A polymorphic repetitive-sequence PR1 family. Evidence for meiotic instability.

When EcoRI digests of mouse genomic DNA were subjected to Southern blot analysis with the polymorphic repetitive sequence PR1 as a probe, one satellite-like band of 3.5 X 10(3) base-pairs, designated as PR1 family B, was detected in BALB/c-strain mice, but not in the DDD/1- or MOA-strain mice. Analysis of recombinant phage clones revealed that the repeating unit of the PR1 family B was 13.5 X 10(3) base-pairs long. This family consisted of a tandem array of repeating units and occupied as much as 2% of one BALB/c chromosome. Since the BALB/c-specific PR1 family B is not present in DDD/1 or MOA mice, the unpaired portion of the BALB/c chromosome may be looped out in a synaptonemal complex during meiosis in F1 hybrids of the BALB/c strain with DDD/1 or MOA. To determine the fate of this extra DNA, we examined the genotypes of the F1 hybrid mice and of the segregating populations. Although the PR1 patterns of F1 and most N2 mice are consistent with typical Mendelian inheritance, some N2 progeny showed an abnormal 3.5 X 10(3) base-pair band of unexpectedly reduced intensity. This indicated that the extra DNA of PR1 family B occasionally underwent recombination during meiosis in F1 mice, resulting in its apparent excision. Examination of PstI digests supported this interpretation.