Corn Genetics IA
Data Collection and Processing:
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. It states that during gamete formation the segregation of the alleles of one
allelic pair is independent of the segregation of the alleles of another allelic pair.1
Two parent corns were crossed, (P in Figure 1 below), one corn is recessive for both
traits (wrinkly and yellow) and the second one is homogenous dominant for both traits
(black smooth). It was instructed that back color was dominant to yellow, and that
smooth phenotype was dominant to wrinkly.
Figure 1: The figure showing two dihybrid crosses P and F1, showing the expected
genotype ratio of F2. First dihybrid cross (P) is the cross of two homozygous for color
and the shape of the kernels, ears of corn. One ear corn is yellow wrinkly (right one is
P), one ear corn is black smooth (left one in P). The second dihybrid cross is the cross
of the offspring from the first cross (F1), all offspring are heterozygous for both pairs
of genes, color and shape, black and smooth.
Phillip McClean. Mendel’ Law of Independent Assortment,
(http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel3.htm)
1
RAW DATA:
Table 1: Personal data table summarizing the number of kernels of each of four
observed phenotypes (black smooth, black wrinkled, yellow smooth, yellow
wrinkled) counted in an individual corn ear (n=1).
Phenotype
Number of kernels ±1
Ratio out of 16
Black smooth
346
9.34
Black wrinkly
44
1.18
Yellow smooth
160
4.32
Yellow wrinkly
43
1.16
Total
593
16
Table 2: Group data table summarizing the number of kernels of each of four
observed phenotypes (black smooth, black wrinkled, yellow smooth, yellow
wrinkled) counted in several corn ears (n=9).
Phenotype
Number of kernels ±1
Ratio out of 16
Black smooth
3486
9.16
Black wrinkly
911
2.39
Yellow smooth
1231
3.24
Yellow wrinkly
459
1.21
Total
6087
16
Qualitative observations:
Some of the kernels appeared to be yellow color but with some black dots. They were
classified as specky and were counted as yellow kernels.
PROCESSED DATA:
In the cross offspring, the F1 generation was produced and then they were crossed to
produce the second-generation offspring, F2. The punnet squares for both crosses are
represented below.
Cross 1:
Parent 1
Phenotype: black smooth
Genotype: BBYY
Possible gametes: BY
BY
Parent 2
Phenotype: yellow wrinkly
Genotype: bbyy
Possible gametes: by
by
BbYy
Possible offspring genotype: BbYy
Possible offspring phenotype: black smooth
Cross 2: two offspring from the Cross 1 are crossed
Parent 1
Phenotype: black smooth
Genotype: BbYy
Possible gametes: By, by, bY, BY
Parent 2
Phenotype: black smooth
Genotype: BbYy
Possible gametes: By, by, bY, BY
By
BY
bY
by
By
BByy
BBYy
BbYy
Bbyy
BY
BBYy
BBYY
BbYY
BbYy
bY
BbYy
BbYY
bbYY
bbYy
Bbyy
BbYy
bbYy
bbyy
by
F2 Offspring genotype ratio:
1 bbyy : 1 BBYY : 1 BByy : 1 bbYY : 2 bbYy : 2 BBYy : 2 BbYY : 2 Bbyy : 4BbYy
F2 Offspring phenotype ratio:
1 yellow wrinkly : 3 black wrinkly : 3 yellow smooth : 9 black smooth
If the genes responsible for the color and the shape of the kernels follow the law of
independent assortment, the observed ratio of phenotypes of the corn kernels will not
be significantly different from 9:3:3:1 expected ratio of phenotypes.
To test whether the observed results are significantly different from the expected
results the chi-squared test was performed. The chi-squared test was chosen, because
the test is performed at the categorical level for an association between two variables.
2
The chi-squared value for the observed results is calculated according to this
formula:3
𝜒2 = ∑
(𝑂 − 𝐸)2
𝐸
If the chi-squared value for the observed results is less than the chi-squared value for
the expected results at the chosen significance level and the right number of degrees
of freedom for the experiment, 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 chi-squared value for
the expected results at the chosen significance level and the right number of degrees
of freedom for the experiment, then the null hypothesis H0 is rejected and the
alternative hypothesis H1 is accepted.
