In cattle, RR = red, Rr = roan, and rr = white. What are the predicted color phenotypes for the offspring from crosses between: (a) a red bull and a white cow (b) a red bull and a roan cow (c) a roan bull and a roan cow?  

a) 100% roan

b) 50% red, 50% roan

c) 50% roan, 25% red, 25% white

Explanation:

a) A red bull has the phenotype RR, a white cow has the phenotype rr. Let's do a punnett square. In its gametes, the bull only has R alleles to pass on, and the cow only has r alleles to pass on:

                               Bull

                        R               R

                r      Rr             Rr

Cow    

               r       Rr             Rr

The offspring can only be Rr, so all of them will be roan. (100% roan).

b) A red bull has the phenotype RR, a roan cow has the phenotype Rr. Let's do a punnett square. In its gametes, the bull only has R alleles to pass on, and the cow has an R allele and an r allele to pass on:

                               Bull

                        R               R

               R      RR            RR

Cow    

               r       Rr             Rr

50% of the offspring will be RR, and 50% will be Rr. So half of the offspring will be red, and half will be roan.

c) A roan bull has the phenotype Rr, a roan cow has the phenotype Rr. Let's do a punnett square. In its gametes, the bull has an R allele and an r allele to pass on, and the cow also has an R allele and an r allele to pass on:

                               Bull

                        R               r

               R      RR            Rr

Cow    

               r       Rr             rr

25% of the offspring will be RR, 50% will be Rr, and 25% will be rr. So half of the offspring will be roan, a quarter will be red, (RR), and a quarter will be white (rr)