Let's look at an example of inheriting TWO genes... A pea plant is heterozygous for both seed shape and pod color. R is the allele for the dominant, round seed characteristic; r is the allele for the recessive, wrinkled seed characteristic. G is the allele for the dominant, green pod characteristic; g is the allele for the recessive, yellow pod characteristic. The full heterozygous genotype for the two genes of this pea plant is RrGg. If this plant self-fertilizes, what is the probability of the offspring being both round and yellow?
Method 1: do a separate monohybrid cross for each gene and calculate combined probability for inheriting both traits:
Genotype for seed shape gene:
Probability of round offspring:
Genotype for pod color gene:
Probability of yellow offspring:
The Product Rule: the probability of two separate events occurring together can be calculated by multiplying the two individual probabilities of each event occurring alone. Use a decimal for an easy calculation! example: 0.5 x 0.5 = 0.25
Probability of round offspring:
Probability of yellow offspring:
Probability of round and yellow offspring:
Method 2: do a dihybrid cross:
foil method: out first RrGg in last
Read This! In some situations, it’s actually easier to use one dihybrid cross, instead of two monohybrid crosses. When we use a dihybrid cross to calculate the probability of inheriting two separate genes, we will use gametes that now have two genes inside. Doubling the genes will double the possibilities of genes in gametes! This DOUBLES the size of the Punnett Square from 2x2 to 4x4! We can find out the gametes using the FOIL method.
STEP 1: Record the gametes from the parents in the top boxes and left side boxes (in yellow). Since this plant is self-fertilizing, it is both the ’mom’ and ’dad’. RrGg
STEP 2: Fertilization! Do the cross. Keep the two alleles for one gene together. example: SY + sy = SsYy
STEP 3: Interpret the data to determine the genotypes and phenotypes. All genotypes of round and yellow offspring:
Probability of round and yellow offspring: (add up all possibilities / 16)
Check your answer: Did you get the same probability as method 1???