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Hey guys, I'm not sure how many of you are good with genetics, but I'm doing a practice test for a midterm and came across this problem:
You take a yellow bodied short winged female Drosophila from a pure breeding line and mate it with a wild type male. All of the F1 progeny are wild type. You let the F1 progeny mate among themselves and get the
following F2 progeny:
Wild type 1330
Yellow body 4
Grey body 115
Brown body 48
Yellow body, short wings 330
Grey body, short wings 42
Brown body, short wings 125
Short wings 6
Total 2000
The question asks: Define the genes and their phenotypes:
My answer:
y/y yellow
y+/- wild type
g/g grey
g+/- wild type
b/b brown
b+/b- wild type
sw/sw short wings
sw+/- wild type
Answer key:
g/g grey
b/b brown
sw/sw short wings
g/g,b/b yellow
g+/g+,b+/b+ wild type
sw+/sw+ wild type
What I don't understand is how you can deduce yellow bodies is a double mutant of grey and brown from the numbers above.
 
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my question is first of all why do you have 3 alleles coding for the same gene (which is to say, color?)
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On October 18 2009 13:01 Caller wrote:
my question is first of all why do you have 3 alleles coding for the same gene (which is to say, color?)
I don't see why this is a problem - you can have many different alleles that code for the same gene, but they are limited to two in a healthy organism.
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Just look at your F2 values, at least thats how I reason out these problems, not really helpful to say that but this what I saw immediately:
Grey body 115
Brown body 48
vs.
Grey body, short wings 42
Brown body, short wings 125
Numbers don't make sense for grey and brown to be totally independent, it's highly unlikely for wing type to affect body color (unless your prof is a major dick) so I looked for color stuff.
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Its, not 3 alleles. There are 2 genes that need to be dominant to get wild type: grey and brown. However, when you become mutant for both, you get yellow bodes.
The pathway would look like:
------------------- Grey
Yellow -----> Black
-------------------- Brown
At least I think that's what its like.
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Seems to me like some sort of pokemon breeding program
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Wild type 1330
Yellow body 4
Grey body 115
Brown body 48
Yellow body, short wings 330
Grey body, short wings 42
Brown body, short wings 125
Short wings 6
Total 2000
Are these labels right? Its not like this?
Wild type 1330
Yellow body 4
Grey body 115
Brown body 48
Wild type, short wings 330
Grey body, short wings 42
Brown body, short wings 125
Yellow body, Short wings 6
Total 2000
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Yea, I'm sure those are the numbers.
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Well it doesn't really make sense then that the double mutant is more common than either single mutant. Also, what does "Short wings" on its own me?
Ah nvm...linked genes
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would help you if i had more time.. at this moment though i dont want to see any more of drosophilia
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United States47024 Posts
On October 18 2009 12:54 darkmetal505 wrote:
What I don't understand is how you can deduce yellow bodies is a double mutant of grey and brown from the numbers above.
The alternative seems to overcomplicate the problem. If yellow is simply a grey/brown double mutant, the number of progeny classes works out. If yellow were a separate gene, you'd then have to wonder what happens with yellow/brown double mutants, yellow/grey double mutants, and grey/brown double mutants, and figure that you should see more progeny types than are present. Or, you make arbitrary assumptions about double mutants fitting a single-mutant phenotype, which is a shady proposition at best.
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EPT
