10.2 Applications and skills
10.2.1 Nature of Science: The discovery of gene linkage
The discovery of gene linkage sheds light on a number of themes in the nature of science:
- Careful observation of trends and discrepancies – laws are descriptive, but theories explain phenomena. The theory of gene linkage followed the discovery of non-Mendelian ratios.
- Collaboration and prior knowledge – Mendel’s work was published in the German language in 1866, but many scientists were unaware of its significance until much later.
- Experimental models and choice of experimental organisms – it was very easy for Mendel to find phenotypic variation in pea plants. Drosophila is a good model organism because of short reproductive cycles (about 25 generations can be observed in one year).
Bateson and Punnett discover anomolies
- At the turn of the 20th century in England, William Bateson and Edith Saunders were performing large-scale breeding experiments on sweet pea plants.
- The team was unable to find any patterns in inheritance until Bateson learned of Mendel’s work around 1902.
- Bateson recruited Reginald Punnett to his lab, and together they began searching for Mendelian inheritance patterns.
- After thousands of breeding experiments, the team uncovered many discrepanices from the Mendelian ratios.
- In general, there were many more parental phenotypes and many fewer recombinants (non-parental phenotypes) than predicted by Mendel’s law of independent assortment.
- They concluded that there might be some form of ‘coupling’ between genes.
Non-Mendelian ratios and Drosophila genetics
- Thomas Hunt Morgan, an American geneticist, is credited with demonstrating that genes are physical entities located on chromosomes. Until he published in 1910, the chromosome theory of inheritance was not widely accepted.
- Morgan’s work focused on mutations in the common fruitfly, Drosophila melanogaster. After thousands of crosses between wild-type flies, Morgan first observed a white-eyed mutant phenotype, which was more common in male flies than in female flies.
- After testing different hypotheses, he predicted that the white-eye phenotype was related to sex determination.
- He performed crosses to determine whether the ratio of white to red eyes followed the pattern of the inheritance of sex, and concluded that the mutant eye colour gene was sex-linked.
- The lab subsequently produced populations of true-breeding flies with different mutant phenotypes, and was able to associate each mutant phenotype to a locus on one of each of the four Drosophila chromosomes.
Nature of Science
- Making careful observations: Morgan developed the notion of linked genes in order to explain anomalous data that Mendelian ratios could not account for.
- Looking for patterns and discrepancies: both Bateson and Morgan began with the null hypothesis that genes assort independently. Scientific discovery advances by asking new questions based on discrepancies and exceptions to the rules.
Figure 10.2.1h – Thomas Hunt Morgan
Thomas Hunt Morgan (1866–1945) initially predicted that white eyes were fatal in females. His research did not support this hypothesis.
Have you ever omitted data from an experiment that you performed? What criteria did you use to determine if the data was anomalous?
Kohler, Robert (1994) Lords of the Fly: Drosophila genetics and the experimental life. Chicago: University of Chicago Press.