2
Chi-Square
2
Genetics
One of Mendel’s Crosses
P: Pure Purple Flowers x
F1:
F2:
Pure White Flowers
All Purple Flowers
705 Purple Flowers, 224 White
Is this a 3:1 ratio?
Chi-Square
• Statistical test to compare observed
to expected results
Chi-Square
Observed
Phenotype
(O)
Purple
705
White
224
Total
929
Chi-Square
Observed
Phenotype
Expected (E)
(O)
Purple
705
3/4*929=696.75
White
224
1/4*929=232.25
Total
929
929
Chi-Square
Observed Expected
Phenotype
(O)
(E)
O-E
Purple
705
696.75
8.25
White
224
232.25
-8.25
Total
929
929
0
Chi-Square
Observed Expected
Phenotype
O-E (O-E)2
(O)
(E)
Purple
705
696.75
8.25 68.06
White
224
232.25
-8.25 68.06
Total
929
929
0
Chi-Square
2
Observed Expected
(O-E)
Phenotype
O-E (O-E)2
(O)
(E)
E
Purple
705
696.75
8.25 68.06 0.098
White
224
232.25
-8.25 68.06
Total
929
929
0
Calculated X2 = 0.388
0.29
0.388
df=degrees of
freedom =
# categories - 1
df=2-1=1
probability =
0.05
If X2 calculated < X2 table, Observed consistent with Expected
(data fits ratio)
0.388<3.84
If X2 calculated > X2 table, Observed not consistent with Expected
(data not in ratio)
Lab 2
• Chi-Square Monohybrid Cross Corn
• Two trait crosses
• Chi-Square Dihybrid Cross
• Chose Genetic Disease