The Chromosome Theory

In 1903 Walter S. Sutton was a professor at Columbia University in New York City. During his studies, he noticed a strong similarity between Mendel’s principles and his own observations of meiosis in grasshoppers. These similarities led Sutton to propose the chromosome theory. It states that hereditary factors, or genes, are carried on chromosomes.

First Sutton noticed that the chromosomes in each grasshopper cell line up in pairs during meiosis. He also observed that the members of each chromosome pair are homologous, or alike in shape and size.

Next Sutton observed that homologous chromosomes segregate during meiosis. As a result, each gamete receives one half of the chromosomes - one member from each pair. In addition, he saw that the way in which members of one pair segregate seems to have no effect on how the members of another pair segregate. Finally, he observed that a sperm and an egg cell, each carrying half the number of chromosomes found in body cells, unite during fertilization. Therefore, the resulting zygote has a complete set of homologous chromosomes.

After Sutton observed meiosis, he realized that chromosomes behave according to Mendel’s principles of heredity. This fact, along with certain other observations Sutton made, contributed to his belief that chromosomes carry the information of heredity. The major points that supposed Sutton’s hypothesis include the following:

1.Egg and sperm cells provide the only physical link between one generation and the next. For this reason, the hereditary material must be carried in these cells.

2.Hereditary material is probably located in a cell’s nucleus rather than in its cytoplasm. Sutton knew that both parents contribute equally to the genetic makeup of the offspring. With this idea in mind, Sutton noted that sperm cells have far less cytoplasm than egg cells have. However, the nuclei of the two cell types are about the same size.

3.During meiosis, chromosomes tend to behave according to Mendel’s principles. Following Mendel’s principle of segregation, each pair of homologous chromosomes separates independently of one another. Each gamete thus receives only one of the genes of the allelic pair.

Other Genetic Terminology

Since the time of Mendel, the language of genetics has become more precise. As you know, scientists use the term gene instead of factorto describe the unit of heredity. They also use the term allele to refer to either member of a pair of genes that determines a single trait. Alleles can be either dominant or recessive.

The pairs of alleles in the cells of an organism make up its genotype. These pairs of genes are represented with capital and lowercase letters, such as YY, Yy, and yy. A trait that is actually expressed in an organism’s appearance is called a phenotype. Although environment may affect many visible traits, most phenotypes are largely determined by an organism’s genotype.

By 1900, biology was finally prepared to accept Mendel’s findings. Within a single year, his results were rediscovered simultaneously and independently by three scientists working in three different European countries. Each of them had done similar experiments and was searching the scientific literature to seek confirmation of his results.