Mary-Claire King studied genetics in the US in the twenty-first century. King identified two genes associated with the occurrence of breast cancer, breast cancer 1 (BRCA1) and breast cancer 2 (BRCA2). King showed that mutated BRCA1 and BRCA2 genes cause two types of reproductive cancer, breast and ovarian cancer. Because of King’s discovery, doctors can screen women for the inheritance of mutated BRCA1 and BRCA2 genes to evaluate their risks for breast and ovarian cancer. King also demonstrated the genetic similarities between chimpanzees and humans and helped to identify victims of human rights abuses using genetics. King's identification of the BRCA genes and their relationship to breast and ovarian cancer, both reproductive cancers, has allowed physicians to screen thousands of women for the genes and for those women to choose to undergo preventative cancer treatment to lower their risk of cancer.

Calvin Blackman Bridges studied chromosomes and heredity in the US throughout the early twentieth century. Bridges performed research with Thomas Hunt Morgan at Columbia University in New York City, New York, and at the California Institute of Technology in Pasadena, California. Bridges and Morgan studied heredity in Drosophila, the common fruit fly. Throughout the early twentieth century, researchers were gathering evidence that genes, or what Gregor Mendel had called the factors that control heredity, are located on chromosomes. At Columbia, Morgan disputed the theory, but in 1916, Calvin Bridges published evidence that, according to Morgan, did much to convince skeptics of that theory. Bridges also established that specific chromosomes function in determining sex in Drosophila.

Alfred Henry Sturtevant studied heredity in fruit flies in the US throughout the twentieth century. From 1910 to 1928, Sturtevant worked in Thomas Hunt Morgan’s research lab in New York City, New York. Sturtevant, Morgan, and other researchers established that chromosomes play a role in the inheritance of traits. In 1913, as an undergraduate, Sturtevant created one of the earliest genetic maps of a fruit fly chromosome, which showed the relative positions of genes along the chromosome. At the California Institute of Technology in Pasadena, California, he later created one of the first fate maps, which tracks embryonic cells throughout their development into an adult organism. Sturtevant’s contributions helped scientists explain genetic and cellular processes that affect early organismal development.

From 1913 to 1916, Calvin Bridges performed experiments that indicated genes are found on chromosomes. His experiments were a part of his doctoral thesis advised by Thomas Hunt Morgan in New York, New York. In his experiments, Bridges studied Drosophila, the common fruit fly, and by doing so showed that a process called nondisjunction caused chromosomes, under some circumstances, to fail to separate when forming sperm and egg cells. Nondisjunction, as described by Bridges, caused sperm or egg cells to contain abnormal amounts of chromosomes. In some cases, that caused the offspring produced by the sperm or eggs to display traits that they would typically not have. His research on nondisjunction provided evidence that chromosomes carry genetic traits, including those that determine the sex of an organism.

In 1910, Thomas Hunt Morgan performed an experiment at Columbia University, in New York City, New York, that helped identify the role chromosomes play in heredity. That year, Morgan was breeding Drosophila, or fruit flies. After observing thousands of fruit fly offspring with red eyes, he obtained one that had white eyes. Morgan began breeding the white-eyed mutant fly and found that in one generation of flies, the trait was only present in males. Through more breeding analysis, Morgan found that the genetic factor controlling eye color in the flies was on the same chromosome that determined sex. That result indicated that eye color and sex were both tied to chromosomes and helped Morgan and colleagues establish that chromosomes carry the genes that allow offspring to inherit traits from their parents.

In 1913, Alfred Henry Sturtevant published the results of experiments in which he showed how genes are arranged along a chromosome. Sturtevant performed those experiments as an undergraduate at Columbia University, in New York, New York, under the guidance of Nobel laureate Thomas Hunt Morgan. Sturtevant studied heredity using Drosophila, the common fruit fly. In his experiments, Sturtevant determined the relative positions of six genetic factors on a fly’s chromosome by creating a process called gene mapping. Sturtevant’s work on gene mapping inspired later mapping techniques in the twentieth and twenty-first centuries, techniques that helped scientists identify regions of the chromosome that when mutated cause organisms to develop abnormally and to create treatments to cure those kinds of disorders.

Launched in 2002, the International HapMap Project was a collaborative effort among scientists from around the world to create a map of common patterns of genetic variation in the human genome. HapMap stands for haplotype map. A haplotype is a stretch of DNA nucleotides, or letters, that individuals inherit as a block because they lie relatively close together along a chromosome. For any particular region of a chromosome, there may be multiple different haplotypes present among humans, each characterized by a slightly different DNA sequence. By collecting and sequencing the DNA of initially 270 individuals from several different geographic regions, HapMap scientists were able to identify common haplotypes that exist among those individuals, as well as reliable markers to distinguish them. That collection of haplotypes and identifying markers—the HapMap—provided a shortcut for researchers who wanted to identify associations between those inherited DNA variants and particular human traits, especially common, complex diseases like heart disease and cancer.

The 1,000 Genomes Project, which began in 2008, was an international effort to create a detailed and publicly accessible catalog of human genetic variation to support medical studies aimed at exploring genetic contributions to disease. Project scientists sequenced the entire genomes of 2,504 individuals from around the world—more than the 1,000 originally planned. The Project extended the results of the International HapMap Project, a prior effort at cataloging human genetic variation that ran from 2002 through 2010. Whereas the HapMap identified common genetic variants, meaning specific DNA sequences present in five percent or more of individuals in a population, the 1,000 Genomes Project identified genetic variants present in as few as one percent of individuals in a population. By assembling a larger catalog of DNA sequence variation than had previously existed, the 1,000 Genomes Project paved the way for researchers to more precisely locate disease-related genetic variation passed from parent to child.

A genome-wide association study, or GWAS, is a method for identifying variations in DNA that may contribute to the development of a particular trait, such as a disease. A GWAS relies on identifying statistical correlations between many, often thousands of, DNA markers and a particular trait. Scientists employ GWASs to try to identify the genetic contributions to complex traits, such as common human diseases. Complex traits are ones that scientists suspect are the result of multiple genes and environmental inputs acting together, in contrast to simple, Mendelian disorders that result primarily from the disturbance of a single gene. The genetic variants identified through a GWAS typically account for only a small proportion of the expected genetic contribution to a complex trait, which scientists refer to as the missing heritability problem. Since 2006, scientists have conducted thousands of GWASs aimed at identifying the genetic contributions to complex traits and have identified many thousands of genetic variations that correlate with those traits, although as of 2025, because of the missing heritability problem and other limitations, the concrete contributions of GWASs to medicine have so far been modest.