Dot point - cross breading experiments

[kitty_n08]

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- explain how cross breeding experiements can identify the relative position of linked genes...



how to answer this dot point? :S

[malkin86]

which number is it? I might have something, how to find it though...

[Tommy_Lamp]

By observing which genes stay together the most. From my notes:
Crossbreeding experiments are used to produce linkage maps. Linkage maps measure the distance between two genes and the relative position of linked according the likely hood that the genes will be inherited together. Genes that tend to stay together during crossing over are put close together on genetic linkage maps. Genes that frequently tend to separate are further apart.
The following indicates that Gene D is closer to gene S then to gene V. Gene D crosses over with Gene S 2% of the time and crosses over with Gene V 12% of the time.
V………………………………............. .................................D……………� ��….S

[whatusername?]

theres one thing i dont understand about these cross breeding experiments which aim to give us a chromosome map. Lets look at a three factor cross:

AaBb/ aabb............recomination rate 6%- so these two are 6 map units apart

therefore it may be ....B...(6)...A.... or .....A...(6)...B..
(ie B may be either side of A, so it isnt an accurate map yet)

so we do another two crosses:

AaCc/ aacc...........recombination rate 12%

BbCc/ bbcc.............recombinatino rate 18%

therefore, C is 12 units from A, and B is 18 units from C
so.....C.......(12).........A...(6)...B
according to the textbooks, we now have our relative position of genes- a chromosome map. but, can't the genes be arranged this way aswell?:

....B...(6)...A......(12)......C

so our second two crosses havent got us any closer to a real chromosome map: the same problem as crossing A and B alone is still present- both mirror images are equally plausible

so, if an exam says: explain how cross breeding experiments can identify the relatvie position of genes. we can theoretically just say the recombination rates from a single cross (ie A and B) show how far apart the genes are.
in reality this will be wrong- they want you to say that a 3rd factor (ie C) must be crossed against both to get an accurate relative arrangement. but the arrangement of A and B is no more clear than before. we just know the relative position of C as well. If you had never heard the words "roof" and "floor" before, this situation is kinda like knowing that the roof and floor of a house are 3m apart, but not knowing if its the roof you're standing or if its the floor- either way they're still 3 m apart.
anyone care to explain?