Data Collection and Processing
The purpose of this experiment was to determine whether the genes responsible for the color and
the texture of the corn kernels follow the Mendel’s law of independent assortment.
Two crossings occurred. First two parent corns, named P1 and P2, were crossed. P1 is recessive
for both traits (wrinkly and yellow) and P2 is dominant for both traits (black smooth). During this
cross, the offspring O1 is produced.
Cross 1:
Parent 1 (P1)
Parent (P2)
Phenotype: black smooth
Phenotype: yellow wrinkly
Genotype: BBYY
Genotype: bbyy
Possible gametes: BY
Possible gametes: by
Punnet Square of Cross 1.
by
BY
BbYy
Possible offspring genotype: BbYy
Possible offspring phenotype: black smooth
In the second the crossings two parents (P3 and P4) were crossed, both being the offsprings of the
first cross (O1) in oder to produce the second generation of offsprings (O2)
Cross 2:
Parent 1 (P3 )
Parent 2 (P4)
Genotype: BbYy
Genotype: BbYy
Phenotype: black smooth
Phenotype: black smooth
Possible gametes: By, by, bY, BY
Possible gametes: By, by, bY, BY
Punnet Square of Cross 2
By
BY
bY
by
By
BByy
BBYy
BbYy
Bbyy
BY
BBYy
BBYY
BbYY
BbYy
bY
BbYy
BbYY
bbYY
bbYy
Bbyy
BbYy
bbYy
bbyy
by
Possible offspring (O2) genotype (with ratio):
1 bbyy : 1BBYY : 1BByy : 1bbYY : 2 bbYy : 2 BBYy : 2 BbYY : 2 Bbyy : 4BbYy
Possible offspring (O2) phenotype (with ratio):
1 yellow wrinkly : 3 black wrinkly : 3 yellow smooth : 9 black smooth
Raw Data:
Table 1: The number of kernels of each of the four different phenotype (black smooth, black
wrinkled, yellow smooth and yellow wrinkled) counted by the experimenter in one corn ears
(n=1).
Phenotype
Number of kernels
(±1)
Ratio of the
phenotypes (out of 1)
Black smooth
Black wrinkly
Yellow smooth
Yellow wrinkly
Total
371
143
111
28
653
0.568
0.219
0.170
0.043
1
Ratio of the
phenotypes (out of
16)
9.088
3.504
2.720
0.686
16
Table 2: The number of kernels of each of the four different phenotype (black smooth, black
wrinkled, yellow smooth and yellow wrinkled) counted by the whole group in several corn ears
(n=9).
Phenotype
Number of kernels
(±1)
Ratio
Black smooth
Black wrinkly
Yellow smooth
Yellow wrinkly
Total
3486
911
1231
459
6087
0.572
0.150
0.202
0.075
1
Ratio of the
phenotypes (out of
16)
9.16
2.39
3.24
1.21
16
Qualitative Observations:


Some of the kernels fell off and were not counted by the testers.
A small portion of kernels were yellow with black dots. Those kernels were considered
speckled and were counted as black because they had to consist at least some of the gene
that makes the aleurone layer black.
Processed data:
Chi-Squared Significance Test
The purpose of this experiment is to determine whether the genes responsible for color and the
shape of the corn kernels follow the Mendel’s law of independent assortment.
To test whether the observed results are significantly different from the expected results the chisquared test was used. This test is appropriate because the association between two categorical
variables was tested.
The chi-squared (𝜒 2 ) can be computed using the following formula:
(𝑂 − 𝐸)2
𝜒 = ∑
𝐸
2
The chi-squared value has to be compared with the chi squared value for the expected results at
the chosen significance level and the right number of degrees of freedom, known as the critical
value.
If the chi-squared value for the observed results is less than the critical value then the null
hypothesis H0 is accepted and the alternative hypothesis H1 is rejected.
If the chi-squared value for the observed results is more than the critical value then the null
hypothesis H0 is rejected and the alternative hypothesis H1 is accepted.
𝐷𝑒𝑔𝑟𝑒𝑒𝑠 𝑜𝑓 𝑓𝑟𝑒𝑒𝑑𝑜𝑚 = (№ 𝑜𝑓 𝑐𝑙𝑎𝑠𝑠𝑒𝑠) − 1
Therefore, the number of the degrees of freedom for this experiment is 3.
