<P> 2 . Chromosome Walking </P> <P> This is significant in that it allows for the detailed mapping of specific regions of the genome . Whole human chromosomes have been examined, such as the X chromosome, generating the location of genetic markers for numerous genetic disorders and traits . </P> <P> YACs are significantly less stable than BACs, producing "chimeric effects": artifacts where the sequence of the cloned DNA actually corresponds not to a single genomic region but to multiple regions . Chimerism may be due to either co-ligation of multiple genomic segments into a single YAC, or recombination of two or more YACs transformed in the same host Yeast cell . The incidence of chimerism may be as high as 50% . Other artifacts are deletion of segments from a cloned region, and rearrangement of genomic segments (such as inversion). In all these cases, the sequence as determined from the YAC clone is different from the original, natural sequence, leading to inconsistent results and errors in interpretation if the clone's information is relied upon . Due to these issues, the Human Genome Project ultimately abandoned the use of YACs and switched to bacterial artificial chromosomes, where the incidence of these artifacts is very low . In addition to stability issues, specifically the relatively frequent occurrence of chimeric events, YACs proved to be inefficient when generating the minimum tiling path covering the entire human genome . Generating the clone libraries is time consuming . Also, due to the nature of the reliance on sequence tagged sites (STS) as a reference point when selecting appropriate clones, there are large gaps that need further generation of libraries to span . It is this additional hindrance that drove the project to utilize BACs instead . This is due to two factors: </P> <P> 1) BACs are much quicker to generate, and when generating redundant libraries of clones, this is essential </P>

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