From Heraclitean Fire by Erwin Chargaff.
"The great names in the biology of the last hundred years are Darwin, Mendel and Avery.
"Darwin's influence on thought and action was almost instant. He is, in many respects, the Richard Wagner of science; and it is not an accident that a susceptible mind such as Nietzsche's fell victim to both.
"Mendel's fame took a long time to establish itself; but once genetics was recognized as a distinct, though popularly misunderstood, science, it became as rapidly and shamelessly vulgarized as did Darwinism. (...)
"Avery's influence was of an entirely different order. It was exerted only within the biological sciences; his name still widely unknown. Whereas Mendel's successors were able to demonstrate that the heredity rules discovered by him were due to distinct units of inheritance which had physical reality, being localized in the chomosomes, Avery's findings pointed to the chemical nature of those units, the genes. "
"Early in 1944 somebody told me about a paper he had seen in the Journal of Experimental Medicine. This was the celebrated paper by Oswald T Avery, Colin MacLeod and Maclyn McCarty entitled 'Studies on the chemical nature of the substance inducing transformation of Pneumococcal types'.
..."it was clear that the virulent smooth cells contained some principle that could transform, permanently and inheritably, avirulent rough cultures into something resembling the smooth virulent donor organism. Avery and his collaborators set out to isolate, purify, and identify this principle. They succeeded; and these are the words with which they concluded their paper:
'The evidence presented supports the belief that a nucleic acid of the desoxyribose type is the fundamental unit of the transforming principle of Pneumococcus Type III.'
"It is difficult for me to describe the effect that this sentence, and the beautiful experimentation that had given rise to it, had upon me. (...)
"The new finding made it (...) extremely probable that the genes contained, or consisted of, DNA. I believe that few people would now deny that this is one of the most important discoveries in biology."
Oswald Avery 1877-1955
At first I thought Avery might potentially be an exception to commenter dearieme's assertion that there has been a complete lack of American first rate scientists (making the assumption that Chargaff was correct in saying that Avery was indeed a first-rater - a genius - which is of course debateable, although it cannot be denied that Chargaff did have extremely high standards and was not one to toss around praise willy-nilly).
However, Avery was in fact Canadian by birth (http://en.wikipedia.org/wiki/Oswald_Avery), and of English descent (naturally).