The number of degrees of freedom for this experiment is 3. (Degrees of freedom =
number of classes –1)
For 3 df and 5% significance level the chi squared value is 7.82
The null hypothesis H0: The observed results for this experiment are not significantly
different from the expected results.
The alternative hypothesis H1: The observed results for this experiment are
significantly different from the expected results.
2
3
Merson-Davies. Student Guide for Internal Assessment In Biology
Merson-Davies. Student Guide for Internal Assessment In Biology
Table 3: Personal data table summarizing the expected values and chi-squared values
of the four observed phenotypes (black smooth, black wrinkled, yellow smooth,
yellow wrinkled) of an individual corn ear (n=9).
Phenotype
Expected ratio
Expected value from
the total number of
kernels of 593
(E-O)2/E
Black smooth
9
334
0.431
Black wrinkly
3
111
40.441
Yellow smooth
3
111
21.631
Yellow wrinkly
1
37
0.973
Total
16
593
63.476
The chi-squared value is 63.476 which is greater than 7.82; therefore, the null
hypothesis H0 for individual results is rejected.
Table 4. Group data table summarizing the expected values and chi-squared values of
the four observed phenotypes (black smooth, black wrinkled, yellow smooth, yellow
wrinkled) of an group corn ears (n=1).
Phenotype
Expected ratio
Black smooth
9
Expected value from
the total number of
kernels of 6087
3424
Black wrinkly
3
1141
46.363
Yellow smooth
3
1141
7.099
Yellow wrinkly
1
380
16.424
Total
16
6087
71.008
(E-O)2/E
1.123
The chi-squared value is 71.008 which is greater than 7.82; therefore, the null
hypothesis H0 for group results is rejected.
The null hypothesis H0 is rejected for both individual and group results; therefore,
there is a significant difference between the expected and the observed results. So, the
genes responsible for color and the shape of the corn kernels do not follow the
Mendel’s law of independent assortment of genes.
Conclusion and Evaluation:
Conclusion:
The purpose of the experiment was to identify whether the genes responsible for color
and the shape or the texture of the corn kernels follow Mendel’s Law of Independent
Assortment. The corn kernels were represented in two colors: yellow and black. Since
the endosperm is always yellow and pericarp is colorless, the color of the kernel came
from the color of the aleurone layer, the tissue around the endosperm. 4 It was either
black or colorless. The gene for black color was assumed to be dominant. Corn
kernels had to two observable textures: smooth and wrinkly. When the corn kernels
that are high in sugars die, lose water and kernels wrinkle. Sugary trait is recessive, so
the wrinkly phenotype or ‘sugary’ was assumed to be recessive. The other kernels that
appeared smooth contained starch in their endosperms, and the genes for starch in the
endosperm is dominant. In other words,
‘starchy’ genes are dominant, and ‘sugary’ are
recessive. 5
Since the dihybrid cross was performed, with
two simple dominant-recessive pairs of genes,
the expected ratio for the four combinations of
phenotypes (black smooth, black wrinkly,
yellow smooth, yellow wrinkly) was 9:3:3:1.
The observed ratios for the group were 9.16 :
4
5
Corn Dihybrid Genetics. (n.d.). Carolina BioKits.
Corn Dihybrid Genetics. (n.d.). Carolina BioKits.
2.39 : 3.24 : 1.21, and for the individual results were 9.34 : 1.18 : 4.32 : 1.16. The
observed values were compared with expected and tested for random variability with
the chi-squared test to eliminate the possibility of the random error.
The null hypothesis H0: The observed results for this experiment are not significantly
different from the expected results.
The alternative hypothesis H1: The observed results for this experiment are
significantly different from the expected results.
The null hypothesis H0 was rejected for both individual and group results; therefore,
there is a significant difference between the expected and the observed results. So, the
genes responsible for color and the shape of the corn kernels did not follow the
Mendel’s law of independent assortment of genes.
Nevertheless, in one of the corn ears examined the observed ratio of the same
phenotypes was 8:3:3:1 and the experimenter accepted the null hypothesis for the
experiment, since the chi-squared value was less than the critical value. Therefore,
one of the experimenter’s individual data supported the hypothesis that color and the
texture of the corn kernels follow Mendel’s Law of Independent Assortment.
