| ppyyRr | 1/4 (pp) x 1/2 (yy) x 1/2 (Rr) | = | 1/16 |
| ppYyrr | 1/4 x 1/2 x 1/2 | = | 1/16 |
| Ppyyrr | 1/2 x 1/2 x 1/2 | = 1/8 = | 2/16 |
| PPyyrr | 1/4 x 1/2 x 1/2 | = | 1/16 |
| ppyyrr | 1/4 x 1/2 x 1/2 | = | 1/16 |
| PpYyRr | (ignored) | ||
| Chance of at least two recessive traits | = | 6/16 (3/8) | |
Sum law: in a multihybrid cross, the probability of obtaining all combinations of alleles, such as two recessive traits, is the sum of the probabilities of each individual combination.
The probability for each individual trait can be predicted from a monohybrid cross.
So, the probability of at least two recessive traits in this trihybrid cross is the sum of the probabilities of the 5 possible combinations.