Index >> Sex Chromosomes and Sex linked Inheritance

Sex-Chromosomes and Sex-Linked Inheritance
The sex-chromosomes (X and Y) are of unequal size, shape and/or staining quality (hence are heteromorphic). The X-chromosomes of man and Drosophila have been found to be straight, rod-like and comparatively larger than Y chromosome. The Y chromosome of man and Drosophila is smaller than X chromosomes however, in Y chromosome of Drosophila, one end remains slightly curved or bent to one side. Hess et.al., (1968) have demonstrated the presence of lampbrush-loop-like structures with specific shapes, in the Y -chromosome of spermatogonial cells of male Drosophila flies. The loop of Y chromosome Contains genes for fertiltiy of male sex (see Sybenga, 1972). In Lychnis (Melandrium), the X and Y chromosomes are rod-shaped and X chromosomes are smaller than Y chromosomes.

The genes which reside in differential or non-homologous regions of X and Y chromosomes always inherit together according to the inheritance of their respective chromosomes, because, the genes of differential regions of X and Y chromosome never undergo crossing over. Such genes are called completely sex-linked genes. The completely sex linked genes may be of following types.
1. Holandric genes- The genes which remain confined to differential region of Y chromosome only, are called holandric or Y-linked genes. They have no homologous allele in the X chromosome. The Y -linked genes inherit along with Y chromosome and they phenotypically expressed only in male sex.
2. Sex-linked genes-The genes which reside in the differential region of X chromosome only are called sex-linked or X-linked genes. The X-linked genes have no corresponding allele in Y chromosome.