biol139-lecture20-2011

Biol139 lecture20 2011

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Unformatted text preview: BIOL 139 Map distances Map Chapter 5 con’t Chapter pp 135 - 140 pp pp 151 - 157 igenetics pp Mapping: Locating Genes Along a Chromosome Mapping: • assigning genes a relative position along the chromosome • this position is called the locus locus • determining a gene’s locus is called mapping determining Two-point crosses Two-point • crosses that trace 2 genes at one time crosses Comparisons between different two-point crosses Comparisons can help to establish relative gene positions can A.H. Sturtevant : was able to determine an order for the A.H. genes he investigated on the Drosophila X chromosome, using this approach chromosome, Mapping genes by comparisons of two-point crosses Mapping Recombination data 3 genes: y, w, m Greatest distance should separate genes on outside y-w = 1.1 m.u. y-m = 34.3 m.u. w-m = 32.8 m.u. Distance of 34.3 between y + m would indicate on outside That leaves w in middle. But is w closer to y or m? But The distance between y + w of 1.1 would suggest w very close to y And the distance between w + m of 32.8 would confirm this Though does not strictly add up Mapping genes by comparisons of two-point crosses Mapping • crosses that trace 2 genes at one time crosses Once orientation for these 3 genes is determined can add a 4th (r) y-r = 42.9 m.u. * r is outside of y-m since the distance is greater (42.9) y-r w-r = 42.1 m.u. But is it closer to y or m? w-r m-r = 17.8 m.u. Distance between m + r is 17.8 which would indicate m-r outside of y and m and closer to m than y Limitations of two-point crosses 1. In crosses involving genes lying close together it may be difficult to determine the correct order e.g., if the distance a-b is 28.2 m.u., and the distance a-c is 29.3 m.u. , it would be hard to confidently say that b is between a and c,due to the possibility of sampling error a a bc cb Limitations of two-point crosses Limitations 2) The actual distances on the map do not always add up A single 2 point cross is not accurate for determining the distance between genes that are far away from each other -have to do a series of 2 point crosses using other genes that lie between OR…… Three-point crosses Thr...
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