3.3 Meiosis, 10.1 Gene Linkage and 10.2 Inheritance

Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells. (3.3 – Core)

Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies.  (10.1 – AHL)

Looking for patterns, trends and discrepancies—Mendel used observations of the natural world to find and explain patterns and trends. Since then, scientists have looked for discrepancies and asked questions based on further observations to show exceptions to the rules. For example, Morgan discovered non-Mendelian ratios in his experiments with Drosophila. (10.2 – AHL)

Both core and HL topics on meiosis focus on the importance of making observations and accurately interpreting them.  The HL NOS, with its references to Morgan, illustrates how this process helps expand and redefine our understanding of biology. Morgan is an interesting character and worth spending some class time on.

When Mendel’s genetic studies were rediscovered, scientists set about replicating and confirming them. However, one group (William Bateson, Edith Rebecca Saunders, and Reginald Punnett, he of the eponymous square) persistently found phenotypic ratios that were far more common than could be predicted based on Mendelian inheritance patterns. Examining pea plants during a dihybrid cross (link to AHL Topic 10.2), they received statistically significant deviations from the predicted, as seen below:

Characteristics of the F2 Generation (Bateson et al., 1905)
Characteristics of the F2 Generation (Bateson et al., 1905)

They surmised that the alleles must be coupled somehow, but could not explain how.  Enter Professor Thomas Hunt Morgan.  Five years later, 1910, he was experimenting on fruit flies, Drosophila melanogaster, and found a white-eyed male in one of his studies (fruit flies normally have red eyes). Further experimentation found that instead of an expected 1:1:1:1 ratio of red-eyed females, red-eyed males, white-eyed males, and white-eyed females, he observed the following phenotypes in his F2 generation: 2,459 red-eyed females, 1,011 red-eyed males and 782 white-eyed males. The lack of white-eyed females led him to hypothesise that the gene must be linked to the sex factor.  This later led to the concept of gene linkage: genes on the same chromosome do not assort independently.

Columbia University Fly Room - note bunches of bananas! © 2013 The American Philosophical Society
Columbia University Fly Room – note bunches of bananas!
© 2013 The American Philosophical Society

This was groundbreaking – it meant that genes were concrete, real objects that could be located on chromosomes  and their inheritance and behaviour mapped, predicted and analysed. Interestingly enough, Morgan had initially rejected the idea that genes were located on chromosomes, believing that data generated through passive observation could not be trusted; another instance of scientists having to reject old ideas in favour of new and compelling evidence. Morgan would go on to win the Nobel Prize in Physiology or Medicine  “for his discoveries concerning the role played by the chromosome in heredity” (Nobelprize.org; 2014).

We will discussing a lot more about Prof. Morgan in our next unit, Inheritance.

Extra: One of the prescribed TOK Essay Titles for May 2015 was: “There are only two ways in which humankind can produce knowledge: through passive observation or through active experiment.” To what extent do you agree with this statement?  You could consider drafting a short response using Morgan as a specific example.

References:

Lobo, I. & Shaw, K. (2008) Discovery and types of genetic linkage. Nature Education 1(1):139. Web. 17 Mar 2015.

Miko, I. (2008) Thomas Hunt Morgan and sex linkage. Nature Education 1(1):143. Web. 17 Mar 2015.

“The Nobel Prize in Physiology or Medicine 1933”. Nobelprize.org. Nobel Media AB 2014. Web. 17 Mar 2015.

Images:

Bateson, W., et al. (1905) Characteristics of the F2 Generation. Reports to the Evolution Committee of the Royal Society. Nature Education. Web. 17 Mar 2015.

n.a. (1913). Columbia University Fly Room. The American Philosophical SocietyNature Education. Web. 17 Mar 2015.

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