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.

Johann Gregor Mendel studied plants and their patterns of inheritance in Austria during the nineteenth century. Mendel experimented with the pea plant, Pisum, and his publication, 'Versuche uber Pflanzenhybriden' (“Experiments on Plant Hybridization”), published in 1866, revolutionized theories of trait inheritance. Mendel’s discoveries relating to factors, traits, and how they pass between generations of organisms enabled scientists in the twentieth century to build theories of genetics.

In the 1930s, George Beadle and Boris Ephrussi discovered factors that affect eye colors in developing fruit flies. They did so while working at the California Institute of Technology in Pasadena, California. (1) They took optic discs (colored fuchsia in the image) from fruit fly larvae in the third instar stage of development. Had the flies not been manipulated, they would have developed into adults with vermilion eyes. (2) Beadle and Ephrussi transplanted the donor optic discs into the bodies of several types of larvae, including those that would develop with normal colored eyes (brick red), and those that would develop eyes with other shades of red, such as claret, carmine, peach, and ruby (grouped together and colored black in the image). (3a) When implanted into normal hosts that would develop brick red eyes, the transplanted optic disc developed into an eye that also was brick red. (3b) When implanted into abnormal hosts that would develop eyes of some other shade of red, the transplanted optic discs developed into eyes that were vermilion. Beadle and Ephrussi concluded that there was a factor, such as an enzyme or some other protein, produced outside of the optic disc that influenced the color of the eye that developed from the disc.

Fruit flies of the species Drosophila melanogaster develop from eggs to adults in eight to ten days at 25 degrees Celsius. They develop through four primary stages: egg, larva, pupa, and adult. When in the wild, female flies lay their fertilized eggs in rotting fruit or other decomposing material that can serve as food for the larvae. In the lab, fruit flies lay their fertilized eggs in a mixture of agar, molasses, cornmeal, and yeast. After roughly a day, each egg hatches into a larva. The larva eats the material it finds itself in, and for four days it grows into stages of increasing size, called first-, second-, and third-instar stages. This figure shows a third-instar larva. Each larva has sections of tissue called imaginal discs, from which various parts of the adult anatomy develop. This figure shows the imaginal discs that will develop into antennae (colored purple), eyes (colored red), brain (colored blue), and wings (colored green). After four days, the larva turns into a pupa by making a casing, similar to caterpillars, and grows within the casing. After a four-day metamorphosis, the adult fly then emerges from its pupal casing. Adult males look somewhat different from adult females, as the males have darker rear abdomen segments than do females. The warmer the temperature around the eggs, the faster the flies develop to adults.

This diagram shows the life cycle of Neurospora crassa, a mold that grows on bread. N. crassa can reproduce through an asexual cycle or a sexual cycle. The asexual cycle (colored as a purple circle), begins in this figure with (1a) vegetative mycelium, which are strands of mature fungus. Some of the strands form bulbs (2a) in a process called conidiation. From those bulbs develop the conidia, which are spores. Next, (3a) a single conidium separates from its strand and elongates until it forms mycelium. The sexual cycle (colored as an orange circle) also starts with the (1b) vegetative mycelium. The strands develop into a structure called the proto-perithecium, and reproduction involves the proto-perithecium interacting with the conidia from a different mycelium. Reproduction also involves two mating types, called type A and type a. In reproduction, type A pairs with type a, and a conidium can be of either type, as can a proto-perithecium. A proto-perithecium fertilized by a conidium of the opposite mating type (2b) will develop into a perithecium. Inside the perithecium, croziers develop and mature into asci. (3b) In a maturing ascus, there are two nuclei (one represented as a white circle and one as a black circle), one of which comes from the conidium and the other from the proto-perithecium. Each nuclei has only one set of chromosomes (haploid). The two haploid nuclei fuse into a diploid nucleus (represented as a half black half white circle). The nucleus then divides, separating into two nuclei each with one set of chromosomes. Those nuclei duplicate themselves (represented as two white circles and two black circles), and then all the nuclei duplicate themselves again (represented as four white circles and four black circles). This process yields eight haploid ascospores within a mature ascus. Ascospores are spores, and function for the mold as do seeds for plants. The mature perithecium releases its ascospores (4b), which germinate and grow into mycelium. In the 1930s and 1940s, George Beadle and Ed Tatum collected the spores of irradiated N. crassa to study how genes produced enzymes.

In 1935, George Beadle and Boris Ephrussi developed a technique to transplant optic discs between fruit fly larvae. They developed it while at the California Institute of Technology in Pasedena, California. Optic discs are tissues from which the adult eyes develop. Beadle and Ephrussi used their technique to study the development of the eye and eye pigment. (1) The experimenter dissects a donor larva, which is in the third instar stage of development, and removes the optic disc (colored red) with a micropipette. Because the antenna disc is attached to the optic disc, they are often removed and transplanted together. (2) The experimenter then implants the optic disc into a host larva, in the part of the host that will develop into an adult abdomen. As the host larva matures to adulthood, the implanted optic disc develops into an eye inside the body cavity of the adult. (3) The adult host has an eye within its body, which Beadle and Ephrussi found by dissecting the adult hosts. If the antenna disc was also transplanted, sometimes the resulting eye developed with an antenna attached.

During the mid-nineteenth century, Johann Gregor Mendel experimented with pea plants to develop a theory of inheritance. In 1843, while a monk in the Augustian St Thomas's Abbey in Brünn, Austria, now Brno, Czech Repubic, Mendel examined the physical appearance of the abbey's pea plants (Pisum sativum) and noted inconsistencies between what he saw and what the blending theory of inheritance, a primary model of inheritance at the time, predicted. With his experiments, which he recored in "Versuche uber Pflanzenhybriden" ("Experiments in Plant Hybridization") in 1865, Mendel discredited the blending theory of inheritance, and from them he proposed laws for inheritance patterns. Despite the fact that Mendel's work did not define all aspects of inheritance, his ideas and laws contributed to later concepts of traits, specifically that offspring inherit traits from their parents via genes, that an offspring has at least two genetic factors for any given qualitative trait, and that the offspring inherits the genetic factors in equal proportion from both parents.