FIG. 4.

Viable sir2α null offspring arise from crosses between heterozygous animals. Males and females that were genotyped as heterozygous for the mutated sir2α alleles were mated, and the DNA from their offspring was used for Southern blots to allow genotyping of individual animals. The DNA was digested with NsiI and KpnI, and the blots were probed with the 3′ probe shown in Fig. 1. In the litter shown in the upper panel, the three expected genotypes were present; lane 2 has two wild-type sir2α alleles, lane 5 has two mutant alleles, and lanes 1, 3, and 4 are heterozygous. The table shows the numbers of offspring of each genotype from these crosses of heterozygous parents. The strain labeled 129/CD1 is outbred and has a mixed genotype, while the strain labeled 129 includes animals in which the targeted sir2α allele was maintained on the 129/Sv background.