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. 

Figure 10.2.1a – Sweet pea breeding experimentsFigure 10.2.1a – Sweet pea breeding experiments

  • 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.

Figure 10.2.1b – Wild-type female (left) and mutant white-eyed male (right). The white-eyed mutant was the first discovered in Morgan’s lab.Figure 10.2.1b – Wild-type female (left) and mutant white-eyed male (right). The white-eyed mutant was the first discovered in Morgan’s lab.

  • After testing different hypotheses, he predicted that the white-eye phenotype was related to sex determination.

Figure 10.2.1c – Inheritance of sex (and white eyes) in Drosophila melanogasterFigure 10.2.1c – Inheritance of sex (and white eyes) in Drosophila melanogaster

  • 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.

Figure 10.2.1d – Some Drosophila mutant phenotypesFigure 10.2.1d – Some Drosophila mutant phenotypes

  • 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.

Figure 10.2.1e – Drosophila chromosome mapFigure 10.2.1e – Drosophila chromosome map

  • On the gene map shown, the distance between two loci on a chromosome is determined by the percentage variation between the expected Mendelian ratio and the observed experimental ratio of phenotypes.  

Figure 10.2.1f – Bateson and SaundersFigure 10.2.1f – Bateson and Saunders
William Bateson (1861–1926) and Edith Rebecca Saunders (1865–1945) looked for Mendelian ratios in pea plants as well as other organisms.

Figure 10.2.1g – DrosophilaFigure 10.2.1g – Drosophila
Drosophila melanogasterwild-type female and male.

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 MorganFigure 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.

TOK

Have you ever omitted data from an experiment that you performed? What criteria did you use to determine if the data was anomalous?

Further reading

Kohler, Robert (1994) Lords of the Fly: Drosophila genetics and the experimental life. Chicago: University of Chicago Press.