Female tabbies are not particularly rare, but orange or ginger female tabbies are much rarer than males. About 1 in every 5 ginger tabbies is female. This is because ginger coloring is a recessive gene found in the X chromosome.  Females have the XX chromosome, while males have the XY chromosome. This means that in order to get a ginger male tabby, only one of his parents needs to be ginger. But in order to get a female ginger tabby, both of her parents would need to be ginger.  So while female tabbies are not rare, it is much more unique to find a female ginger tabby than it is to find a male.