An unstable family of large DNA elements in the center of the mouse t complex.

We have cloned 363 kb (X 10(3) bases) from a novel, locally dispersed family of 11 large DNA elements, called T66 elements, within the center of complete mouse t haplotypes. Homologies among individual members of the T66 family are observed along a repeated unit of at least 75 kb in length. Individual T66 homology units are classified into three subfamilies through hybridization studies with a series of diagnostic subfamily-specific probes. The organization and number of elements in wild-type forms of chromosome 17 are very different from those found within t haplotype forms of this chromosome. The number of T66 elements present within individual chromosomes is highly polymorphic among both inbred strains of mice and among independently derived t haplotypes. Wild-type chromosomes have between five and nine T66 elements distributed between two loci that are separated by a genetic distance of at least three map units, whereas t haplotypes have between 9 and 11 T66 elements within a single cluster. Many of the rare recovered products of recombination between a t haplotype and a wild-type form of chromosome 17 have resulted from recombination within or near the T66 regions present on each chromosome. Molecular and genetic data lead to the speculation that portions of individual T66 homology units could be involved in t haplotype effects on sperm differentiation.