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 is inherited irrespective of the sex of the individual and obviously the genes responsible for these characters must be situated on 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 resiprocal 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.
ADVERTISEMENTS:
In sex linked inheritance a particular character always follows a particular sex chromosome as the gene for the former is located on the latter. Sex linkage may be classified into the following categories, depending upon which chromosome carries the genes. This are-
1. XY linked characters (genes)
2. Y linked characters (genes)
3. X linked characters (genes)
ADVERTISEMENTS:
Inheritance of Y linked Characters: As has already been pointed out, some genes are exclusively linked to Y chromosome. A few instances of these in man are – a) Porcupine man in whom the entire surface of the skin will be warty and coarse (lchtyosis hystrix) b) hypertrichosis hair on the lobes of the ear c) Kerotoma dissipatum-skin disorder affecting the hand and feet d) Occurrence of Webbed toes in some individuals etc.
In plants, the Y chromosome contains genes for maleness and its absence induces sterility. Unfortunately not much detail is available on the Y linked somatic characters in plants.
Inheritance of sex linked or X linked genes: The genes or characters linked to the X chromosome are generally termed sex linked as they are the most prevalent sex linked characters. In order to understand the pattern of inheritance of X linked characters one should understand the inheritance pattern of the X chromosome.
In female individuals (which are chromosomes, while the males (which are hetrogametric) have only one X chromosome in the body cells. During reproduction the eggs will have only one X chromosome where as the sperms will be 50% X and 50% Y. When an egg is fertilized by an X sperm the resultant female (XX) individual will have one X from the mother and the other X from the father.
ADVERTISEMENTS:
When a Y sperm fertilizes the egg the resultant male offspring will get its X chromosome from the male individual again goes to the female (daughter). In this way an X chromosome and the genes linked with it will alternatively go to male and to female. The following chart summarises, the above explanation.
This type of inheritance of X chromosome which alternatively finds place in the male and female generations is called the criss cross inheritance. We shall study a few examples of this type of inheritance in Drosophila and man.