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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

 

÷ 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!