Sutton Proposes That Chromosomes Carry Hereditary Traits

Date December, 1902

Walter S. Sutton determined that Mendel’s inherited traits of organisms are physically located on chromosomes.

Locale Columbia University Medical School, New York, New York

Key Figures

• Walter S. Sutton (1877-1916), American geneticist and surgeon
• Theodor Boveri (1862-1915), German biologist
• Gregor Mendel (1822-1884), Austrian monk and botanist
• Carl Erich Correns (1864-1933), German geneticist

Summary of Event

Beginning in 1856, an Austrian monk named Gregor Mendel initiated a series of experiments with garden peas that would revolutionize biology in the twentieth century. Mendel cross-pollinated different lines of peas bred for certain characteristics (purple or white flower color, wrinkled or smooth seeds, and the like). From his extensive experiments, he concluded that each garden pea plant carries two copies of each characteristic trait (flower color, seed texture, and so on). He called these traits genes.

Diploid organisms have two copies of each gene that may or may not be identical. Different forms of the same gene are called alleles. For example, the gene for flower color may have two alleles, one conferring purple flower color and the other conferring white flower color. In most cases, one allele is dominant over other alleles for the same gene. For example, a plant having two purple flower-color alleles has purple flowers; a plant having two white flower-color alleles has white flowers. A plant having one purple and one white flower-color allele, however, has purple flowers; the dominant purple allele masks the white allele. It was later demonstrated that the encoding of proteins by genes is responsible for dominance relationships among alleles.

Because most plants and animals reproduce sexually through the fusion of pollen (or sperm) with egg, Mendel discovered the pattern of transmission of these genetic traits (that is, inheritance) from parents to offspring. Although each individual has two copies of every gene, an individual can transmit only one copy of each gene (that is, only one of two alleles) to each of its offspring. One copy of each gene is transmitted from the male parent and one copy is transmitted from the female parent to each of their offspring. If a parent has two different alleles for a given gene, there is a fifty-fifty probability of either allele’s being transmitted. This is true for each of thousands of different genes conferring different characteristic traits.

The inheritance of either allele operates by chance, as in flipping a coin, and that is what makes each individual of a species unique. Mendel summed up the random chance inheritance of different alleles of a gene in two principles. The first principle, allelic segregation, maintains that the different alleles for a given gene separate from each other during the formation of germ line cells (that is, sperm and egg). The second principle, independent assortment, maintains that different alleles of different genes arrange themselves randomly during germ cell production. For example, a person inherits two alleles of the gene for eye color and two alleles of the gene for hair color, one of the two alleles for each gene coming from either parent. Each parent can contribute any combination of the alleles for each gene, such as a brown eye-color allele and blond hair-color allele, blue eye-color allele and blond hair-color allele, and so on, depending on which alleles each parent possesses. Independent assortment requires that the genes in question lie on different chromosomes. It was only by luck that Mendel studied traits located on different chromosomes, because he did not know that chromosomes are the physical carriers of genetic traits. When Mendel published his results in the 1866 Proceedings of the Brünn Society for Natural History, they were scarcely noticed. Twenty years passed before the importance of his research was understood.

From 1885 to 1893, the German biologist Theodor Boveri researched the chromosomes of the roundworm Ascaris. Chromosomes are molecules composed mostly of deoxyribonucleic acid (DNA) and protein. They are located within the nuclei of the cells of all living organisms. Boveri studied under Pierre-Joseph van Beneden, who discovered that sperm and egg each carry one-half of the chromosomes needed to begin a new organism.

In the late 1890’s, Walter S. Sutton studied chromosomes of the grasshopper Brachyostola magna under Clarence Erwin McClung, discoverer of the sex-determining X chromosome. Sutton constructed very detailed diagrams of Brachyostola chromosomes during various phases of organismal development, including chromosomal behavior during mitosis (chromosome doubling and separating prior to cell division) and meiosis (chromosome dividing and splitting in sperm and egg production). Both Sutton and Boveri were independently attempting to understand chromosomal structure and function. The breakthrough came in 1900, when the German biologist Carl Erich Correns rediscovered Mendel’s work with garden peas. Correns boldly proposed that the chromosomes of living organisms carry the organisms’ inherited traits. Unfortunately, he did not provide an exact mechanism, nor did he provide detailed experimental data to support his hypothesis. Correns’s hypothesis eventually caught the attention of both Sutton and Boveri. With Correns’s hypothesis, Mendel’s data, and his own research on Brachyostola chromosome behavior, Sutton began to derive a mechanism for the chromosomal transmission of inherited traits.

