In guinea pigs the coat may be rough or smooth. Certain rough-coated guinea pigs when crossed with smooth-coated ones produce all rough- coated offspring. Other rough-coated guinea pigs when crossed with smooth-coated ones produce equal numbers of rough-coated and smooth-coated offspring. Smooth-coated guinea pigs crossed together always produce smooth-coated offspring. Explain these results. Write the genotypes for all animals concerned in the crosses.