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Unformatted text preview: 495 15.1 Changes in chromosome number chromosome 1 and gene B on chromosome 2, the ratio is 4 A:4 B, or 1:1. In contrast, in an aneuploid, the ratio of genes on the aneuploid chromosome to genes on the other chromosomes differs from the wild type by 50 percent; 50 percent for monosomics; 150 percent for tri- somics. Using the same example as above, in a trisomic for chromosome 2, the ratio of the A and B genes is 2A:3B. Thus, we can see that the aneuploid genes are out of balance. How does this help us answer the ques- tions raised? In general, the amount of transcript produced by a gene is directly proportional to the number of copies of that gene in a cell. That is, for a given gene, the rate of transcription is directly related to the number of DNA templates available. Thus, the more copies of the gene, the more transcripts are produced and the more of the corresponding protein product is made. This relationship between the number of copies of a gene and the amount of the genes product made is called a gene-dosage effect. We can infer that normal physiology in a cell de- pends on the proper ratio of gene products in the eu- ploid cell. This ratio is the normal gene balance. If the relative dosage of certain genes changesfor example, because of the removal of one of the two copies of a chromosome (or even a segment thereof )physiologi- cal imbalances in cellular pathways can arise....
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.
- Spring '08