If someone develops a mutation for purple ears, we’d expect about one-quarter of their grandchildren to have ears just as purple as granddad’s, but early thinkers might have thought that all the grandchildren would tend to have ears that were only 25% purplish. Is this the difference between “discrete” and “continuous” heredity?
In another thread a Doper comments
This came as a surprise to me, and I asked for a cite. I realize Darwin’s views evolved considerably between the first and last editions of Origin of Species, but it was others who pointed out flaws in “continuous” heredity, with Darwin then almost retreating into Lamarckism!
Can anyone post a quote from Darwin demonstrating an understanding of “discrete heredity”? I realize any such quote might be ambiguous and then the thread should move to IMHO, but GQ seems appropriate to first find the quote.
… In Mendel’s copy of Origins, [Mendel] did make occasional marks and margin notations. Mendel marked one passage where Darwin discusses the uniformity of hybrids in the F1 generation and the variability of their F2 offspring. Darwin’s explanation for this was that there was some alteration in the reproductive system, some mutational effect. This explanation differs substantially from Mendel’s explanation of independent assortment of independent traits or alleles. Also, Mendel directly contradicted Darwin’s claim in Origin that changing conditions of life were the cause of variation in domesticated species.
As I recall, Darwin never knew anything about discrete genetics - his default position was continuous genetics (blending inheritance), but he recognized the difficulties that that presented (and considered those difficulties one of the most serious challenges to the idea of natural selection). Stephen Jay Gould looks at this issue in an article about Fleeming Jenkin (Fleeming Jenkin - Wikipedia) who was one of the people who identified the problems with blending inheritance -
Thank you, Isilder and Andy L. It’s reassuring to see my surprise validated.
I’m no biologist and, although I love browsing Darwin’s beautiful writing, I’ve never read Origin cover-to-cover. (My backed-up must-read list is getting longer and longer and, anyway, it’s less fun reading stories when one already knows the ending. )
Of course some genetics is much closer to continuous. There are numerous genes that affect height, skin color, etc. so that children often are “mixtures”.
The Wiki article about Fleeming Jenkin was indeed interesting. However, whoever wrote was serious mistaken about it taking 100 years for Mendel’s work to be recognized. According to the Wiki on Mendel, it was more like 30. At any rate, when I took biology in HS in 1952 Mendelian genetics was already considered commonplace. And we talked freely about dominant and recessive traits, terms coined by Mendel (at least in German).
Yeah, I noticed that, and should have commented on it. I have no idea how they could have come up with 100 years. Heinlein was talking about Mendelian genetics in his 1942 “Beyond this Horizon” and this Google Ngram shows that Mendel and Mendelian came to common use in 1920 or so Google Ngram Viewer: Mendelian,Mendel
Mendelian genetic principles, and Mendel’s own work, were rediscovered around 1900. Key scientists involved in this were Hugo de Vries, in the Netherlands, William Bateson, in Britain, and Carl Correns in Germany. De Vries seems to have been the first to rediscover the basic Mendelian principles of discrete inheritance, in the 1890s, originally quite independently of any knowledge of Mendel, and because of this, he was always rather reluctant to acknowledge Mendel’s priority. Correns and others also rediscovered the principles shortly after, and seem to have been more aware of, perhaps even guided by, Mendel’s work. However, it was Bateson, building upon de Vries’ work, who probably did most to establish the new genetics on a solid foundation, establish its relevance to Darwinain evolutionary theory, and to popularize the idea that Mendel deserved the primary credit for founding the science.
Of course, Darwin was long dead before any of these events.
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