Genetics is the study of how genes are transmitted across generations, which includes the genetic information that produces an individual's traits, physical characteristics, and diseases. This recent branch of biology has transformed medicine, agriculture, biotechnology, and pharmacology.

DNA (deoxyribonucleic acid), which contains the blueprint necessary for the development of all living things, is central to heredity and is often the focus of genetics. Genetic testing and genetic engineering are two of the most prominent subfields of the discipline. Genetic testing allows doctors to identify conditions that may require treatment, while genetic engineering allows for the alteration or creation of specific DNA characteristics.

Beginning in the 4th century BCE, the ancient Greeks wondered about the transmission of hereditary traits between generations, suggesting that specific body parts of children were inherited from their parents' corresponding body parts. However, sometimes children looked less like their parents and more like another relative, according to Aristotle (384 to 322 BCE). He further believed that it was the father who passed along the physical characteristics, while the mother created the appropriate environment, which aided the child's development. Aristotle's ideas, none of which were accurate, included the belief that genetics is passed along via blood. It would be centuries later, especially with the invention of the microscope, which allowed scientists to examine genetic characteristics more closely.

In 1824, French scientist René Joachim Henri Dutrochet (1776 to 1847) concluded that plant and animal tissues were composed of cells, thereby verifying a hypothesis held by scientists since the late 1600s. Additional research in 1838 to 39 by the German botanist Matthias Jakob Schleiden (1804 to 1881) and the German cytologist Theodor Schwann (1810 to 1882) confirmed that cells were the basic unit of life. Both discoveries comprise the cell theory. Later scientists expanded the theory by identifying the various parts of cells, including the nucleus and mitochondria (the nucleus contains the cell's genetic material and regulates the cell's functions, while the mitochondria produce the cell's energy). Additionally, the emerging field of cytology studied the formation, structure, and function of cells and their relationship to cell inheritance. Charles Darwin (1809 to 1882), the English naturalist, proposed that inherited characteristics are passed from one generation to the next, leading scientists to believe that genetic information is contained in what would later be called germ cells (sperm and eggs).

In the 1760s, Carl Linnaeus (1707 to 1778), a Swedish taxonomist, was credited with a theory known as hybridization, which showed that a different form of life could be created by combining various kinds of the same species, plants, and animals. Subsequent generations would also inherit these new forms' characteristics. In the late 1700s, French physicians proposed the concept of heredity to explain the physical similarities between parents and their children. By the early 19th century, scientists revealed that physical traits from one parent dominated traits from the other parent and that reproduction between species could be controlled, leading researchers to develop a unitary theory of genetics.

In 1859, the English naturalist Charles Darwin published On the Origin of Species by Means of Natural Selection, in which he described his theory of natural selection (living beings are more likely to survive and reproduce if they possess inherited traits that allow for adaptation to their environment), leading to the modern understanding of genetics. However, Darwin was unable to explain the mechanisms underlying hereditary characteristics that would enable life forms to change over time.

The discovery of inherited dominant and recessive genes was credited to an Austrian monk, Gregor Mendel (1822 to 1884). Using pea plants, Mendel identified various traits of the plants – color, height, and flowers – which were inherited from one generation to the next. Mendel further concluded that traits (aka phenotypes) came in pairs (one from each parent) and that one trait, because it appears more often, will dominate in the successive generation. Furthermore, the transfer of the traits occurred in patterns. These discoveries comprised what came to be known as the Mendelian principles of genetics. Initially not recognized by other scientists, Mendel's findings would re-emerge in 1900, thereby establishing genetics as a scientific discipline.

The end of the 19th century and the beginning of the 20th century witnessed additional discoveries central to the study of genetics. The chromosome, which contains the genetic material, was discovered by Walther Flemming (1843 to 1905), a German biologist, in 1882 who also identified mitosis (cell division); in 1906, William Bateson (1861 to 1926), an English biologist, was the first person to use the term genetics to describe the study of heredity; Wilhelm Johannsen (1857 to 1927), a Danish biologist, argued that environmental factors along with heredity determined gene variation and devised the terms genes, genotype (the genetic composition of a cell), and phenotype (the physical characteristics resulting from the interaction of the genotypes with the environment).; and an American geneticist, Walter Stanborough Sutton (1877 to 1916), whose research, using grasshoppers, helped to clarify the role of the chromosomes in sexual reproduction. The results of his study, published in 1902, proved that chromosomes exist in pairs, confirming that sperm and egg cells each have one pair of chromosomes.

