cross breeding experiment

[whatusername?]

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i asked this question in another thread but no one answered. possibly because the last post in that thread was from 2004 =/ Hopefully this new one will attract more attention.
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............recombination 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 6 map units 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 relative 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 being in a house and knowing that the roof and floor of the house are 3m apart, but not knowing whether what you are standing on or what is above your head is called the roof or floor- either way they're still 3 m apart.
anyone care to explain?

[axlenatore]

Originally Posted by whatusername?

i asked this question in another thread but no one answered. possibly because the last post in that thread was from 2004 =/ Hopefully this new one will attract more attention.
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............recombination 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 6 map units 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 relative 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 being in a house and knowing that the roof and floor of the house are 3m apart, but not knowing whether what you are standing on or what is above your head is called the roof or floor- either way they're still 3 m apart.
anyone care to explain?

If they asked explain how the relative position of genes can be found on chromosomes, you just have to say you did a test cross, ie cross breeding experiement by multiplying with a homozygous recessive (pretty sure thats the one you cross), this is done in a large sample space, thus allowing you to obtain percentages for the parental and recombinant types, with the percentage of recombinat types been the distance between the genes in centimorgans on a chromosome, and that is the relative position of a gene, ie where it is in relation to other genes.