An essential step toward the direct assignment of a gene onto a particular chromosome (or a region within the chromosome) depends on the feasibility of correlating the change which occurred in a chromosome and specific alterations observed in gene product(s) or gene orders (relationship between certain linkage groups). Changes involving a whole chromosome or even part of a chromosome (particularly an autosome) in mammalian species are usually incompatible with life. Therefore only heteroraorphic chromosome markers (Donahue et al., 1968), small deletions (Ferguson-Smith et al., 1973), balanced translocations (Miller and Miller, 1972) and made use of sex chromosomes (for the sex-linked gene) were useful among the pioneer studies in the field of gene mapping of mammalian species. However, usefulness of a chromosome marker depends on the close proximity of the marker to the gene to be mapped. In man this has been found not to be the case in many instances (Human Gene Mapping 2, 1975). Viable deletions and translocations are also rare in natural conditions. Therefore a much more fruitful approach to mammalian gene mapping was later developed using a parasexual system of interspecific somatic cell hybrids (Ephrussi, 1972). The newly formed hybrid cells have a tendency to lose their chromosomes. 

C. C Lin

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 6. Round tables, , 385–395, 1982
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