Females heterozygous for the bad hair day mutation have patches of fur missing from the coat, male hemizygotes are almost bald, and female homozygotes are nearly as bald as hemizygous males. Hairs from heterozygous females are morphologically normal, while those from hemizygous males are short and thin. There is increased pigment in the hairs of both hemizygotes and heterozygotes.
The bad hair day mutation arose spontaneously at The Jackson Laboratory in 2004 in the B6.V-Lepob/J strain when that strain was at generation N34. Two backcrosses were done to C57BL/6J and this strain has been maintained by sibling inbreeding heterozygous females and wildtype males or widltype females and hemizygous males since then.