In the late 1980s, Peter Goodfellow in London, UK led a team of researchers who showed that the SRY gene in humans codes a protein that causes testes to develop in embryos. During this time, scientists in London and Paris, including Peter Koompan and John Gubbay, proposed that SRY was the gene on the Y chromosome responsible for encoding the testis-determining factor (TDF) protein. The TDF is a protein that initiates embryo to develop male characteristics. Looking for evidence that SRY was the TDF, Goodfellow and colleagues examined people who were anatomically female, but whose cells had Y chromosomes. Females normally have cells with two X sex chromosomes (XX), while males normally have cells with one X and one Y chromosome (XY). Goodfellow's team discovered that individuals with Y chromosomes developed as female instead of as male due to inactive SRY sequences on the Y chromosome. Goodfellow and colleagues compiled the results of their experiment in a paper titled Genetic Evidence Equating SRY and the Testis-Determining Factor in 1990. Their results showed that the SRY gene is necessary for male characteristics to develop in embryos, and that SRY encodes the TDF protein.

The Sex-determining Region Y (Sry in mammals but SRY in humans) is a gene found on Y chromosomes that leads to the development of male phenotypes, such as testes. The Sry gene, located on the short branch of the Y chromosome, initiates male embryonic development in the XY sex determination system. The Sry gene follows the central dogma of molecular biology; the DNA encoding the gene is transcribed into messenger RNA, which then produces a single Sry protein. The Sry protein is also called the testis-determining factor (TDF), a protein that initiates male development in humans, placental mammals, and marsupials. The Sry protein is a transcription factor that can bind to regions of testis-specific DNA, bending specific DNA and activating or enhancing its abilities to promote testis formation, marking the first step towards male, rather than female, development in the embryo.

Early 1990s research conducted by Peter Koopman, John Gubbay, Nigel Vivian, Peter Goodfellow, and Robin Lovell-Badge, showed that chromosomally female (XX) mice embryos can develop as male with the addition of a genetic fragment from the Y chromosome of male mice. The genetic fragment contained a segment of the mouse Sry gene, which is analogous to the human SRY gene. The researchers sought to identify Sry gene as the gene that produced the testis determining factor protein (Tdf protein in mice or TDF protein in humans), which initiates the formation of testis. Koopman's team published their results in 1991 in Male Development of Chromosomally Female Mice Transgenic for Sry gene. Their results showed that Sry gene partly determines the sex of an embryo and is the only gene on the Y chromosome necessary for initiation of male development in mice.

Studies in Spermatogenesis is a two volume book written by Nettie Maria Stevens, and published by the Carnegie Institution of Washington in 1905 and 1906. In the books Stevens explains the research she conducted on chromosomal sex determination in the sperm and egg cells of insect species while at Bryn Mawr College, near Philadelphia, Pennsylvania. Studies in Spermatogenesis described early examples of chromosomal XY sex-determination.

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.

The Y-chromosome is one of a pair of chromosomes that determine the genetic sex of individuals in mammals, some insects, and some plants. In the nineteenth and twentieth centuries, the development of new microscopic and molecular techniques, including DNA sequencing, enabled scientists to confirm the hypothesis that chromosomes determine the sex of developing organisms. In an adult organism, the genes on the Y-chromosome help produce the male gamete, the sperm cell. Beginning in the 1980s, many studies of human populations used the Y-chromosome gene sequences to trace paternal lineages. In mammals, the Y-chromosomes contain the master-switch gene for sex determination, called the sex-determining region Y, or the SRY gene in humans. In most normal cases, if a fertilized egg cell, called a zygote, has the SRY gene, the zygote develops into an embryos that has male sex traits. If the zygote lacks the SRY gene or if the SRY gene is defective, the zygote develops into an embryo that has female sex traits.

In humans, sex determination is the process that determines the biological sex of an offspring and, as a result, the sexual characteristics that they will develop. Humans typically develop as either male or female, primarily depending on the combination of sex chromosomes that they inherit from their parents. The human sex chromosomes, called X and Y, are structures in human cells made up of tightly bound deoxyribonucleic acid, or DNA, and proteins. Those are molecules that contain the instructions for the development and functioning of all life forms, including the development of physical traits and body parts that correspond with each biological sex. Humans who inherit two X chromosomes typically develop as females, while humans with one X and one Y chromosome typically develop as males. Sex determination is the beginning of the development of many characteristics that influence how a human looks and functions as well as the societal expectations that other humans have for each other.

Curt Jacob Stern studied radiation and chromosomes in humans and fruit flies in the United States during the twentieth century. He researched the mechanisms of inheritance and of mitosis, or the process in which the chromosomes in the nucleus of a single cell, called the parent cell, split into identical sets and yield two cells, called daughter cells. Stern worked on the Drosophila melanogaster fruit fly, and he provided early evidence that chromosomes exchange genetic material during cellular reproduction. During World War II, he provided evidence for the harmful effects of radiation on developing organisms. That research showed that mutations can cause problems in developing fetuses and can lead to cancer. He helped explain how genetic material transmits from parent to progeny, and how it functions in developing organisms.

In 1973, Ronald Ericsson developed the Ericsson method, which is a technique used to separate human male sperm cells by their genetic material. Ericsson, a physician and reproduction researcher, developed the method while conducting research on sperm isolation in Berlin, Germany, in the early 1970s. He found that the sperm cells that carry male-producing Y chromosomes move through liquid faster than the cells that carry female-producing X chromosomes. As a result of his findings, Ericsson suggested suspending a semen sample in a viscous liquid made from albumin protein, and collecting only sperm that quickly pass through the liquid. Shortly after Ericsson described his method, researchers demonstrated that it was effective for sex selection. However, later studies contested those results. Despite that, the Ericsson method is still utilized by couples in 2018 as a means of sex selection and was the first sperm separation technique used in combination with artificial insemination to enable people to select the sex of their children.

In the book Your Baby’s Sex: Now You Can Choose, David Michael Rorvik and Landrum Brewer Shettles describe methods that couples can use prior to and during conception that will increase the chances of producing a child of their desired sex. Rorvik, a science writer, and Shettles, an obstetrics and gynecology researcher and physician, co-wrote the book. Shettles developed the methods detailed in the book during the 1960s. Although the authors claim a high success rate, some researchers have contested the validity of the methods proposed in Your Baby’s Sex: Now You Can Choose. Despite contradicting evidence for the effectiveness of the methods, the book itself has remained popular throughout its forty consecutive years in print. Since its original publication, Your Baby’s Sex: Now You Can Choose has reached a large audience, with over 1.5 million copies of the book sold worldwide, while adding to the controversy about the ethics of sex selection research.