The transmission ratio distortion seen in males heterozygous for a mouse /-complex has been explained on the basis of trans-acting distorter genes, having a harmful effect on a responder gene. The /-complex form of the responder is relatively resistant to these harmful effects and hence is preferentially transmitted. Animals homozygous for the /-complex responder would be expected to show equal transmission of the two homologous chromosomes, but this is not always so. Studies described in this paper have shown differences among complete /'s in their transmission when opposite a constant responder carrying partial /-haplotype. In addition, the proximal partial haplotypes / A49 and t wl8 , both derived from t wb but of different lengths, behave differently when opposite a responder. The three central partial haplotypes, t lowli , t l0W2H and t low3H , also differ, in that t low3H shows lower transmission than t lowli or t l0W2H when opposite either wild-type, or another responder, or distorter genes. These results can be explained either on the basis of differences in the responder region of various haplotypes, including the possibility of varying numbers of copies of the relevant sequences, or on the basis of differences in cis-acting (as opposed to trans-acting) distorter genes.
Probes for loci situated near one end of the proximal (Tcp-1) and distal (Qa-2, 3) inversions of the mouse t complex have been hybridized to chromosomes of mice with and without t complexes and with morphologically distinguishable chromosome 17s. Both the probe for Tcp-1 and that for Qa-2, 3 hybridized to clearly different positions on t and non-t chromosomes, thus making visible the extent of the two inversions. The proximal inversion extends from roughly the junction of bands A1 and A2 to band A3, and the distal inversion from band A3 to band C. Thus, the whole t complex extends from the band A1-A2 junction to band C, and is therefore somewhat larger than previously thought, and occupies about 1.2% of the genome. A probe for complement component 3 (C3-1), genetically known to be several cM distal to the t complex, was found by in situ hybridization to lie in band E1. The proximal part of chromosome 17 is one of the best known parts of the mouse genome, at both the genetic and molecular levels. It may soon be possible to correlate the length of the t complex in terms of chromosomal distance with its physical length in megabases.
The hypoxanthine phosphoribosyltransferase locus (Hprt) of the mouse has been localized by in situ hybridization to band XA6. Comparison of the distributions of known loci on the genetic and cytogenetic maps of the X-chromosome suggests some chiasma localization with a relatively high frequency of chiasmata in the F bands. In the A bands there appear to be fewer known loci than expected, but no evidence has been found so far of excessive chiasma formation.
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