Brachyostola magna is a diploid organism—that is, it contains two copies of every chromosome within each cell of its body. During the process of mitosis for a single cell, these chromosomes duplicate, so that there are four copies of each chromosome. By the end of mitosis, the four copies of each chromosome have been separated into two groups of two chromosomes each. The cell then divides into two cells, with each of the two newly formed cells having two copies of each chromosome similar to their single parent cell. If inheritable traits are located on chromosomes, then mitosis would appear to be a very effective mechanism for properly duplicating and dividing the genes up into daughter cells during cell division. Likewise, during the process of meiosis for a sperm or egg cell, the chromosome number is halved from two copies to one copy of each chromosome per cell. This would explain van Beneden’s observations that sperm and egg cells each contain one copy of each chromosome and Mendel’s assertion that each parent can contribute only one copy of each gene to its offspring. The behavior of chromosomes during meiosis explains Mendel’s principles of allelic segregation and independent assortment.

Sutton’s conclusions were reported in an article titled “On the Morphology of the Chromosome Group in Brachyostola magna,” which appeared in the December, 1902, issue of the Biological Bulletin. Sutton proposed that chromosomes physically contain genes and that the process of meiosis for sperm and egg production is the basis of Mendel’s two principles. He supported his propositions with detailed diagrams of Brachyostola chromosomes during various stages of meiosis.

Boveri followed in 1903 with the same conclusion from his earlier observations of Ascaris chromosomes. Together, the two scientists’ findings culminated in the chromosomal theory of inheritance, one of the basic tenets of genetics and modern biology. The chromosomal theory of inheritance makes four assertions. First, the fusion of sperm and egg is responsible for reproduction—the formation of a new individual. Second, each sperm or egg cell carries one-half of the genes for the new individual, or one copy of each chromosome. Third, chromosomes carry genetic information and are separated during meiosis. Fourth, meiosis is the mechanism that best explains Mendel’s principles of allelic segregation and independent assortment.

Significance

The Sutton and Boveri chromosomal theory of inheritance represents a major landmark in the history of biological thought. It reestablished Mendelism and provided a definite physical mechanism for inheritance. It demonstrated the molecular basis of life and thereby launched two successive waves of biological research in the twentieth century: the pre-World War II genetic and biochemical revolution and the postwar molecular biology revolution that continues today. The theory has also been very useful for the study of human genetic disorders.

Following Sutton’s and Boveri’s work, many geneticists began concentrating on chromosomes and how chromosomal genes actually confer individual traits. The English biochemist Sir Archibald E. Garrod coined the phrase “inborn errors of metabolism” to describe certain inherited genetic disorders, such as phenylketonuria and alkaptonuria. He attributed these genetic disorders in certain individuals to the lack of production of specific enzymes (proteins). He correctly hypothesized that a defective gene in an affected individual causes a defective enzyme resulting in a particular disorder. In the 1930’s and 1940’s, George Beadle, Edward Tatum, and Boris Ephrussi verified that one gene encodes one enzyme by studying eye-color mutations in the fruit flyDrosophila melanogaster and biochemical/nutritional mutations in the pink bread mold Neurospora crassa.

Thomas Hunt Morgan and his associates generated hundreds of mutations in Drosophila melanogaster and mapped these mutations to specific chromosome locations, thereby verifying Sutton and Boveri’s theory. The researchers generated mutations by using either chemicals or high-frequency ionizing radiation (for example, ultraviolet light and X rays). This work demonstrated that exposing living organisms to radiation and certain chemicals can cause chromosome and gene damage, often resulting in severe abnormalities and/or death in the exposed individuals and their descendants.

Chromosome studies also proved useful as a tool for understanding evolution. Evolution is the mutational changes that occur in organisms over time, thereby giving rise to new types of organisms and new species that are better adapted to the environment. Theodosius Dobzhansky, Alfred H. Sturtevant, and other eminent geneticists obtained evidence supporting Darwinian evolution through chromosome studies of many different species, including Drosophila and Zea mays (corn).

