Clarence Erwin McClung’s suggestion that the “accessory,” or “X,” chromosome determines sex represented a significant contribution to the evolution of a chromosomal theory of inheritance.
Since ancient times, observers have recognized that although offspring generally resemble their parents, they differ from them in particular characteristics. Most early attempts to explain these similarities and differences were not very successful. In 1866, Gregor Mendel,
Meanwhile, cytologists (scientists who study cells) were taking advantage of improved instruments and staining techniques to observe the behavior of the chromosomes (darkly staining bodies in the cell nucleus) during cell division.
Investigators also observed that a second type of cell division occurs when gametes (eggs or sperm) are formed during meiosis. Instead of producing daughter cells identical with the parent, the number of chromosomes is reduced by half (haploid). When the egg and sperm fuse during fertilization, the set of chromosomes contributed by the egg match with the set contributed by the sperm, restoring the double (diploid) number. The chromosomes of the offspring, then, are derived equally from the egg and the sperm, assuring that each diploid pair of chromosomes in the offspring is composed of a maternal and a paternal representative. Mendel’s “factors” seemed to be distributed in the same way as the chromosomes, suggesting that the factors either were the chromosomes themselves or were located on the chromosomes.
Chromosomes as the physical agents of heredity could explain why offspring show characteristics of both parents (because half of the chromosomes are contributed by the male and the other half by the female). As reasonable as such a hypothesis appeared, if it were to be confirmed, a specific trait would have to be traced from a parent’s chromosomes to those of the offspring. Although no trait had been traced from parent to offspring by the beginning of the twentieth century, investigators found a likely candidate in sex inheritance. Clarence Erwin McClung played an important role in this investigation. Although McClung had a wide range of biological interests—including microtechnique, vertebrate paleontology, and histology—he became interested in the role of the chromosomes when, as a graduate student at the University of Kansas, he spent the summer of 1898 working with William Morton Wheeler at the University of Chicago. Wheeler suggested that McClung study chromosome behavior in sperm production in a species of grasshopper. This introduction, as well as the semester McClung spent at Columbia University studying with cytologist Edmund Beecher Wilson, directed his attention to the behavior of the chromosomes.
McClung’s studies resulted in a paper titled “A Peculiar Nuclear Element in the Male Reproductive Cells of Insects” (1899). This “nuclear element” had been described in 1891 by Hermann Henking, who followed the behavior of an unusual structure (he did not recognize it as a chromosome) in sperm formation of the fire wasp Pyrrhocoris. This structure, described by Henking as a “peculiar chromatin-element” (“X”) or an atypical “nucleolus,” did not appear to pair up with a partner during meiosis as did the other chromosomes. Henking noted that the male bug had an uneven number of chromosomes and the female an even number. During sperm formation, one-half of the sperm received the “X” body and the other half did not receive it. The half receiving it ended up with twelve chromosomes, whereas the half without it had only eleven. Henking, however, did not recognize the significance of the unpaired chromosome. McClung accomplished this step when, working on the formation of sperm in the grasshopper Xiphidion, he observed the “X” element that Henking had described. In his 1899 paper, he coined the term “accessory chromosome” for Henking’s “X” structure. In papers of 1901 and 1902, McClung postulated that the accessory chromosome
At the beginning of the twentieth century, biologists were divided on the issue of the inheritance of sex, with some asserting that it is determined by heredity and others arguing that it is determined by environment. One popular theory suggested that the fertilized egg exists initially in a sort of balanced state and that environmental factors, such as temperature or amount of food, determine whether it develops into a male or a female. Other investigators, however, insisted that external conditions do not affect sex ratios and that the determination of sex occurs at the time of fertilization or shortly thereafter. McClung’s observations were based on the second view.
The “accessory” chromosome was present in all cells of the organism and was present in one-half the gametes of males. Therefore, at fertilization one-half the gametes contained the accessory chromosome and the other half lacked it. Because sex was the only characteristic that McClung could think of that would divide a species into two equal groups, he concluded that the accessory chromosome might be a sex determiner. Miscounting the number of chromosomes in the female Xiphidion, he erroneously concluded that spermatozoa possessing an accessory (X) chromosome were male determiners. In fact, the opposite situation is correct. A sperm bearing an accessory (X) chromosome is female-determining, and a sperm bearing either no accessory or a Y chromosome is male-determining.
Although McClung’s misunderstanding of the way in which the accessory chromosome acts to determine sex was soon modified, his results did not resolve the controversy over the role of chromosomes in determining sex. Even McClung considered his ideas about the role of the accessory chromosome as a mere working hypothesis, not proof that sex is chromosomally determined. He recognized that his evidence was largely circumstantial, dependent on the presence or absence of the accessory chromosome in males or females. Given the tremendous variability within the animal kingdom and the scarcity of data on the subject, McClung was unwilling to generalize from his specific evidence to all animals. Even though he considered his evidence in grasshoppers compelling, he was not convinced of its truth for all species.