According to Jean Gayon in From Mendel to epigenetics: History of genetics, the first half of the century is dominated by classical genetics: "the gene is simultaneously a unit of function and transmission, a unit of recombination, and of mutation" (225). Thomas Morgan's (1866 to 1945) trailblazing research using fruit flies demonstrated the relationship between chromosomes and genes in the transfer of physical traits between generations, resulting in the chromosomal theory of inheritance, the idea that genes, which are found in chromosomes, are the fundamental components of heredity. He established that specific chromosomes carry specific genes, which may not always transfer to succeeding generations. Morgan is credited with the theory of genetic recombination: genetic material can transfer to DNA molecules, thus forming a new gene. His work earned him the title "father of classical genetics" and the 1933 Nobel Prize in Medicine.

Classical genetics research dominated the first half of the 20th century. Nucleotides, the basic building blocks of DNA, were discovered in 1929 by the American chemist Phoebus A. Levene (1869 to 1940). Another American chemist, Linus Pauling (1901 to 1994), researching issues concerning sickle-cell anemia, offered explanations for hereditary genetic disorders often caused by changes in proteins. Classical genetics sought to better understand the structure of DNA with the certainty that DNA was the key component of all genetic material. By the 1950s, modern genetics set out to understand how genes function and how they operate in the hereditary process.

In 1953, two molecular biologists, James Watson (1928 to 2025), an American, and Francis Crick (1916 to 2004), an Englishman, aided by Rosalind Franklin's image of DNA, were the first to map the double-helix structure of DNA. Their discovery proved that DNA carries the genetic material of heredity and that its structure, along with other substances, is self-replicating. The discoveries laid the foundation for modern molecular biology. Both men were awarded the Nobel Prize for Medicine in 1962.

In 1956, Vernon M. Ingram (1924 to 2006), an American biochemist, referred to as the "father of molecular medicine," revealed in his research on sickle-cell anemia that the mutation of a single letter in the DNA genetic code was sufficient to cause a hereditary medical disorder, offering the possibilities for prevention and treatment for similar diseases such as hemophilia and cystic fibrosis. Shortly thereafter, the first human chromosome abnormality was identified in people with Down syndrome. This genetic disorder, the result of an extra chromosome, was discovered by direct examination of chromosomes.

The sex of children and chromosomal abnormalities causing congenital disabilities were among the areas of research that occupied scientists in the following decades. Biotechnology also emerged as a field that explored the identification of mutations leading to DNA fingerprinting. Further research led to the development of human insulin & human growth hormone (1979), cloning (1981, which resulted in the creation of a sheep named Dolly), and human gene therapy (1990).

Genetics refers to the study of a single gene at a time, while genomics is the study of all genetic information contained in a cell. Beginning in 1990 and ending in 2003, the Human Genome Project (HGP) identified all of the genetic information contained in human DNA, roughly 30,000 to 35,000 genes. In 2006, Roger D. Kornberg (born 1947), an American biologist, discovered transcription, the process by which DNA converts into RNA (ribonucleic acid), a molecule essential to bodily functions that transmits genetic information to various parts of the body. Illnesses, such as cancer or heart disease, often result from RNA complications.

In 2022, scientists completed the first entire map of the human genome. Controversies about the direction of genetic research, human cloning, stem cell research, and genetically modified food and crops have dominated the field in recent years.

Within roughly 100 years, the field of genetics has produced new medicines, crops that resist pests and diseases, the identification of new hereditary diseases, helped couples experience parenthood, and identified DNA links among all humans (especially by uncovering common ancestors, Mitochondrial Eve & Y-chromosomal Adam). Genetic research has allowed people to trace their ancestral migrations, especially important for African-Americans, due to the transatlantic slave trade, searching for their homelands and people in Africa.

Scientists and doctors working in the field are regularly confronted with ethical and social concerns related to their research. Should genetic engineering be used to modify humans to enhance their physical abilities? How about creating "designer babies" in which prospective parents can choose eye or hair color, height, or skin tone? Are these issues usual and customary or examples of doctors "playing God?" At no other time in human history have people possessed the abilities to dramatically and profoundly alter or control their present and future physical conditions. The field of genetics offers endless possibilities of overcoming nature in beneficial ways, but at what cost?