Curt Jacob Stern (1902-1981)

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.

Stern, the first son of Anna Stern and Earned S. Stern, was born to a middle-class Jewish family in Hamburg, Germany, on 30 August 1902. Stern's father was an antiques dealer and sold dental supplies, and Stern's mother was a teacher. Shortly after Stern's birth, the family moved to Berlin, Germany. In 1918, Stern graduated from secondary school at Gymnasium Hindenburgschule in Berlin at the age of sixteen. Early graduation was common during the war because it enabled young men to be drafted into the military two years early. However, World War I ended in 1918, and Stern did not join the army and continued his education.

During his early education, Stern pursued microscopic studies of pond water, and he later studied zoology at the University of Berlin in Berlin. After obtaining his bachelor's degree in zoology in 1920, Stern began his doctoral work at the University of Berlin. He conducted his doctoral research on the process of mitosis in the microbial eukaryote Heliozoa in the lab of Max Hartmann, who studied protozoa. Mitosis results in the transfer of genetic material from the parent cell to daughter cells, which enable tissues to grow or repair themselves. Scientists label most of the cells in eukaryotic organisms as somatic cells, which include every cell other than sex cells. Mitosis ensures that each cellular generation has the same genetic information as the previous generation. Stern received a PhD from the University of Berlin in 1923 at the age of twenty-one.

Stern began postdoctoral work in 1924 at Columbia University in New York City, New York, in Thomas Hunt Morgan's lab, often called the Fly Room. The Rockefeller Foundation, also in New York City, funded promising European postdoctoral researchers to spend a year or more researching in the United States. Stern received one of the first of those fellowships. Morgan's team studied the mechanisms of heredity and evolution in fruit flies from the genus Drosophila. Morgan had helped to show that genes are located on chromosomes and are the basis of Gregor Mendel's 1866 theory of heredity, work the earned Morgan the Nobel Prize in Physiology or Medicine in 1933.

Morgan helped to describe the process of meiosis of germ cells, which produce sperm or egg cells. Scientists distinguish germ cells from somatic cells, the latter of which include all kinds of cells except germ cells. Like its somatic cells, an organism's germ cells generally have a complete set of chromosomes. Many kinds of organisms, including humans and fruit flies, have two sets of chromosomes in their somatic and germ cells. In those organisms, called diploids, one set is inherited from the mother and the other from the father. Germs cells go through a process called meiosis, in which they divide into two sex cells, sperm cells in males or egg cells in females, and each of those cells has only one set of chromosomes. During the first stage of meiosis, however, the chromosomes in the germ cells align up next to each other such that each maternal chromosome pairs with one paternal chromosome. In a process called crossing over, the paired chromosomes separate parts from their own strands, exchange those parts, and incorporate the new parts in place of the old parts. Via the process of crossing-over (recombination), germ cells produce gametes that have different chromosomes than either of the organism's parents, increasing genetic diversity across generations. Stern relied on the processes of meiosis and recombination in his Columbia experiments.

In the Fly Room, Stern collaborated with Calvin Bridges, also at Columbia University, to describe the relative placement of five genes on the left end of the second chromosome in Drosophila melanogaster. Especially in the eyes and wings of those fruits flies, researchers had identified five normal characteristics (wild-type), but for each of those characteristics, some flies had abnormal characteristics (mutant). They hypothesized that a normal gene caused a normal characteristic to develop, and an abnormal gene caused the abnormal characteristics to develop. Bridges had earlier shown that the hypothesized genes were likely on the second chromosome of Drosophila. Stern and Bridges collaborated to determine the locations on the second chromosome of those genes, in relation to each other.

Stern and Bridges mated normal and mutant individuals in different combinations to observe the physical characteristics (phenotypes) of the offspring. Stern and Bridges observed the phenotypic results of genetic recombination in multiple generations of Drosophila offspring. They used that information to develop a linkage map showing which characteristics are most often inherited together, and they established the relative order of the genes on the left end of the chromosome. When two genes are inherited together in a chromosome, scientists describe them as linked. Stern and Bridges reasoned that the greater the frequency of genetic recombination between known genetic markers, the further apart they were on the chromosome.

