Sentence:  While Mendel was able to start his experiments with <WSD>truebred</WSD> tall and <WSD>truebred</WSD> dwarf peas, neither of which had any trace of the other in its past breeding history, his results were naturally more uniform and positive than can possibly be attained with Beagles, large and small... 
Meaning of the word marked with <WSD> tags:  Purebred.