Thomas Morgan

Born: September 25, 1866; Lexington, Kentucky
Died: December 4, 1945; Pasadena, California
Fields of study: 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: XR and Xr. 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 (XrY) results in all first filial (F1) generation flies, XRXr females and XRY males, having red eyes. Matings between the F1 flies demonstrated that the red-eyeinducing 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. Anormal 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. Afemale 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 XrL 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 XRLchromosome. 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 (XrlY). 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.