In Drosophila, four genes on the X chromosome are sites of recessive mutations causing yellow body (y), white eyes (w), miniature bristle (m), and small wings (sm). In a test cross of F1 triple heterozygous female (obtained by crossing two pure-breeding P1 stocks) with (m y w)/Y males, seven classes of progeny were recovered and counted among 400 F2 flies.
yellow white 155
miniature 131
yellow miniature 49
white 47
white miniature 8
yellow 6
wild 4
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total 400
A. What were the genotypes of the pure breeding P1 stocks?
B. Draw a genetic map of these genes, showing order and distances.
In two further crosses, the small wing gene was studied.
testcross parents % wild type in F2
(sm +) / (+ w) x (sm w)/Y 2.25
(sm +) / (+ m) x (sm m)/Y 4.5
C. Place sm on your map.
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My attempt:
yellow white 155 yw+ - Parental
miniature 131 ++m - Parental
yellow miniature 49 y+m - Single Cross 1
white 47 +w+ - Single Cross 1
white miniature 8 +wm - Single Cross 2
yellow 6 y++ - Single Cross 2
wild 4 +++ - Double Cross
yellow white miniature 0 ywm - Double Cross
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total 400
A. P1: (yw+)/(yw+) x (++m)/Y
B.
Compared Parental with Double Crossed:
yw+ ywm
++m +++
m differs therefore m is in the middle
Compared Parental with Single Cross 1, in corrected order:
y+w ++w
+m+ ym+
y differs, therefore SC1 is a y-m cross
and SC2 then is a m-w cross
y-m: (49+47+4)/400 * 100 = 25m.u.
m-w: (8+6+4)/400 * 100 = 4.5 m.u.
y m w
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25 m.u. 4.5m.u.
I'm not sure if I have done it correctly and also I have no clue as to how to do part C. I'm stuck :/
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