1. Probability
Probability is defined as the likelihood of an event occurring. The practice of genetics uses the concept of probability to predict the outcome of phenotypic expression based on the alleles present

2. Punnett Square
A punnett square can be used to predict the allelic frequencies when true breeding parents are crossed.
draw a square having twice the number of side boxes as number of alleles.
segregate the alleles on the side of the boxes (if more than one trait you must segregate as groups)
recombine the alleles in the boxes
count the ratios
genotype ratio for the F1 cross is 1:2:2:1
phenotype ratio is 3:1 with the dominant trait (purple) expressed in the hybrids.

3. dihybrid crosses
For dihybrid crosses the results are still predictable and are as follows:
R - round, r – wrinkled
Y - yellow, y – green
Ratios:
Phenotypic ratio is 9:3:3:1

4. Multiplication- addition rule:
You can use the allelic frequencies to predict the outcome of a cross.
Each trait is calculated by its own punnett square to create the probability
Given the Genotypes RrYy & rrYy, what is the frequency of offspring exhibiting both recessive traits?
rr = 1/2
yy = 1/4
1/2 x 1/4 = 1/8
Chance for both dominant?
Rr = 1/2
YY = 1/4
Yy = 1/2
1/2 x (1/4 + 2/4) = 3/8