CORN GENETICS & CHI SQUARE ANALYSI S
In this exercise, you will examine an ear of corn and determine the type of cross and genes responsible for the coloration and texture of the corn kernels ike the one show below. There are four grain phenotypes in the ear. Purple and smooth (A), Purple and Shrunken (B), Yellow and Smooth (C), Yellow and Shrunken (D).
Monohybrid Cross
1.Count the number of purple and yellow kernels in five of the rows on your ear of corn and record the number on the chart. Be sure to use the same five rows for each calculation.
2.Count the number of smoot h and shrunken seeds on the same five rows and record on the chart.


Number of 
K ernal Percentage 





Kernels 
(divide count by total) 





Kernal Coloration 



Purple 
267 






Yellow 
121 






Total (for 5 rows) 
481 






Kernal Texture 



Smooth 
306 






Shrunken 
175 






Total (for 5 rows) 






3.What are the probable phenotypes of the parents with regard to coloration?
4.What are the probable phenotypes of the parents with regard to texture?
Dihybrid Cross
5. We will now consider a dihybrid cross, which is a combination of the two monohybrids. Your ear of corn may be a result of a cross between plants that were both heterozygous for color and texture (Pp x Ss). Work out this cross in the Punnet square below.
6.Calculate the phenotypic ratios for each type of seed. Purple & smooth _______________
Purple & shrunken ______________
Yellow & smooth _______________
Yellow & shrunken ______________
7.Now count the number of each in your five rows on the ear of corn.

Number Counted 
Ratio: Number counted / total 



Purple & smooth
Purple & shrunken
Yellow & smooth
Yellow & shrunken
TOTAL
8. Did you obtain a 9:3:3:1 ratio? If you did not, then the genes may be found on the same chromosome and do not assort independently. To determine if the deviations from your observed data are due to chance alone or if the data is significantly different, you need to use a chi square test.
First calculate the expected number you should have gotten based on your total
number assuming a 9:3:3:1 ratio. 




Calculate the individual chi squ are values for each row and add them 
all together to 

determine your overall chi square value. 










Expected Nu mber 
Observed 

Ã· expected 

Number 












Purple & smooth 
Total x 9/16 = 








Purple & 
Total x 3/16 = 



shrunken 














Yellow & smooth 
Total x 3/16 = 








Yellow & 
Total x 1/16 = 



shrunken 
















CHI SQUARE VALUE ==== ====> 



(add the numbers from the rows above) 






9. Now determine if your chi s quare value is a good fit with your data. Your degrees of freedom (df) is the number of possible phenotypes minus 1. In your case, 4  1 = 3. Find the number in that row that is closest to your chi square value. Circle that number.
Good Fit Between Ear & Data

df 
.90 
.70 
.60 
.50 
.20 
.10 









1 
.02 
.15 
.31 
.46 
1.64 
2.71 









2 
.21 
.71 
1.05 
1.39 
3.22 
4.60 









3 
.58 
1.42 
1.85 
2.37 
4.64 
6.25 









4 
1.06 
2.20 
2.78 
3.36 
5.99 
7.78 








Poor Fit 

.05 
.01 
3.85 
6. 64 
5.99 
9. 21 
7.82 
11.34 
9.49 
13.28 
10. Explain what it means to have a "good fit" or a "poor fit". Does you chi square analysis of real corn data support the hypothesis that the parental generation was PpSs x PpSs?
PROBLEM SET
Chi Square Problem: A large ear of corn has a total of 433 grains, including 271 Purple & starchy , 73 Purple & sweet, 63 Yellow & starchy, and 26 Yellow & sweet. These numbers are entered in Columns 1 and 2 of the following Table 4.
Your Tentative Hypothesis: This ear of corn was produced by a dihybrid cross (PpSs x PpSs) involving two pairs of heterozygous genes resulting in a theoretical (expected) ratio of 9:3:3:1.
Objective: Test your hypothesis using chi square and probability values. SHOW ALL WORK!