Thursday 7 July 2016

It's the Y of course

If you do genetics in college, possibly if you do it in high school, you get to hear about Thomas Hunt Morgan and his research on Drosophila melanogaster, the fruit fly.  I've written about his use of milk bottles as convenient 'free' incubators for a sample of flies. In passing, as a bit of a jape, I noted his uncanny resemblance to DH "Mellors" Lawrence. And just to put things in a time-frame, note that this Morgan was only one generation separated from his famous uncle: Civil War General John Hunt "Beardy" Morgan. Genetics students in the old days were also taught that Morgan didn't work alone: he was supported by three brilliant students who did all the work. I guess this was to inspire us to work longer hours, and think deeper thoughts on behalf of our mentors and teachers.  The students had sort of clichĂ© personas so that we could get a sound-byte fix on who they were and what they achieved.

  • Calvin Bridges was the golden boy, strikingly good looking, who pioneered the use of polytene chromosomes to help map genes. He died young of  "died of heart failure due to complications from a heart valve infection" . . . ahem, read that as syphilis: he parlayed his good looks into a parallel career as a serial womaniser.
  • AH Sturtevant was the steady one, who produced the first evidence that some gene variants were inherited together. With Bridges, he followed Morgan from Columbia to Cal Tech in 1928.
  • Hermann Muller, was Cinderella to these two favored students. He was the ideas man who got serially hacked off when his contributions weren't acknowledged in publications. There was a hint of anti-semitism in his treatment by Morgan but we were encouraged to demonise Muller because a) he emigrated to Germany and then the USSR in the 1930s and b) was a advocate of eugenics.
Is that all? Did Morgan have other students? Well, yes, he did; but we students in the 1970s never heard about them. Notably Nettie Stevens was not mentioned.  It is not impossible to believe that, as Morgan was a bit leery about Muller because of his Jewish mother, he also didn't rate Dr Stevens because she had two X chromosomes. That metaphor for 'female' is particularly appropriate because Nettie Stevens was the first person to note that males and females had a different karyotype (complement of chromosomes).  She was working at the time in Morgan's Lab in Bryn Mawr before Morgan was head-hunted by Columbia.

She wasn't looking at the human karyotype: we didn't even get the correct number of our chromosomes until Tjio and Levan counted them in 1955!  Stevens was concentrating her attention on Tenebrio molitor the common mealworm beetle. She noticed that female beetles had 20 large chromosomes while males had 19 and a speck. Doubtless she didn't believe her eyes or her counts until she'd done it again and again to be sure to be sure. It is very difficult now to appreciate how earth-shaking this ultimately correct idea was at the time. Sex-determination was treated as a philosophical, as much as a biological problem and despite her compelling evidence, Morgan and others clung to a role for external forces in the process for several subsequent years. Also reflect that Mendel's theory of particulate inheritance had only been [re-]discovered in 1901, 4 years before Stevens' seminal paper. Her microscope [L] is in a glass case in Bryn Mawr, partly to show how poorly designed it is ergonomically and how hard it was for Stevens to be comfortable while peering. Morgan went on to get the 1933 Nobel Prize in Physiology and Medicine for his work on chromosomes. He shared the cash with Bridges and Sturtevant.  Even if he'd had a mind to, he couldn't have included Nettie Stevens in the bonanza because she was dead - 1912, breast cancer aged 50.  She was late to the academic scene and did her best work in her 40s, having been born on this day, 7th May 1861, 155 year ago.

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