Law of Independent Assortment
The inheritance of one trait doesn't influence the inheritance of another trait.
The inheritance of coat color and tail length for a cat do not influence the outcome of one another. A cat with a brown coat is equally likely to inherit a long tail or a short tail.
A good way to visualize the independent inheritance of separate traits is a dihybrid cross punnett square.
This punnett square uses BB Bb and bb to represent different alleles for coat color. SS Ss and ss are used to represent different alleles for tail length.