Table 1.

Backcross and F1 intercross experiments with animals from the WUC1 strain

Cross	Number of crosses	Number of litters w/affected	Number of litters w/o affected	Number of offspring/litter		% affected
				In affected	In unaffected
WUC1 × WUC1	26	26	0	9.3  ± 2.94		26.34  (n = 186)
WUC1 × F1 (backcross)	17	12	5	9.9  ± 2.91	8.8  ± 3.42	26.89  (n = 119)

WUC1 is the strain defined by those breeding pairs that generate offspring with the flathead phenotype. In WUC1 breedings, 26% of the offspring are affected. In addition there appears to be no embryonic lethality because the litter sizes from these breedings or other breedings that do not generateflathead animals are similar. The backcross experiments were breedings between unaffected littermates from litters that contained affected animals and their parents. In these breedings, approximately two-thirds of the litters contained affected animals. Together, this is consistent with a single autosomal recessive gene with 100% penetrance segregating at 1:2:1.