A Brief History of Genetics

Adapted by John Christmas

Northern Virginia Community College

Early Concepts of Heredity

Ancient Greeks – believed in “pangenesis”, in which miniature versions of body parts were transmitted during sexual reproduction;

Darwin, despite his evolutionary theories believed in a modifiesd sort of pangenesis, via “gemmules”

Gregor Mendel–studied inheritance of seven traits in pea plants and first used the term dominance and recessiveness;

Laws of independent assortment and segregation based on his work.

Proposed similar but separate inheritable characters, one from each parent, later to be called genes.

Mendel’s Contribution

Mendel examined complete dominance; phenotypes of heterozygote Aa and dominant homozygote Aa are indistinguishable.Law of independent assortment – independent assortment of alleles in metaphase during gamete formation;Law of segregation – the separation of alleles into separate gametes.

Law of Independent Assortment

Law of Segregation

Later Concepts1900 - Not until 34 years after its publication did Mendel’s work receive additional attention, with publications in 1900 by three German Botanists: Hugo de Vries, Carl Correns, and Erich von Tsernak; 1902 - Walter Sutton first integrated the concepts of chromosomes with Mendel’s laws, in studies of grasshopper reproduction and cell division and concluded that Mendel’s heritable factors must be on the chromosomes.1906 - William Bateson coined the term genetics, noting that “a new and well-developed branch of physiology has been formed.” Gene came into usage in 1909.1907 – T.H. Morgan began his work with fruit flies, ultimately mapping gene locations.

Wilhelm Johannsen1909 - The term “gene” first used by Wilhelm Johannsen, not knowing quite what it was, just wanting a distinction from Darwin’s “gemmules”.Johannsen noted that:

“The word gene…is completely free from any hypotheses, it expresses only the evident fact that in any case, many characteristics of the organism are specified in the gamete by means of special conditions and conditions and determine which are presented in unique separate, and thereby independent ways—in short precisely what we wish to call genes.”

T. H. Morgan’s Fruit Flies 1907-1930s

Frederick Griffith, 1928Transformation of Bacteria

Still Later Concepts

By 1930s, DNA shown to be a long molecule with four different bases, ACT and G. However, as late as 1933 that there was no consensus amongst geneticists as to what the genes are.

George Beadle and Edward Tatum 1941 (Nobel Prize 1958)

“What is Life” Erwin Schrodinger 1946

Argued for “a hereditary code-script” embedded in the molecular fabric of chromosomes and that understanding life would mean understanding the gene…that life could be thought of as storing and passing on biological information.” His work had a marked influence on the thinking of both James Watson and Francis Crick.

Rosalind Franklin, 1953Headed a X-Ray crystallography unit at King’s College in London;

Did extensive work to elucidate structure of DNA;

Franklin resisted model building unlike Pauling and Watson and Crick.

Linus Pauling, 1953

Prominent American biochemist who wrote ”The Nature of the Chemical Bond”;Elucidated the structure of amino acids and proteins, and associated helical structures;Proposed a 3-chain helical structure for DNA, but with sugar-phosphate groups on the inside;Evelynn Fox Keller (2000) notes that had Pauling used all available literature, his model might have been correct; Watson and Crick are sure that he was on to it.

James Watson and Francis Crick, 1953

Collaborated at Cambridge, England.

Before beginning their work, it was known that DNA consisted of multiple copies of nucleotides, A,C,T,G

Utilized R. Franklin’s work extensively

The Double Helix, James D. Watson, 1980

My doodling of the bases on paper at first got nowhere.…not until the middle of next week however did a nontrivial image emerge. Suddenly I realized the potentially profound implications of a DNA structure in which the adenine residue formed hydrogen bonds similar to those found in crystals of adenine….I thus started wondering whether each DNA molecule consisted of two chains with identical base sequences held together by hydrogen bonds [although because of the different shapes of purines and pyrimidines the backbone would have to to show minor in-an-out buckling depending on whether pairs of purines or pyrimidines were in the center].

James Watson“The Double Helix”, 1980

“Despite the messy backbone my back pulse began to race. If this was DNA, I should create a bombshell by announcing its discovery. The existence of two intertwined chains with identical base sequences could not be a chance matter. Instead, it would strongly suggest that one chain in each molecule had at some earlier stage served as the template for the synthesis of the other chain. Under this scheme, gene replication starts with the the separation of its two identical chains. Then two new daughter strands are made on the two parental templates.”

James Watson’s Journals

After deducing on February 28, 1953 that the proper structure required a pairing of pyrimidines and purines, not similar bases, Watson noted in his journal that:

“For two weeks [following the initial pairing scheme] I got nowhere misled by an error in my nucleic acid textbook”

DNA Replication

Alfred Hershey and Martha Chase, 1952

Matthew Meselson and Edward Stahl, 1957