Mendel and the re-discoverers of genetics Correns, de Vries and Tschermak identified discrete traits whose patterns of inheritance followed interpretable rules.  But most traits found in animals and plants were continuous, not discrete, and tended to mix in offspring - when you cross a Labrador and a poodle (two breeds of dogs), you get a labradoodle, not Labradors that give a 3:1 ratio of Labradors and poodles in the next generation. Explain the concept of a gene resulting from the work of Mendel, and use it to explain the appearance of labradoodles. Keep in mind not all dogs are Labradors or poodles.