Thomas Hunt Morgan, using Drosophila
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Thomas Hunt Morgan, using Drosophila
Thomas Hunt Morgan did the experiments on Drosophila following the chromosomal theory of inheritance, which had long been introduced in Mendel’s general laws. Before the experiment, no research had successfully shown direct evidence indicating that traits were carried on chromosomes. Morgan chose to work with fruit flies because they were easily bred and were readily available and harmless. The flies also had a short generation time, and the four pairs of chromosomes could be easily studied under a microscope.
Morgan used red and white-eyed fruit flies in his study. He mutated the flies with white eyes and then breaded them with red-eyed flies. The Drosophila had a pair of autosomes and also another couple of sex chromosomes. By the time Morgan was aware of the X and the Y chromosomes and associated with gender. In the flies, the red-eyed (XW) was dominant to the white-eyed (Xw) flies (Morgan, 2017). The study was based on the assumption that the gene for eye color was on the X chromosome. The trait was, therefore, identified as the X-link became the first to be identified. The flies were divided into generations known as offspring. Morgan took a male white-eyed fly and bred with several red-eyed females. The offspring for the first experiment was named F1. He then took the offspring of F1 and bred amongst themselves. The resulting family was named F2.
The experiment’s result is that the mating between the white-eyed male and the red-eyed females produced all red-eyed female offspring and white-eyed male offspring. The coupling between the members of F1 produced F2 flies, which consisted of a mixture of red-eyed flies and white-eyed flies. The direct evidence of linkage can be seen as chromosome inheritance is seen in the second generation. Morgan also bred the F1 red-eyed females with the original white-eyed mutant, to establish the truth of Mendel’s patterns. The male to female ratio for the experiment was 1:3, which was similar to Mendel’s abstract (Morgan, 2017). Mendel’s ratio is only significant for modern use and not in the traditional sense. With continued interbreeding, several other variations resulted. Morgan concluded that the X chromosome had discrete hereditary units. He also adopted another term, gene, which he borrowed from Wilhelm Johannsen Botany.
The three hypotheses of his experiments were that sex-limited characters formed part X-chromosomes of females. Traits that correlate with specific factors on the X chromosome are called X-linked. Males display all the X-linked traits since they only have one X chromosome, and the other is Y (Gleason, 2017). For females to exhibit a trait, they must have the X-linked trait on both X chromosomes. The second hypothesis was that, during the breeding experiment, generation one of all the flies consisted of males only having white eyes since the gene which controls eye color was on the X chromosome, the only X chromosome in males. The third hypothesis was that the females in the first generation did not exhibit the white eye because the trait was only present in one of the X chromosomes. Morgan mated some specific groups of flies to test his hypothesis. If there were a correlation between the predicted results and the actual results, then his hypothesis would have been supported. This would be the first experiment to show direct evidence of linkage.
References
Gleason, K. (2017). Sex Limited Inheritance in Drosophila (1910), by Thomas Hunt Morgan”. Embryo Project Encyclopedia (2017-05-22). ISSN: 1940-5030 http://embryo.asu.edu/handle/10776/11509.
Morgan, T. H. (2017). Sex Limited Inheritance in Drosophila. Science (1910): 120–2. http://www.jstor.org/stable/pdf/1635471.pdf.