Table 1. Expected frequencies of four genotypes under the liability and fitness models in a backcross population.

Genotype	Relative fitness ()	in liability model	in fitness model
S1S2/S1S2	Φ(a1+a2+i)		(1−r)/D
S1s2/S1S2	Φ(a1−a2−i)		rv/D
s1S2/S1S2	Φ(−a1+a2−i)		ru/D
s1s2/S1S2	Φ(−a1−a2+i)		(1−r)x/D

D=(1−r)(x+1)+r(u+v);