Two loci: A and B
At each loci two alleles: A1/A2 and B1/B2

D = pA1B1 - pA1 pB1

If (D > 0) Dmax = min( pA1 (1-pB1), (1-pA1) pB1 )
If (D < 0) Dmax = min( pA1 pB1, (1-pA1) (1-pB1) )
D' = D / Dmax

r2 = D2 / (pA1 pA2 pB1 pB2)