ABSTRACT
The downless mutation arose spontaneously in the late 1950s on the A/H strain. It closely resembled the crinkled mutation; however, crossing the two mutants established that downless was not at the crinkled locus. The sleek mutation arose spontaneously in a stock carrying the Robertsonian translocation Rb(4.6)2Bnr at the MRC Radiobiology Unit in Harwell, UK It is allelic with downless and phenotypically very similar. Sleek is a dominant mutation that has been mapped to Chromosome 10. Heterozygous downless are normal because the mutation is recessive; however, sleek heterozygotes, because it is a dominant mutation, are similar to the homozygotes except that they have some hair on their tails and a smaller reduction in the number of vibrissae. In the downless mutation, only the awl hairs are present, and they exhibit irregularities in width and septation. The primary defect is restricted to the epidermis since explants composed of heterozygote tail skin epidermis with either heterozygote or homozygote dermis developed hair follicles.
