Laboratory Exercise 11 - Mendelian Genetics
(Monohybrid Cross, Dihybrid Cross and χ2 Analysis)
I. Monohybrid Cross: Count the yellow and purple kernels for 3 ears of corn from the “3:1” collection. These are
the result of a monohybrid cross (two heterozygous parents) and we expect a ratio of 3 dominant phenotypes to
1 recessive phenotype.
Purple Counted Yellow Counted
Ear 1
Ear 2
Ear 3
Total
Overall Total (Add both
Totals from above)
II. Test Cross: Count the yellow and purple kernels for 3 ears of corn from the “1:1” collection. These are the result
of a test cross (two heterozygous parents) and we expect a ratio of 1 dominant phenotype to 1 recessive
phenotype.
Purple Counted Yellow Counted
Ear 1
Ear 2
Ear 3
Total
Overall Total (Add both
Totals from above)
1
III. Dihybrid Cross: Count the kernels for 3 ears of corn from the “9:3:3:1” collection. These are the result of a
dihybrid cross (two heterozygous parents for two traits) and we expect a ratio of 9 dominant/dominant: 3
dominant/recessive: 3 recessive/dominant: 1 recessive/recessive.
Purple / Smooth Purple / Rough Yellow / Smooth Yellow / Rough
Ear 1
Ear 2
Ear 3
Total
Overall Total (Add
all Totals from
above)
2
Refer to http://waynesword.palomar.edu/lmexer4.htm for a nice discussion of chi-square analysis of a
dihybrid cross.
Table 1. Probability Values from (http://waynesword.palomar.edu/lmexer4.htm)
Good Fit Between Ear & Data Poor Fit
df .90 .70 .60 .50 .20 .10 .05 .01
1 .02 .15 .31 .46 1.64 2.71 3.85 6.64
2 .21 .71 1.05 1.39 3.22 4.60 5.99 9.21
3 .58 1.42 1.85 2.37 4.64 6.25 7.82 11.34
4 1.06 2.20 2.78 3.36 5.99 7.78 9.49 13.28
3
Chi-Squared Analysis
I. Monhybrid Cross (expected 3:1 phenotypic ratio)
Phenotype Observed # Expected Difference Difference Standardized Squared
(O) Results (E) (d) =(E – O) Squared (d2) Deviation (d2/E)
Purple
Yellow
χ2=
χ2= sum (d2/E)
II. Test Cross (expected 1:1 phenotypic ratio)
Phenotype Observed # Expected Difference Difference Standardized Squared
(O) Results (E) (d) =(E – O) Squared (d2) Deviation (d2/E)
Purple
Yellow
χ2=
χ2= sum (d2/E)
III. Dihybrid Cross (expected 9:3:3:1 phenotypic ratio)
Phenotype Observed # Expected Difference Difference Standardized Squared
(O) Results (E) (d)=(E – O) Squared (d2) Deviation (d2/E)
Purple /
Smooth
Purple /
Rough
Yellow /
Smooth
Yellow /
Rough
χ2=
χ2= sum (d2/E)
4
How to Calculate Expected Values
I. Monohybrid Cross (3:1)
1. Use the Overall Total number of kernels counted.
2. Take that total and multiply it by 3/4 (0.75). This will give you the expected value for the purple
kernels.
3. Take the total from #1 and multiply it by ¼ (0.25). This will give you the expected value for the yellow
kernels.
II. Test Cross (1:1)
1. Use the Overall Total number of kernels counted.
2. Take that total and multiply it by ½ (0.5). This will give you the expected value for the purple kernels.
3. Take the total from #1 and multiply it by ½ (0.5). This will give you the expected value for the yellow
kernels.
III. Dihybrid Cross (9:3:3:1)
1. Use the Overall Total number of kernels counted.
2. Take that total and multiply it by 9/16 (0.5625). This will give you the expected value for the
purple/smooth kernels.
3. Take the total from #1 and multiply it by 3/16 (0.1875). This will give you the expected value for the
purple/rough kernels.
4. Take the total from #1 and multiply it by 3/16 (0.1875). This will give you the expected value for the
yellow/smooth kernels. This should be the same number as you calculated for # 3 above.
5. Take the total from #1 and multiply it by 1/16 (0.0625). This will give you the expected value for the
yellow/rough kernels.
