Sample Lab Writeup for Virtual Flylab I

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Problem 1: Investigate the purple eye trait

 
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                          Results of Cross #1                        
  
                              Ignoring Sex                              
 
                                Parents                                 
 
                        (Female: PR) x (Male: +)                        
 
                               Offspring                                
 
Phenotype                                Number   Proportion    Ratio    
  +                                       998      1.0000      1.000    
 Total                                    998 
 
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A cross of a purple eyed female with wild-type (+) male gave all wild-type (+).
My hypothesis is that purple eyes is a standard recessive mendelian trait.
If so, the genotype of the purple eyed female was prpr and the 
genotype of the wild-type male was pr+pr+.
Therefore the F1 wild type progeny should all have the genotype prpr+.
If so, a cross of the F1 should give a 3:1 ratio of wt:purple eyes. 

Results of Cross #2                           
   F1                        (Female: +) x (Male: +)
                             prpr+   x  prpr+
 
F2                               Offspring                                
 
Phenotype   Genotype                    Number   Proportion    Ratio    
+           prpr+ or pr+pr+               751      0.7327      2.741    
PR          prpr                          274      0.2673      1.000    
Total                                    1025 
 

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As there was no difference between the sexes I've combined the male and female numbers. 
The ratio of 751 wt : 274 purple is close
to the predicted 3:1 so I'm going to tentatively accept my hypothesis (boy, I wish I knew how to do a 
statistical test to prove this, maybe next week). The
genotype of the purple F2 flies should be prpr while the wt F2 flies should be
a mixture of pr+pr+ and prpr+.
As the wt F2 are a mixture, I think I'll test
my hypothesis that the F1 are all prpr+ with a couple of backcrosses. First
I'll cross a wt F1 with a wt parent. As this will be prpr+ x pr+pr+ all of
the progeny will get a pr+ allele and they all should be be wt.                          
 
Results of Cross #3                           
 
                     F1   (Female: +) x Parent (Male: +)                     
                            prpr+     x  pr+pr+   
 
Backcross                               Offspring                                
 
Phenotype     Genotype                 Number   Proportion    Ratio    
  +           prpr+ or  pr+pr+          975      1.0000      1.000    
 Total                                 975 
 
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I couldn't find my original wt male (must have flew off), so I got another
one from the stock, they're true breeding so the stocks all have the same
genotype. The result was as expected, all wt progeny, so the wt allele is
dominant. This cross really didn't tell me very much so I'll try the
back-cross to the purple eyed parent. As that parent should be prpr this will
be a test cross, F1 prpr+ x Parent prpr, and should give a 1:1 ratio of
purple:wt progeny. As I haven't seen this ratio yet being able to predict a
new result like that will be very strong evidence that my hypothesis is
correct.                         
 
 Results of Cross #4                           
 
                     F1   (Female: +) x Stock (Male: PR)                    
                            prpr+     x  prpr
   
 
                               Offspring                                
 
Phenotype     Genotype                  Number   Proportion    Ratio    
+             prpr+                       463      0.4889      1.000    
PR            prpr                        484      0.5111      1.045    
 Total                                    947 
 
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Yes! As predicted I got a 1:1 ratio of purple:wt. The purple flies must be
prpr and the wt flies should all be prpr+. I repeated the initial crosses
with the sexes reversed and got the same results (data not shown). I conclude
from the disappearance of purple in the F1, the 3:1 ratio of wt:purple in the
F2, and the 1:1 ratio in the wt F1 cross with a purple-eyed fly, that purple 
eyes are a standard recessive Mendelian trait.
 
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