Thomas Hunt Morgan

Born: September 25, 1866; Lexington, Kentucky Died: December 4, 1945; Pasadena, California Fields of study: Genetics

Contribution: Morgan's studies popularized the use of the fruit fly Drosophila for the study of animal genetics. He is credited with discovering the first sex-linked trait in Drosophila and with demonstrating how new characteristics could be passed on to successive generations. Morgan and his students showed that chromosomes exchanged genes, a process known as crossing over. In 1933, Morgan was awarded the Nobel Prize for Physiology or Medicine for his work on Drosophila genetics. In his classic paper of 1910, Thomas Hunt Morgan described very rare white-eyed flies that appeared spontaneously in a red-eyed population. Because these mutants were always males, Morgan suspected that the gene controlling eye color was linked to the X chromosome that influenced the development of sex. The observation that white-eyed females could be produced from certain matings, however, indicated that the white-eyed trait was not limited to males.

Morgan's experiments demonstrated for the first time that a gene controlling eye color was linked (or limited) to the X chromosome. Red and white eyes are caused by different alleles of the same gene. These alleles on the X chromosome are represented in the following manner: X and X. Any gene linked to the X chromosome is called a sex-linked gene. In Drosophila, two X chromosomes generally result in a female fly, whereas one X chromosome results in a male. Morgan found that the R allele inducing red eyes is dominant over the r allele that allows white eyes to develop when it is the only allele a fly has.

The Y chromosome that pairs with the X chromosome in males (XY) lacks sex-linked genes. Thus, mating red-eyed females (XRXR) to white-eyed males (XY) results in all first filial (F1) generation flies, XX females and XRY males, having red eyes. Matings between the F1 flies demonstrated that the red-eye-

inducing allele and the white-eye-promoting allele always remained associated with the X chromosome. This suggested that the alleles were linked to the X chromosome.

In 1913, A. H. Sturtevant, working in Morgan's laboratory, reported on mutations linked together on a fruit fly's X chromosome. Sturtevant demonstrated that recombination between two X chromosomes could separate genes controlling different traits. In addition to using the alleles that determined eye color, Sturtevant used alleles that influenced wing formation. A normal wing forms under the influence of the L gene, but a miniature wing is associated with the l allele of the L gene. Genes linked on the X chromosome may be shown as follows: XRL. Sturtevant observed recombination when he characterized the offspring from certain crosses. A female fly with red eyes and normal wings, XRlXrL, usually produces two types of eggs. One type of egg has the XRl chromosome, whereas the other type of egg has the XL chromosome. Very infrequently, when there is a crossover between the X chromosomes, rare eggs are produced with recombinant X chromosomes, one type of egg has the Xrl chromosome, whereas the other type of egg has the XRL chromosome. When these recombinant eggs fuse with a sperm carrying only a Y chromosome, recombinant male flies result, those that have normal eyes and normal wings (XRLY) and those that have white eyes and miniature wings (XlY). By using various mutant flies, Morgan and Sturtevant discovered that they could both order a number of different genes on the X chromosome and determine how far they were from each other. The farther a gene is from another gene, the greater the number of recombinant offspring. The pattern of offspring was used to determine the sequence of genes on the X chromosome. Finding flies with mutations in different genes was essential for determining the sequence of genes and the distances between them.

—Jaime Stanley Colome

Morgan Drosophila

Thomas Hunt Morgan popularized the use of the fruit fly Drosophila melanogaster for the study of genetic mutations. (Library of Congress)

Homeotic genes are found in all multicellular organisms. Homeotic genes similar to those found in Drosophila control the development of segments most visibly exemplified by the vertebrae and the bones in animals' appendages. Mutations in homeotic genes or their controlling sites affect the development of segments. Segments can be eliminated or modified by homeotic gene controlling site mutations.

One well-studied homeotic gene in Drosophila is the gene antennapedia, antp. Certain mutations in the controlling sites for the antennapedia gene result in legs developing rather than head antennae. Another homeotic gene is ultrabithorax, ubx.

Some mutations in the controlling sites for ultrabithorax gene result in a second pair of wings developing where the pair of halteres normally develop. Halteres are tiny, winglike appendages that all flies have, which promote stable flight. Other mutations in the controlling sites for ubx produce a second pair of winglike structures that are half haltere (anterior portion), half wing (posterior portion). By studying mutations and the altered traits, scientists have discovered that controlling site mutations change when and where proteins are synthesized. For example, if a protein is to be produced in seven segments along the anterior-posterior axis of an animal, there must be at least seven different controlling sites that can respond to the different activators and repressors produced in each segment.

Numerous studies suggest that antennapedia and ultrabithorax are transcriptional repressor-activators that not only repress the development of legs and wings, but also stimulate the development of antennae and halteres, respectively. The study of Drosophila mutants is beginning to clarify how antennae and mouth parts evolved from leglike appendages and how halteres evolved from wings. The study of genes and controlling sites has led to the understanding of their role in the maintenance, development, and evolution of every organism.

—Jaime Stanley Colomé See also: Asexual reproduction; Breeding programs; Cleavage, gastrulation, and neurulation; Cloning of extinct or endangered species; Copulation; Courtship; Determination and differentiation; Development: Evolutionary perspective; Estrus; Fertilization; Gametogenesis; Hermaphrodites; Hydrostatic skeletons; Mating; Parthenogenesis; Pregnancy and prenatal development; Reproduction; Reproductive strategies; Reproductive system of female mammals; Reproductive system of male mammals; Sexual development.

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