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6. When the offspring of this cross (the F2 generation) begin to emerge, they should be killed
(by over-anesthetizing), removed, and the sex and characteristics determined and recorded.
Counting should go on until all flies have emerged and have been counted (usually about eight
days, depending on the temperature). Counts should be made daily.
7. The results should look like this:
wild type (+) 439 males
452 females
891 total
vestigial (vg) 159 males
142 females
301 total
total flies counted = 1192
The wild type flies outnumber the vestigial wing flies by about three to one. The sex of the flies does
not appear to affect the ratio (in this case). The crosses can be diagrammed to predict this in a very
simple manner.
Since the F1 generation is the result of two homozygous strains of flies, the wild type fly has only
one possible characteristic in the gametes, wild (+). The vestigial fly also has only one possible type
of gamete, vestigial (vg). The F1 generation can therefore be diagrammed:
vg
+ vg + vg
vg
+
+ + vg + vg
–
The zygotes (fertilized eggs) all have both the wild and the vestigial genes. Since the wild type is the
dominant factor, all of the F1 flies will appear to be wild type, even though they are not homozygous
strains.
