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BIOL 239 (290)
Lecture

Set 9.docx

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Department
Biology
Course
BIOL 239
Professor
Christine Dupont
Semester
Winter

Description
Set 92 Twopoint crossesComparisons between different twopoint crosses can help establish relative gene positionsAH Sturtevant was able to determine an order for the genes he investigated on the Drosophila X chromosome using this approachwhat weve been doing is a two point crosspoints the genes places on the chromosomewere going to look at three genes nowtwo point comparing recombination frequency in breeding experiments Two genes two alleles each giving wild type and mutant phenotypeusually wildtype is dominant3 Mapping genes by comparisons of twopoint crosses Ex genes y w v m rhe took all these genes and he did a bunch of two point crosses he found flies that had mutant alleles for these different geneshe did 10 different experiments to get the recombination frequencieswe did w and y and looked at recombination frequencies and we now know thats a map unitwe did w and m with recombination frequency of 328 We dont know where m is Now we knowthe experiment between y and m determines thishe knows where they lay in what order and how far apart they arehe did that for all the otherspain staking process4 Limitations of twopoint crosseswhat he was finding was that the breeding experiments werent completely accurate all the time There was two reasons for this1In crosses involving genes lying close together it may be difficult to determine the correct order Ex if the distance ab is 282 mu and the distance ac is 293 mu it would be hard to confidently say that b is between a and c due to the possibility of sampling errorwhen you did the breeding experiments you got pretty much the same numbers of recombinant number when you did the two point cross for a and b as you did when you did the two point cross for a and c you couldnt tell which side they were on The mapping units werent accurate enough2The actual distances on the map do not always add up for genes that are far apartthere was a discrepancy between what he got for the two point cross for genes far apart vs what he got when he just added up the numbers he got for the two point cross for the genes between them5 Mapping genes by comparisons of twopoint crosses
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