The genes (characters) in individuals then may be classified under two categories – these are (a) characters which do not show any difference in inheritance pattern in reciprocal classes and (b) characters which are inher­ited differently in reciprocal crosses. The characters of the first category are on the autosomes, while those of the second category are on the sex chro­mosomes. Such characters which are located on the sex chromosomes are called sex linked characters (sex linked genes) and the inheritance is called sex linked inheritance.