In 1928, Frederick Griffith discovered that some substance, which he called the “transforming factor,” could be passed from dead smooth Diplococcus pneumoniae to live rough Diplococcus pneumoniae, converting the rough bacterium into a smooth bacterium. Obviously, the transforming factor was a chromosome, but the chemical composition of the genetic material in the chromosome was unknown. Some biologists believed that the chromosome genetic material was DNA; others thought that it was protein. In 1944, Oswald T. Avery isolated and purified the transforming factor from Diplococci, discovering it to be DNA. In 1953, Alfred Hershey and Martha Chase provided compelling evidence for DNA as the chromosome’s genetic material in experiments using radioactively marked viruses. The same year, James D. Watson and Francis Crick proposed a physical model for DNA and a chromosomal mechanism by which DNA duplicates itself.

The study of chromosomal abnormalities is invaluable to the identification of human genetic disorders. An individual’s chromosomes can be studied from blood cell samples, and fetal chromosomes can be obtained from amniotic cells extracted from the mother’s womb (amniocentesis). Many disorders, such as Down syndrome, can be identified before birth. The Sutton and Boveri chromosomal theory of inheritance has pervaded all areas of biology and medicine.

Bibliography

Cummings, Michael. Human Heredity: Principles and Issues. 6th ed. Monterey, Calif.: Brooks/Cole, 2002. This highly illustrated text aimed at nonscience students presents the complex topic of heredity clearly, without oversimplifying the concepts discussed. Also addresses the social, cultural, and ethical implications of the use of genetic technology.

Gardner, Eldon J., and D. Peter Snustad. Principles of Genetics. 7th ed. New York: John Wiley & Sons, 1984. This introductory genetics textbook for undergraduates is a detailed, comprehensive survey of the field. Discussions of key concepts are very clear and are supported by excellent diagrams and illustrations. The first three chapters, “Historical Perspectives,”“Mendelian Genetics,” and “Cell Mechanics,” discuss the scientific details of Sutton’s and Boveri’s work.

Goodenough, Ursula. Genetics. 2d ed. New York: Holt, Rinehart and Winston, 1978. This introductory genetics textbook for undergraduates is clearly written, with careful explanations of difficult topics. Includes outstanding diagrams, extensive references, and brief historical sketches of prominent geneticists. Chapter 3, “The Meiotic Transmission of Chromosomes and the Principles of Mendelian Genetics,” describes the genetic basis of the chromosomal theory of inheritance.

Lewis, Ricki. Human Genetics: Concepts and Applications. 5th ed. New York: McGraw-Hill, 2002. This text designed for students who are not science majors explains what genes are, how they function, and how they interact with the environment. Includes discussion of how the understanding of genetics has changed since completion of the Human Genome Project.

Moore, John Alexander. Heredity and Development. New York: Oxford University Press, 1963. This introductory textbook provides an uncomplicated presentation of genetics. The work of Sutton and Boveri is described, along with Mendel’s two principles of inheritance and chromosome behavior during meiosis.

Pai, Anna C. Foundations for Genetics: A Science for Society. 2d ed. New York: McGraw-Hill, 1984. This genetics textbook presents a very down-to-earth discussion of the subject for the layperson. Describes Mendelian inheritance and the Sutton and Boveri chromosomal theory of inheritance and presents very clear explanations of chromosome behavior, mitosis, and meiosis.

Raven, Peter H., et al. Biology. 7th ed. New York: McGraw-Hill, 2004. Outstanding introductory biology textbook for undergraduate biology majors. Clearly written and features beautiful photographs and other illustrations. Provides an excellent presentation of patterns of inheritance for lay readers.

Sang, James H. Genetics and Development. New York: Longman, 1984. This textbook for advanced undergraduate and graduate students is a concise but thorough summary of how genes control the development of living organisms. Describes numerous important experiments and includes an extensive reference list. Chapter 3, “Active Chromosomes,” describes the organization of genes on chromosomes.

Starr, Cecie, and Ralph Taggart. Biology. 5th ed. Belmont, Calif.: Wadsworth, 1989. This introductory biology textbook provides a very clear, concise summary of all biological disciplines. Includes excellent photographs and illustrations. Chapter 13, “Chromosomal Theory of Inheritance,” is an outstanding discussion of Sutton and Boveri’s theory and its relationship to Mendelian inheritance.