For 3 degrees of freedom and standard 5% significance level the chi squared value is 7.82
The Hypothesis for this experiment are:


The null hypothesis H0: The observed results for this experiment are not significantly
different from the expected results. The genes responsible for color and the shape of the
corn kernels follow the Mendel’s law of independent assortment of genes.
The alternative hypothesis HA: The observed results for this experiment are significantly
different from the expected results. The genes responsible for color and the shape of the
corn kernels do not follow the Mendel’s law of independent assortment of genes.
Table 3: The number of the observed kernels phenotypes for the individual corn (n=1) and the
chi-squared significance test.
Phenotype
Black smooth
Black
wrinkly
Yellow
smooth
Yellow
wrinkly
Total
Number of
Kernels
counted (O)
(+/-1)
371
143
111
28
653
Ratio of the
phenotypes
observed
(out of 16)
9.088
Expected
ratio of the
phenotypes
(out of 16)
9
3.504
3
2.720
3
0.686
1
16
16
Expected
Number of
Kernels (E)
(𝑶 − 𝑬)
𝑬
367.3
0.037
122.4
122.4
40.8
653
3.453
1.068
4.022
8.581
The chi-squared value is 8,581 which is greater than 7.82. The null hypothesis H0 is rejected for
individual results.
Table 4: The number of the observed kernels phenotypes for the individual corn (n=11) and the
chi-squared significance test.
Phenotype
Black smooth
Black
wrinkly
Yellow
smooth
Yellow
wrinkly
Total
Number of
Kernels
counted (O)
(+/-1)
3857
1054
Ratio of the
phenotypes
observed
(out of 16)
9,156
2,502
1342
3,186
487
1,156
6740
16
Expected
ratio of the
phenotypes
(out of 16)
9
3
3
1
16
Expected
Number of
Kernels (E)
(𝑶 − 𝑬)
𝑬
3423.94
1141.31
54.774
6.680
1141.31
35.289
380.44
29.849
6087
126.591
The chi-squared value is 126,591 which is significantly greater than 7.82. The null hypothesis H0
is rejected as well for the group results.
Conclusion
The purpose of this experiment was to identify whether the genes responsible for the color and
the shape of the corn kernels follow the Mendel’s Law of Independent Assortment.
The corn kernels color was observed to be either black or yellow. Since the color of the
endosperm is always yellow and the pericarp is colorless, the appearing color of each kernel is
determined by the color of the aleurone layer. Aleurone is a layer of tissue around the
endosperm. It was either black, which would make the whole seep appear black, or colorless,
which would the whole kernel appear yellow. The gene for the black color was assumed to be
dominant.
However, a small portion of kernels were yellow with
black dots. The primary reason of the occurance of these
dots is result of the jumping genes. Those are the elements
that can move between the locations on the genome. 1
The corn kernels shape was observed to be either
“smooth” or “wrinkly”. Kernels that were found to be
“smooth” are likely to contain high concentration of starch
in their endosperms, while the kernels that were found to
be “wrinkly” are likely to instead contain a high
concentration of sugar in the endosperms. When the latter
die, they dehydrate and become wrinkly. The gene for the
“starchy” (or “smooth”) kernel was assumed to be
dominant and the “sugary” (or “wrinkly”) gene to be
recessive.
Since the simple dihybrid cross was performed with,
assumingly, two simple dominant-recessive pairs of genes
(Black and white; and “starchy” and “sugary”), the
expected ratio for the four phenotypes was expected ratio
expected to be 9 : 3 : 3 : 1 (where Black&Starchy :
Black&Sugary : Yellow&Smooth : Yellow&Wrinkly).
However, the observed ratios were 7.6 : 2.0 : 2.7 : 1.0 for
the group results and 13.2 : 5.0 : 4.0 : 1.0 for the individual results. After the chi-squared test was
performed the H0, which reads as “The observed results for this experiment are not significantly
different from the expected results. The genes responsible for color and the shape of the corn
kernels follow the Mendel’s law of independent assortment of genes”, was rejected.
The values of the chi-squared test for the personal and group data were, respectively, 8,581 and
126,591, which are greater than the critical value of 7.82
1
Pray, L. (2008) Transposons: The jumping genes.
