Understanding Mendelian analysis and heredity.

Holstein cattle are normally black and white.  A superb black and white bull, Nick, was purchased by a farmer for $100 000.  The progeny sired by Nick were all normal in appearance.  However, certain pairs of his progeny, when interbred, produced red and white progeny at a frequency of about 25 percent.  Nick was soon removed from the stud lists of the Holstein breeders.  Explain why, using symbols.