When Gregor Mendel published his theory of inheritance in 1865, it should have started a revolution. Charles Darwin had published The Origin ofsix years earlier. Its major omission was any kind of explanation for the mechanism of heredity, which Mendel's work supplied. However, it would be another 35 years before his theory would be rediscovered and then accepted. What changed?
Sir Paul Nurse: Thebiologists had been looking at cells and discovered chromosomes in between 1865 and 1900. This is a picture from that period of dividing cells in a plant, in an onion root. And what you can see there is that when the cell's divided, they could see strings that we now know to be chromosomes. And scientists like Wiseman began to speculate that maybe the hereditary material might be contained upon these chromosomes.
And then a very interesting observation was made by somebody called van Beneden, working with an obscure little worm called Ascaris. And what he observed, was that this worm had only 4 chromosomes, which made them very easy to count. And, crucially, when the worm made gametes during its reproduction, those four chromosomes were reduced to two. And because he could count them easily, because there were only four, he clearly documented this. Now the reduction from four to two was exactly what Gregor Mendel had predicted if you recall, for his characteristics that were reduced from two down to one. And I think it was this concrete realisation of the abstractions from Mendel that perhaps made it more acceptable to believe the data in 1900 rather than 1865.