Evaluation of Conclusion
There are multiple of reasons which could explain why do the genes for shape and color do not
follow the Mendel’s Law of Independent Assortment.
One of the possible explanations is the polygenic inheritance of the color of the corn kernel. The
black color of the kernel is associated with the protein called anthocyanin. It is possible that there
are multiple genes associated with its production. As was mentioned before, the color of the
aleurone is controlled by the presence of the anthocyanin, and there are more than one gene
involved in its production.
It is also possible that the genes associated with the color are transposable elements, also known
as “jumping genes”, which can move between different locations of the genome. Some kernels
appeared yellow with some black spots. They were marked as black because there is some of the
black color, then at least some of the aleurone must be black and the gene is present. 2
Another possible explanation is the epistasis of this digenic inheritance. Epistasis is the phenomenon in
which the effect of one gene is being concealed by the
effect of the other gene. A common example for this is
the gene for baldness being epistatic to the genes for
the genes of different hair colors (such as blond or red;
see pic.). 3
The ultimate and most likely explanation of the fact
that results not following the Law of Independent
Assortment is the linkage between the genes
responsible for the color and the kernel consistency.
Linked genes are a group of genes, which can be responsible for completely different
characteristics, but are carried on the same chromosome. Unless crossing over occurs right in the
location between the linked genes, the alleles will be inherited together. Therefore, the
assortment of the phenotypes will be dependent on only one chromosome of the parental
gametes. 4
2
Pray, L. (2008) Transposons: The jumping genes.
Miko, Epistasis: Gene interaction and phenotype effects
4
Allot, IB Biology
3
Evaluation of the Method
A number of errors could also occur that would affect the result of experiment.
A random error caused by the human factor could have affected the results. Any of the
experimenters could have miscount the kernels. In order to diminish the effect of this error, the
proof checks or recounts could be introduced. Additionally a complete peeling off and
subsequent counting are likely to give more accurate results due to an easier counting process.
Another random error could be associated with the age of the kernels. As was mentioned by the
instructor the corn kernels used in the experiment are bought from the Carolina Technology
Institution and are very expensive. The kernels are therefore being re-used over a certain period
of time. Despite all the delicacy experimenters treat them with, some of the kernels fall of and
are not counted by the testers. If such incident has occurred in the previous round of
experiments, the testers might not even be aware that the Kernel has originally contained more
kernels that it does now. Plastic bags should be used not only throughout the experiment to
collect and count the individual kernels that fall off, but also to maintain the kernels for the
future experimenters in order to preserve the actual proportions of the phenotypes.
Another systematic error is associated with over counting the black kernels. A pen was used in
order to mark the kernels that have been counted already. However, on the black kernels this pen
was hardly seen due to it itself being of dark color, which could possibly lead to counting the
same black kernels more than once. To avoid such problem a wipe away silver pen could be
used, or a procedure that involved a complete peel off the kernel could be executed.
Lastly, as mentioned before some kernels shared the characteristics of more than one phenotype.
For example, there were the speckled kernels which sometimes appeared to be more yellow than
black while some kernels appeared to be perfectly smooth with only one wrinkle in its structure.
The students have decided that any clear black spot is considered black, and only completely
wrinkly kernels are considered wrinkly. The misattribution of the Kernels could have
significantly affected the ratio of the phenotypes.
Bibliography
Merson-Davies, A. (2008). Student Guide for Internal Assessment in Biology. Oxford: Oxford
Study Courses.
Allott, A. (2007). IB Biology (2nd ed.). Oxford: Oxford University Press.
Chandler, V. (1989). Two Regulatory Genes of the Maize Anthocyanin Pathway Are
Homologous: Isolation of B Utilizing R Genomic Sequences. The Plant Cell Online,1175-1183.
(http://www.plantcell.org/content/1/12/1175.abstract)
Corn Dihybrid Genetics. (n.d.). Carolina BioKits.
Pray, L. (2008) Transposons: The jumping genes. Nature Education 1(1):204
(http://www.nature.com/scitable/topicpage/transposons-the-jumping-genes-518)
Pray, L. & Zhaurova, K. (2008) Barbara McClintock and the discovery of jumping genes
(transposons). Nature Education 1(1):169 (http://www.nature.com/scitable/topicpage/barbaramcclintock-and-the-discovery-of-jumping-34083)