Chromosomal Type-Genetic basis of Sex Determination
It refers to the condition where the genes involvcd in sex dcterrnincltion arc located on specific chromosomes known as the sex chromosomes.
First, we shall see how the genic typc of sex determination evolved into a chromosomal type of sex determination mechanism. In the primitive forms, the only difference between the two sexes was in their gametes. Later in evolution, morphological or phenotypic differcnce developed in the two sexes of a species.
In primitive forms, sex determination was due to genes on autosomes (the genic type). In the process of evolution, gradually, the genes responsible for sex determination got localised on specific chromosomes - the sex chromosomes, These chromosomes were designated as 'X' and 'Y' or 'Z' and 'W', and they can usually bc distinguished morphologically from eacb.other. The remaining chromosomes of the complement are known as. 'autosomes' and are designated 'A'.
The X and Y chromosomes differ from each other in many respects. This is because, there is accumulation of sex determining genes on tlie respective sex chromosomes. Also, there is negligible crossing-over between the X and Y chromosomes. This helps to preserve gene combinations favouring distinct sexual differences. The consequence is that the Y chromosome bears mostly the genes essential for the male determination while all the other genes become inert. These regions got reduced in size in some species and are completely lost in others. This was how the heteromorphic sex chromosome evolved.