Corn Genetics IAData 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.

1 Phillip McClean. Mendel’ Law of Independent Assortment, (http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel3.htm)

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 16Black smooth 346 9.34

Black wrinkly 44 1.18Yellow smooth 160 4.32

Yellow wrinkly 43 1.16Total 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 16Black smooth 3486 9.16

Black wrinkly 911 2.39Yellow smooth 1231 3.24

Yellow wrinkly 459 1.21Total 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: BBYYPossible gametes: BY

byBY BbYy

Possible offspring genotype: BbYyPossible offspring phenotype: black smooth

Cross 2: two offspring from the Cross 1 are crossed

Parent 1 Phenotype: black smoothGenotype: BbYyPossible gametes: By, by, bY, BY

By BY bY by

By BByy BBYy BbYy Bbyy

BY BBYy BBYY BbYY BbYy

bY BbYy BbYY bbYY bbYy

by Bbyy BbYy bbYy bbyy

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

Parent 2Phenotype: yellow wrinklyGenotype: bbyyPossible gametes: by

Parent 2Phenotype: black smoothGenotype: BbYyPossible gametes: By, by, bY, BY

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)E

2

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 Merson-Davies. Student Guide for Internal Assessment In Biology3 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 ratioExpected 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 ratioExpected value from the total number of

kernels of 6087(E-O)2/E

Black smooth 9 3424 1.123Black wrinkly 3 1141 46.363Yellow smooth 3 1141 7.099Yellow wrinkly 1 380 16.424

Total 16 6087 71.008

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 :

2.39 : 3.24 : 1.21, and for the individual results were 9.34 : 1.18 : 4.32 : 1.16. The 4 Corn Dihybrid Genetics. (n.d.). Carolina BioKits.5 Corn Dihybrid Genetics. (n.d.). Carolina BioKits.

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 6 Miko, Epistasis: Gene interaction and phenotype effects

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.

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 7 Chandler, Two Regulatory Genes of the Maize Anthocyanin Pathway Are Homologous: Isolation of B Utilizing R Genomic Sequences8 Pray, The Jumping Genes9 Pray & Zhaurova, Barbara McClintcok and the discovery of jumping genes (transposons)10 Miko, Epistasis: Gene Interaction and phenotype effects11 Corn Dihybrid Genetics. (n.d.). Carolina BioKits.

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

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

12 Allot, IB Biology

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-gene-interaction-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-of-jumping-34083)