5. The closely linked genes have less chances of departure or frequency of crossing over than the widely located genes. 6. And each gene has definite order and location in a linkage group or chromosome, as the crossing over frequency has been found constant for two given linked genes of a species.
For example, various genetic crosses have shown that in Drosophila, among three mutant sex-linked genes (viz., genes for white eyes. yellow body and cut wings), the genes for white and yellow are always found to have 1% crossing over and similarly white and cut genes are· found to have 20% crossing over. The percentage or frequency of crossing over, thus, appears to be closely related to physical distance between genes. When, one knows all the genes, linkage groups and number of linkage groups of a species, it becomes possible for him that by adopting the crossing over as a tool, he may determine the relative distances between the genes in a linkage group and also their order and may give diagrammatic representation of chromosomes showing the genes all points separated by distances proportional to the amount of crossing over.Such a diagrammatic, graphical representation of relative distances between linked genes of a chromosomes is called linkage or genetic map. Because, such a linkage or genetic map is the outcome of crossing over studies, therefore, is also called cross over map.
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