Macrohairless (mhl1-R) is an epidermal-patterning mutant first identified in a Mu-active background. The phenotype appears to be controlled by a single recessive gene. The original mutant displayed "complete" loss of macrohairs on the adaxial epidermal surface. After three generations of introgression to W22 and W23, mhl shows 100% penetrance yet reduced expressivity. The introgressed mutant shows a partial loss of macrohairs. In each of the inbreds there was a varying degree of macrohair loss. In a family segregating for mhl/mhl and mhl/+ there was an average of 1.7 macrohairs/cm2 in the macrohairless plants as compared to an average of 37.7 macrohairs/cm2 in the heterozygous normal siblings.

Macrohairless has been preliminarily mapped to 4L. Homozygous macrohairless mutants were outcrossed by a series of B-A translocations covering 18 of the 20 arms. Crosses by TB-4L uncovered the partial loss of macrohair phenotype associated with the introgressed mutant. None of the B-A translocations uncovered a phenotype displaying a complete loss of macrohairs. This mapping result will be confirmed by growouts of the above mhl/ TB-4L test crossed to mhl/mhl.
 

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