model {
NAA[1:3] ~ dmulti(XAA[1:3], KAA)
NAB[1:3] ~ dmulti(XAB[1:3], KAB)
NBB[1:3] ~ dmulti(XBB[1:3], KBB)
XAA[1] <- (1 - sigma)*p + sigma
XAA[2] <- (1 - sigma)*q
XAA[3] <- 0
XAB[1] <- (1 - sigma)*p/2 + sigma/4
XAB[2] <- 0.5
XAB[3] <- (1 - sigma)*q/2 + sigma/4
XBB[1] <- 0
XBB[2] <- (1 - sigma)*p
XBB[3] <- (1 - sigma)*q + sigma
KAA <- sum(NAA[])
KAB <- sum(NAB[])
KBB <- sum(NBB[])
p ~ dunif(0, 1)
sigma ~ dunif(0, 1)
q <- 1 - p
}

Data:
list(NAA = c(427, 95, 0), NAB = c(108, 161, 71), NBB = c(0, 64, 74))

Inits:
list(p = 0.5, sigma = 0.5)

Node statistics
   node   mean   sd   MC error   2.5%   median   97.5%   start   sample
   p   0.7049   0.02394   2.455E-4   0.6562   0.7053   0.7497   1001   10000
   q   0.2951   0.02394   2.455E-4   0.2504   0.2947   0.3438   1001   10000
   sigma   0.37   0.04158   4.236E-4   0.287   0.3703   0.4509   1001   10000

Posterior densities

[example-7_2_2-popgen0][example-7_2_2-popgen1][example-7_2_2-popgen2]