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In a duplication, a section of a chromosome is present twice, i.e., in duplicate. The nucleus therefore has extra chromosomal material beyond the normal chromosomal complement. Duplications are the various types. In a displaced duplication a segment of the chromosome is repeated some distance away from its normal location. The repetition may take place on the same arm of the chromosome or on the other arm.


Position effect. The duplication of the Bar locus of Drosophila has been examined critically. The wild type of eye of Drosophila is large and has an average of 779 facets. It has two Bar regions, B, one on each X chromosome. If the Bar region undergoes duplication, the size of the eye is reduced. In the presence of a single heterozygous Bar (BB, B), the eye is smaller and averages 358 facets. The mutants Bar (BB, BB) and Double Bar (BBB, B) have four Bar regions each. The eyes of bar average 68 facets and those of Double Bar 45 facets. Thus although Bar and double bar contain four Bar regions, their phenotype effects are different. There is no change in the amount of genetic material, only in the order of the genes. Such phenotypic changes which are the result of differences in the position of genes are called position effects. The expression of the Bar region differs according to its arrangement on the chromosome. The function of the gene depends not only on the quality of the gene itself, but also on the genes that lie next to it on the chromosome.

position effect

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