The hairy ears (Eh) mutation in the mouse originated from neutron irradiation experiments and is associated with chromosomal inversion on chromosome 15. Eh/+ mice have small pinna and extra hairs on the pinna but the phenotypic features of Eh/Eh mice are unclear. In this study we found that Eh/Eh mice died shortly after birth and had a cleft palate caused by impaired growth of palate shelves. Because genes located on the breakpoints of inversion are likely to be responsible for the defects associated with chromosomal inversions, we determined the breakpoints of the Eh inversion. We used a new genetic method that uses recombinant chromosomes resulting from crossing over between two overlapping inversions to determine the breakpoints. Koa is a mouse mutation associated with inversion of chromosome 15, which partially overlaps with the Eh inversion. We made Eh +/+ Koa double heterozygotes and obtained the recombinant chromosomes possessing deletion and duplication of the regions flanked by the breakpoints of both inversions, which were generated by crossing over within the overlapped region of these inversions. By defining the deleted regions we identified the breakpoints of the Eh inversion. We then examined the expression of genes in the vicinities of the breakpoints and found ectopic expression of the Hoxc5 gene and a transcript with unknown function in the developing palate of Eh/Eh mice, which is likely to be responsible for the cleft palate.
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