Lifesci 4 - lecture 2 - mendelian genetics and pedigrees. Product rule : the probability of both events occurring is the product of their individual probabilities the two events must be independent to each other p(a and b) = p(a) x p(b) Sum rule: when a and b are independent events (which can co-occur): probability of a and b occuring = p(a) + p(b) minus shared area [p(a) and. P(b)] = p(a) + p(b) - [p(a) x p(b): when a and b are mutually exclusive events (if a happens, b cannot happen): Probability of a or b occuring = p[a+b] = p(a) + p(b) Sum rule: p(rryy or rryy) = 1/16 + 1/8 = 3/16. Probability of yy = p(rryy) = ( )( ) = 1/16. Probability of yy = p(rryy) = ( )( ) = . Note that one genotype must come from each parent. Since one parent is completely recessive, f1 round, yellow must be heterozygous.