Evidence for unequal crossing over within the mouse T/t complex.

The Tcp-1 gene located within the T/t complex on chromosome 17 of the mouse codes for a major cell surface-associated protein p63/6.9. Previously, we identified two structural alleles of this gene which specify alternate forms of the p63/6.9 protein. The Tcp-1b allele is associated with all wild-type chromosome 17; the Tcp-1a allele is found only with chromosome 17 carrying a complete t haplotype. Normal recombination along a major length of chromosome 17 is suppressed in mice that are heterozygous for any complete t haplotype. Suppression is not complete, however, and rare crossing over between wild-type and t haplotype chromatin does occur. In this report, 15 rare recombinant chromosomes have been analyzed for Tcp-1 alleles. The results indicate that in four independent events the Tcp-1b and Tcp-1a alleles have become associated in cis position in a single DNA molecule. Further genetic analysis provides support for the hypothesis that a significant nonhomology exists between the arrangement of DNA sequences on wild-type and t-carrying chromosome 17. This could account for both the suppression of normal recombination along the stretch of t chromatin and the frequent unequal crossing over when rare recombinational events do take place.