T(In1;5)44H, a complex mouse chromosomal rearrangement with a phenotypic effect

A complex murine chromosomal rearrangement, T(In1;5)44H, was recovered after 5 Gy + 5 Gy (given 24 h apart) spermatogonial X-irradiation. T44H is a paracentric inversion of most of Chromosome (Chr) 1 (1A1-1H6), followed by splitting of the inverted segment through a reciprocal translocation with Chr 5, the latter breakpoints being in 1C2 and 5F. Linkage tests have shown that the probable order on Chr 1 is fz-ln-T44H with 2.4 +/- 2.4 crossover units between ln and T44H. On Chr 5 the probable order is W-T44H-go-bf with 7.1 +/- 4.9 crossover units between T44H and go. All heterozygotes show a marked dilution of coat colour. Heterozygotes of both sexes are fertile, producing small litters with a marked shortage of T44H carriers. The number of live embryos produced from female carriers is significantly lower than from males. Despite the complex nature of the rearrangement, complete chromosome pairing and chiasma formation occur regularly at meiosis. Depending on the strands involved, this leads to the production of either one or two dicentric chromatids per spermatocyte, and their disjunctional fate can be followed into metaphase II. Analysis of chromatid classes at this stage suggests reasons for both the high embryonic mortality and the shortage of liveborn T44H carriers.