Explanation of the Conclusion:
There are several reasons that can explain the fact that the genes for the color and the
shape of the corn kernels do not follow Mendel’s Law of Independent Assortment.
One possible explanation is that the color of corn kernels was influenced by more
than one gene. 6 The color of the aleurone is controlled by the protein called
anthocyanin; it makes the aleurone black. However, there are multiple genes involved
in the anthocyanin production, R and B regulatory genes.7
There is a possibility that some genes are transposable elements or ‘jumping genes’
moving between locations on the genome. Some genes may move between loci and
thus interfere with the proper gene expression. Also, it is possible that the
transposable elements interfere with the genes responsible for the production of the
anthocyanin.8 Corn was the species in which the whole phenomena of transposable
elements were discovered by geneticist Barbara McClintock. 9
Another possibility for explaining the unexpected ratio is the epistasis of this di-genic
inheritance. Epistasis is the process by which genes interact with each other and
produce an entirely different trait.10 In other words, depending on the mechanism and
the loci of the genes, the phenotypes can be combined in an interaction at the
phenotypic level of organization to produce a third outcome. It was noted by the
experimenter’s that some kernels appeared yellow with some black spots. It is
possible that the anthocyanin production was inhibited in the epistasis by some other
genes or certain combination of genes.
6
Miko, Epistasis: Gene interaction and phenotype effects
Chandler, Two Regulatory Genes of the Maize Anthocyanin Pathway Are
Homologous: Isolation of B Utilizing R Genomic Sequences
8
Pray, The Jumping Genes
9
Pray & Zhaurova, Barbara McClintcok and the discovery of jumping genes
(transposons)
10
Miko, Epistasis: Gene Interaction and phenotype effects
7
Last explanation for the results rejecting the Law of Independent Assortment is the
possibility that the genes for color and kernel consistency were linked.11 Linked genes
are pairs of groups of gene, which are inherited together, carried on the same
chromosome. In other words, unless crossing over occurs precisely between the loci
of linked genes, the alleles are inherited together as a pair, which means the
assortment of the phenotypes in the offspring of the di-genic inheritance will be
dependent on only one chromosome of the parental gametes.12
A random human error due to the miscount of the kernels in this experiment is also a
possibility. An experimenter could have simply miscounted the number of kernels.
However, to avoid this error, proof checks by other people should be done in the
investigations involving corn kernel count.
The main systematic difficulty that was encountered in this experiment by the
experimenters is counting the black corn kernels. There was no such problem with
yellow ones, because a pen was used for marking the yellow counted kernels.
However, the black ones were hard to be marked. It resulted in several losses of
counts of the black rough and black smooth kernels. To avoid such problem the
counted black kernels can be marked with little pieces of ducktape, or with small
Post-It® notes. Another suggestion for marking the black kernels is a wipe away
silver pen.
Another problem that was encountered by the experimenters is the fact that corn ears
were very dry, hence they needed to be treated with great care so that no kernels were
11
12
Corn Dihybrid Genetics. (n.d.). Carolina BioKits.
Allot, IB Biology
lost, which results in a inaccurate data. As an improvement, it is possible to peel off
the ears completely, and then count the kernels; however, the ears are planned to be
used in further experiments after this one. Therefore, it is simpler to perform all the
experiments above the plastic bag and eventually count the kernels that fell of the
corn ear.
As is it was mentioned above some corn kernels were yellow with black dots, they
were called ‘specky’. In order to provide consistency and avoid a systematic error
because different experimenters can subjectively judge the color of a specky cornel,
all the specky corn kernels should be designated into the yellow or black category by
the same person. Therefore, random variability due to disparate judgment by different
experimenters is avoided.
References
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.
Merson-Davies, A. (2008). Student Guide for Internal Assessment In Biology.
Oxford: Oxford Study Courses.
Miko, I. (2008) Epistasis: Gene interaction and phenotype effects. Nature
Education 1(1):197 (http://www.nature.com/scitable/topicpage/epistasis-geneinteraction-and-phenotype-effects-460)
Phillip McClean. (2000) Mendel’ Law of Independent Assortment,
(http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel3.htm)
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/barbara-mcclintock-and-the-discovery-ofjumping-34083)