McClung agreed with many of his contemporaries that the environment might play a role in the determination of sex in some species. He also believed in selective fertilization: that the egg could choose whether it was fertilized by a sperm carrying an accessory chromosome or one lacking it. McClung did not have all the answers. His creative speculations, however, based on accurate observations of a specific group of animals, provided his successors with an important tool they could use to establish a basis for the chromosomal theory of sex determination.
McClung’s suggestion that the accessory chromosome in insects functions as a sex determiner represented a significant stage in the evolution of a chromosomal theory of inheritance. His interpretation helped attribute purpose to the movements of chromosomes in general and specifically of the accessory chromosome in sperm formation. Even though his proposal was tentative, he was the first to suggest the relation of a particular chromosome to a particular characteristic. His work not only explained the one-to-one male-female ratio observed in most animals but also, more generally, provided the groundwork for a chromosomal theory of heredity.
McClung’s work was acknowledged soon after it was completed by those who incorporated the work into their own research. Because the function of the behavior of the chromosomes was one of the major topics of interest within the early twentieth century biological “establishment,” cytologists were intrigued by new ideas. Thus McClung’s interpretation of the function of the accessory chromosome as a sex determiner was of vital interest to investigators such as Walter S. Sutton and Theodor Boveri, who were convinced that the chromosomes represented the physical basis for heredity but lacked evidence to prove it. Sutton, who had been a student of both McClung at Kansas and Wilson at Columbia University, published his paper “The Chromosomes in Heredity” in 1903, immediately after McClung’s major interpretive paper appeared in 1902. Sutton demonstrated that the behavior of the paired chromosomes during egg and sperm formation parallel exactly the segregation of the paired Mendelian factors in heredity.
McClung’s contribution on the accessory chromosome’s importance in the inheritance of sex was important to the understanding of the way in which sex is determined. McClung’s interpretations were considered by Nettie Maria Stevens and Edmund Beecher Wilson, who collected additional data and provided extensive hypotheses of sex determination by chromosomes.
From McClung’s cautious interpretation of his data that sex is determined by a chromosome in certain insects, others recognized the need to gather additional data. They soon realized that the situation could become quite complicated. In many groups of animals, males carry only one unpartnered accessory chromosome (designated XO), but in others they have an accessory chromosome plus an additional element, the Y chromosome (designated XY). Females generally have two accessories (designated XX). In some groups of animals, such as birds and some insects, however, the situation is reversed, with males having two accessories (XX) and females one (XO or XY). McClung’s creative interpretation of his data provided other investigators with a starting point to unravel a most complex situation.
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Allen, Garland E. Thomas Hunt Morgan: The Man and His Science. Princeton, N.J.: Princeton University Press, 1978. Although this book is basically concerned with the life and work of geneticist Thomas Hunt Morgan, it provides a valuable context for understanding McClung’s contributions. Includes an excellent bibliographic essay. -
Carlson, Elof Axel. Mendel’s Legacy: The Origin of Classical Genetics. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2004. Based heavily on early twentieth century sources, this book traces the roots of genetics in breeding analysis and studies of cytology, evolution, and reproductive biology. Highly illustrated. -
Cummings, Michael. Human Heredity: Principles and Issues. 6th ed. Monterey, Calif.: Brooks/Cole, 2002. Highly illustrated text aimed at nonscience students presents the complex topic of heredity clearly, without oversimplifying the concepts discussed. -
Dunn, L. C. A Short History of Genetics: The Development of Some of the Main Lines of Thought, 1864-1939. 1965. Reprint. Ames: Iowa State University Press, 1991. Provides context for McClung’s ideas. Useful volume covers concisely the central ideas in classical genetics from Mendel’s paper up to 1939. Includes a glossary and bibliographies of both primary and secondary sources. -
Hughes, Arthur. A History of Cytology. 1959. Reprint. Ames: Iowa State University Press, 1990. One section of this book considers theories of inheritance. Includes a discussion of the history of ideas on sex determination. -
McClung, Clarence E. “The Cell Theory: What of the Future?” American Naturalist 74 (1939): 47-53. McClung puts his own work and that of his contemporaries into context and speculates on the direction that future developments in cytogenetics might take. -
Wenrich, D. H. “Clarence Erwin McClung.” Journal of Morphology 66 (1940): 635-688. This work, published on the occasion of McClung’s seventieth birthday, represents one of the most complete biographical sources available. Also includes a complete bibliography of McClung’s work up to 1940. -
Wilson, Edmund B. The Cell in Development and Heredity. 3d ed. New York: Macmillan, 1937. The first edition of this book, published in 1896, was the outgrowth of a series of lectures Wilson gave at Columbia University in 1892-1893. A classic source, Wilson revised it as new data and interpretations emerged. Includes a massive literature list.
Bateson Publishes Mendel’s Principles of Heredity
Sutton Proposes That Chromosomes Carry Hereditary Traits
Punnett’s Mendelism Includes Diagrams Showing Heredity
Bateson and Punnett Observe Gene Linkage
Hardy and Weinberg Present a Model of Population Genetics
Morgan Develops the Gene-Chromosome Theory
Johannsen Coins the Terms “Gene,” “Genotype,” and “Phenotype”
Sturtevant Produces the First Chromosome Map