The effect of inbreeding on the BB/W diabetic rat

The Worcester colony of BB/W diabetic rats has been inbred by brother X sister matings for 12 generations. The litter size at both birth and weaning was dependent on whether the female was normal or diabetic. At the beginning of the inbreeding, the normal females produced 2.1 more young per litter than did the diabetic females, but the eleventh generation of inbreeding this difference had been reduced to 0.68 young. Diabetic males had little influence on litter size. At weaning, the normal mothers had raised litters that had 3.64 more young than the diabetics in the early generations, but this had dropped to 1.48 in later generations. After 11 generations of inbreeding, there was a 1.95 decrease in litter size from normal matings, but only a 0.86 decrease from diabetic matings. The average loss from birth to weaning was one pup for normal females and two pups for diabetic females. The time of onset (median 85 days) has not changed with inbreeding, nor is the onset pattern and severity different in any of the nine inbred lines. The penetrance in the D X D matings did change from 45% in the first five generations to 60% in the next six, but there has been no significant change in generations 6 through 11. Inbreeding changed the results obtained from outcrossing to nondiabetic lines. All crosses produced no diabetics in the F1, but noninbreds produced less than 2% in the F2 and 4% in the BC, whereas inbreds produced 18 to 32% in the BCs. These latter figures are consistent with the ratio of 7:3 expected with 60% penetrance.