In other work conducted at Columbia, Stern showed that temperature and age affect the occurrence of crossing-over in the first chromosomes of Drosophila. Stern found that the rate of crossing-over increased in female Drosophila as they aged in room temperature, but decreased as they aged in warmer temperatures. Stern concluded that environmental factors during reproduction in Drosophila affect the genetic combinations of offspring, and therefore their phenotypic characteristics.

Stern met a US citizen, Evelyn Sommerfield, in 1925 while working in Morgan's lab. The couple later married in 1931, and had three daughters. Stern returned to the University of Berlin in 1926 and worked as an assistant to Richard Goldschmidt at the Kaiser Wilhelm Institute for Biology. During this time, Stern helped Goldschmidt edit his book Physiologische Theorie der Vererbung (Physiological Theory of Heredity), and was otherwise free to pursue his own research on crossing over. He stayed in Berlin for six years, where he wrote his first two books, Multiple Allelie (Multiple Allelism) in 1930, and Faktorenkoppelung und Faktorenaustausch (Coupling Factors and Factors Exchange) in 1933, and he published dozens of papers.

Stern returned to the United States in 1932. He received a second fellowship from the Rockefeller Foundation to visit the California Institute of Technology in Pasadena, California, where he again joined Morgan's Fly Room, as the lab had moved. In 1933, Adolf Hitler assumed power in Germany. Stern, of Jewish decent, and his wife decided that his return to Germany was not safe, and the couple elected to stay in the United States permanently. Stern accepted a research associate position in 1933 at the University of Rochester in Rochester, New York.

In 1936, Stern published the results of his ten years of research on crossing-over in Drosophila. In his research, Stern studied traits that offspring inherit from their mothers, especially those hypothesized to result from genes on the X chromosome inherited from their mothers. The process of mitosis normally results in two daughter cells identical to the parent cell and to each other. While studying fruit flies, Stern observed that a parent cell sometimes divided and produced daughter cells that were not identical to each other, as one part or tissue of a fruit fly would be normal but would have underdeveloped spots within it. Stern found that the difference between daughter cells occurred during mitosis when crossing-over occurred, a process previously theorized to happen only during meiosis. Stern found that somatic crossing-over does not follow the normal process of mitosis. By observing crossing-over of somatic cells, Stern found that genes on the female X chromosome, genes called sex-linked genes, could segregate themselves into combinations that, if they occurred in gametes, would be lethal if transmitted to offspring.

In 1937 Stern became an assistant professor at the University of Rochester and, in 1941, a full professor and chair of the department of zoology. In 1939, Stern became a US Citizen. During World War II, Stern conducted research for the US government on the effects of radiation. Stern's research studied the effects of exposure to low doses of x-rays and gamma radiation to assess if there was a safe level of exposure. He concluded that there is no safe level of exposure, and that x-rays can be harmful to the body by causing genetic mutations.

In 1947, Stern accepted a position at the University of California, Berkeley, in Berkeley, California. There, he taught courses on general genetics, human genetics, and had many postdoctoral students. In 1948, he joined the US National Academy of Sciences, and the next year he published a textbook titled The Principles of Human Genetics, which had multiple editions. The last edition appeared in 1973, during his last years of tenure at Berkeley. From 1947 to 1951, Stern was editor of the journal Genetics, and he was the 1950 president of the Genetics Society.

Stern contributed to the 1950 UNESCO statement The Race Question, along with other scientists of the time, questioning the scientific validity of genetic theories of race and eugenics. From 1950 to 1955, Stern served in the Advisory Committee for Biology and Medicine of the Atomic Energy Commission as a result of his earlier research on radiation. Throughout the 1950s and 1960s, he increasingly studied human genetics.

In 1963, Stern received the Kimber Genetics Award Medal of the National Academy of Sciences. Throughout his lifetime, Stern published greater than 200 scientific works. In 1970, Stern was diagnosed with Parkinson's disease, a degenerative disorder of the central nervous system. Stern gave his last public lecture at the Thirteenth International Congress of Genetics at Berkeley in 1973. Stern died in Sacramento, California, on 23 October 1981 at the age of seventy-nine due to cardiac failure as a result of Parkinson's disease.

The American Society of Human Genetics commemorated Stern in 2001 by establishing the Curt Stern Award. Given annually, the award recognizes scientists who have made major scientific contributions to the field of human genetics in their first ten years of research.