The laws of inheritance as propounded by Mendel indicate that reciprocal crosses always yield the same result in hermaphrodite individuals.
In other words in a cross between tall and dwarf plants the resultant offspring would be tall irrespective of the fact whether tall parent is male or female.
This means that tallness or dwarfness are inherited irrespective of the sex of the individual and obviously the genes responsible for these characters must be situated or the autosomes.
On the other hand there are certain characters which seem to be associated with a particular sex or sex chromosome and such characters are not inherited in the same pattern in reciprocal classes.
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.
In sex linked inheritance a particular character always follows a particular sex chromosome as the gene for the former is located on the latter.
Discovery of sex linked characters:
The name of TH Morgan (1910) ranks first among the pioneers who have studied the inheritance pattern of sex linked genes. One of the earliest known sex linked characters is hemophilia which appeared only in the males of Royal family of Spain.
In hemophilia, the blood fails to clot on exposure to air (it should normally clot). But it was not known that hemophilia is a sex linked character. It was just thought to be a hereditary character. Morgan who was working on sex linked characters in drosophila melanogaster like the red and white eye opined that hemophilia is also a sex linked character.
Chromosomes and sex linked characters:
In order to understand the Pattern of inheritance of sex linked characters, one should understand the inheritance pattern of sex chromosomes themselves. Among organisms exhibiting sex linked characters there are two sex chromosomes X and Y.
They are genetically identical only partially. When males are the heterogametic sex they have XY and the females XX and when females are heterogametic they have X Y (ZW) and the male XX (ZZ).
At the time of gametogenesis (meiosis) when pairing takes place between X and Y, crossing over is restricted to genetically identical regions only. The genes located in such regions may get exchanged and they do not seem to follow a particular chromosome.
Such genes are said to be partially sex linked. Whereas the genes which are located on the non homologous regions of X and Y chromosomes do not get exchanged and they always follow a particular chromosome.