Morgan Used the Fruit Fly, Drosophila, to Study Genetics

In the early 20th century, the fruit fly,Drosophila melanogaster, achieved fame as the model organism for genetic analysis. Fruit flies are small, easy to feed, and yield a new generation in a few days—a big time saving advantage compared with flowering plants. Larger numbers of individuals can be examined and,most important, many generations can be observed in a single year of experimentation.

Mendel did not actually discover linkage, as the characters he worked with were mostly on different chromosomes. Two were actually on the same chromosome but far enough apart for their linkage to go unnoticed. He was lucky in being able to lay the foundations of genetics without the complications that linkage introduces. T. H. Morgan pioneered work on Drosophila, beginning in 1909. He was largely responsible for discovering and analyzing the phenomena of linkage, sex-determination and sex-linked genes that have been described above.

the total human world population. After World War II, the use of bacteria and their viruses took genetic analysis down to the level of the DNA molecule and allowed the mapping of different mutations within the same gene. Bacteria are not merely ideal for high-powered genetic analysis; they are also convenient for biochemical investigations. It was at this point that Escherichia coli (or, commonly, E. coli), a bacterium found as a harmless inhabitant of the large intestine of man and other animals, came to the forefront of genetic research.

Bacterial genetics gave rise to a standard terminology for naming genes. Consider the biochemical pathway for synthesis of the amino acid threonine (Fig. 1.24). This pathway consists of three steps, catalyzed by three enzymes that are encoded by three genes, thrA, thrB, and thrC.

Note that related genes are all designated by a three-letter abbreviation, which, hopefully, indicates their function. Each separate gene of such a related group is additionally followed by a capital letter of the alphabet. The gene designation is printed in italics, or if written by hand it may be underlined. The wild-type allele is indicated with a "+" sign; e.g., thrA+. A defective allele may have a "-" sign; e.g. thrA~. Different mutations in the same gene receive allele numbers, for example, thrB1, thrB2, thrB57, etc. [This convention does not apply to eukaryotes, in part because the number of genes is much larger and their nomenclature has never been properly standardized. Nonetheless, eukaryotic genes are generally italicized.]

The bacterium E. coli has approximately 4,000 genes (about one tenth as many as a human) arranged within a single circular chromosome (Fig. 1.25). The E. coli chro-

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