlby Theory of Attachment, hereafter ABTA. The ABTA is a theoretical framework that accounts for how and why attachment and separation, particularly between children and mothers, impact psychological and behavioral development. Using her training in clinical psychological assessments, she developed new techniques for assessing security and attachment between infants and mothers. Ainsworth and her colleagues applied those techniques to several long-term studies that provided empirical support for the ABTA. Many of her assessment techniques are still in use as of 2025, and her contributions to the ABTA have enabled researchers to systematically analyze how attachment, security, and separation impact psychological and behavioral development throughout the lifespan, opening new avenues for research and treatment.

Sherri Chessen, also known as Sherri Finkbine, a television host who lived in Scottsdale, Arizona, during the 1960s, sought an abortion after learning that the sedative thalidomide caused fetal deformities. At the time, Arizona law only allowed abortions if the mother’s life was at risk. Chessen anonymously contacted The Arizona Republic, a local newspaper, and a reporter, Julian DeVries, told Chessen’s story in an article titled, “Pill May Cost Woman Her Baby.” Chessen’s identity later became public when the Good Samaritan Hospital in Phoenix, Arizona, where Chessen was to have the abortion, filed a suit to get the state’s approval to authorize the abortion. After her name became public, the hospital refused to perform the abortion, leading Chessen to travel to Sweden for the procedure. Chessen’s case led to widespread discussion about abortion access in the United States, brought the issue of reproductive rights into the national spotlight, and eventually influenced legal reforms, including the US Supreme Court’s decision in Roe v. Wade (1973).

Étienne-Louis Arthur Fallot was a physician working in France during the late nineteenth century who studied and described the four cardiac anatomical defects that cause the congenital anomaly known as the Tetralogy of Fallot. Those four heart defects result in deoxygenated blood recirculating through the body, giving the skin a blue-like color, a process called cyanosis. Numerous physicians and researchers before Fallot had identified and described the anatomical cardiac defects that would eventually be included in the Tetralogy. However, Fallot was among the first to note that the four anatomical heart defects tended to occur together. Through autopsy investigations, Fallot established the Tetralogy as one unified pathology rather than four unrelated anatomical abnormalities, providing a basis for the eventual surgical treatment of the condition, which affects approximately four out of every one thousand births worldwide.

Sir Martin John Evans researched developmental biology in the United Kingdom during the twentieth and early twenty-first centuries. He was among the first to isolate and grow embryonic stem cells in the lab. Embryonic stem cells have the ability to develop into many different cell types. Using those cells, Evans and his colleagues developed methods for introducing changes to the DNA of early mouse embryos. He found that when he introduced those modified embryos into foster mothers, the genetic alterations also appeared in subsequent generations. That finding helped him produce some of the first living mice with desired genetic changes, later dubbed knockout mice. In 2007, Evans received the Nobel Prize in Physiology or Medicine along with Mario Capecchi and Oliver Smithies for their work on introducing specific gene modifications in mice using embryonic stem cells. Evans’s scientific contributions have permitted scientists to better understand the roles of different genes in both embryological development and disease.

Ben Barres researched neurobiology during the late twentieth and early twenty-first centuries as a professor at Stanford University School of Medicine in Stanford, California. His primary research focus was glial cells, which are the most abundant cells of the nervous system that support and protect neurons. There are many kinds of glial cells, and Barres examined their roles in the nervous system as well as in development and disease. Prior to Barres’s work, researchers believed that neurons were the primary players in brain function and that glial cells played a passive role. Barres discovered that glial cells play a key role in creating and eliminating synapses, which are the connections between nerve cells. Barres was transgender and after transitioning at age forty-three spoke frequently about sexism in science. Through his scientific research, Barres brought attention to the function of glial cells in development and disease, and through his activism he became a role model for LGBTQIA+ people in science.

Charles Rotimi is a researcher who studies the etiology of complex diseases and health disparities and advocates for the inclusion of greater racial and ethnic diversity in genomic repositories. In the early 2000s, Rotimi spearheaded the recruiting of African communities for participation in the International HapMap Project. As director of the Center for Research on Genomics and Global Health, or CRGGH, at the National Institutes of Health, or NIH, Rotimi led governmental research on human genetic variation and patterns of disease. Rotimi is a founding member of the Human Heredity and Health in Africa, or H3Africa, initiative, which aims to increase the representation of African populations in global genetic studies. Through his epidemiological research, leadership in advocacy groups for the support of African scientists, and his collaboration in genomic diversity initiatives, Rotimi promotes racial and ethnic representation in genetic research, especially regarding the inclusion of African people and the African diaspora.

Lewis Albert Sayre was an orthopedic surgeon who practiced medicine in New York City, New York, during the second half of the nineteenth century and held a number of leadership positions in his profession. Over the course of his nearly fifty-year career, Sayre developed a number of surgical and nonsurgical treatments of bone problems including scoliosis and other forms of spinal curvature, club foot, and hip-joint disease resulting from tuberculosis infection. He also helped popularize circumcision, or surgical removal of the foreskin of the penis, as a treatment for a variety of medical conditions, including muscle paralysis and epilepsy, based on the theory, discredited as of 2025, that a chronically irritated foreskin could lead to diseases in other parts of the body. By providing a medical justification for circumcision, Sayre helped to embed the surgical procedure within US medicine, even as the medical rationales for its use would change periodically in the decades that followed.

Luigi Luca Cavalli-Sforza (1922–2018) was a researcher whose work explored the relationships between human genetic diversity and historical migrations, integrating genetics and anthropology to determine how humans populated the world. Prior to his work in human genetics, Cavalli-Sforza studied genetic recombination in bacteria and helped determine the system of genetic inheritance within Escherichia coli in the late 1940s. After pivoting his research focus and assuming a long-term teaching and research position at Stanford University in Stanford, California, in 1971, Cavalli-Sforza participated in studies that modeled human migration, focusing on the global spread of agriculture during the Neolithic period. He was also one of the founders in the creation of the Human Genome Diversity Project, or HGDP, an international scientific collaboration launched in the early 1990s to map the genetic diversity of human populations across the globe. Cavalli-Sforza’s interdisciplinary approach to studying human history and human evolution left its mark on the fields of both genetics and anthropology in the twentieth and twenty-first centuries.