Short Answer 
The dihybrid cross involves parental genotypes RB, Rb, rB, and rb, with one parent being heterozygous and the other homozygous dominant. The resulting offspring all display the RB genotype, leading to a uniform phenotype of normal running ability and black coat color due to the dominance of these traits.
Step 1: Understand the Parental Genotypes
The parental genotypes involved in this dihybrid cross are RB, Rb, rB, and rb. These represent the different combinations of alleles inherited from each parent. The first parent is heterozygous for both traits, while the second parent is homozygous dominant for running ability and black coat color.
Step 2: Perform the Dihybrid Cross
In a dihybrid cross, we analyze the inheritance of two traits simultaneously. Here, the cross is made between RrBb (heterozygous for both traits) and RRBB (homozygous for both traits). The genotypes resulting from this cross include RBRB, RBRb, RBrB, and RBrb, highlighting the combinations passed down to the offspring from both parents.
Step 3: Determine Offspring Phenotype and Genotype
All resulting offspring from the cross display the RB genotype, leading to a uniform phenotype among them. Each offspring will exhibit traits of being Normal running and Black mice. This shows that the dominant traits for running ability and coat color uniformly manifest in all the offspring due to the genetic makeup of the parents.