Educators usually teach the Mendelian inheritance model using Punnett squares to determine the probability of an offspring having a particular genotype and phenotype. To find the probability of an outcome of a particular cross, students need to understand the underlying biological concepts of these visual representations. However, this approach becomes more complex for cases with three or more characters and shies away from the authentic integration of mathematical and biological concepts. Therefore it is crucial for students to use mathematical algorithms that Mendel used to understand and solve inheritance problems. In this paper, we propose relating two simple probability rules to the laws of inheritance and using a probability tree diagram to predict the combined frequency of traits in the offspring of crosses. We validate the proposed probability rules for various examples.