Corn Genetics

This is an experiment to prove Gregor Johann Mendels theories of dominance, segregation and independent assortment. Gregor Mendel (1822-1884) was an Austrian monk from Brunn, whose work on pea plants helped him formulate the three basic laws of inheritance the laws of Dominance, Segregation and Independent Assortment (Rhee, 2010). In 1866, he presented his first paper  Experiments on Plant Hybridization  at the Natural Science Society. His work remained unrecognized and unacknowledged for its genius until 1900. Bateson confirmed Mendels work by conducting hybridization experiments while scientists like Hugo de Vries working on Oenothera, Carl Correns working on Xenia and Erich von Tschermak-Seysenegg on pea plants drew conclusions similar to Mendel.

Phenomenon of Dominance
While conducting his experiment Mendel considered one character in pea plants and crossed plants showing contrasting characters. Tall pea plants were crossed with dwarf plants and what he observed was that in the first filial generation or F1 generation, all the plants were tall. On self-fertilizing, some plants in the F2 progeny were tall, while some dwarf. Thus, he concluded that the factor that contributed towards the tall height of the plant must be dominant to have expressed itself in the F1 generation. According to Mendel, the other character that was expressed in the F2 generation was recessive. Thus evolved the homozygous and heterozygous concepts. Each character is represented by two contrasting factors, which are now termed alleles or allelomorphs. Homozygotes have a gene represented by alleles that are identical or present in the same form whereas, heterozygotes have contrasting pair of alleles representing one gene. In the case of heterozygotes, the dominant allele expresses itself over the recessive allele and hence contributes towards the particular dominant feature in the organism. This was what Mendel discovered in his experiment with the F1 generation. In the F2 generation, the presence of homozygotes was revealed as a part of the progeny exhibited the previously hidden character of dwarf height similar to the parents of F1 progeny. Studying other characters in contrasting pairs in pea plants like seed form and position of flowers, Mendel formulated his two theories

LAW OF SEGREGATION This law states that heterozygotes of F1 generation have two contrasting characters or allelomorphs of dominant and recessive character. These remain together but do not mix. At the time of gametogenesis, they separate or segregate and each gamete receives only one allele.

LAW OF INDEPENDENT ASSORTMENT This law states that if the parents differ from each other in two or more pairs of contrasting characters, then the inheritance of one pair of factors is independent of the other pair of factors.

While considering these two laws, geneticists must keep in mind that the first law deals with monohybrid crosses or crosses in which only a single character is studied. The second law, on the other hand, deals with the expression of two or more contrasting characters and hence considers dihybrid crosses or multihybrid crosses.

We will now discuss the rationale for the use of corn in our experiment. Corn exhibits numerous pairs of contrasting phenotypes in corn kernels, which are easy to study for this experiment. There are also many kernels on a cob, hence it was easy to carry out this experiment in our classroom. And most importantly, as corn is an important source of food, it has been used as a model organism quite extensively.

For a student of genetics it is vital to know the difference between phenotypes and genotypes and their relationship.

Phenotype It is the outward manifestation of a physical character be it structure, function or behavior (Brooklyn) that is easily observable in an organism. This is influenced by the expression of genes as well as environmental factors.

Genotype On the other hand, genotype is the internal coding of inheritable genes that are expressed to sustain a living being and that are passed from one generation to the other subject to the laws of inheritance.

Mendels principles of Dominance, Segregation and Independent Assortment will be valid for eukaryotes such as corn. If Mendels principle holds up for corn, then a dihybrid cross is expected to show 9331 phenotypic ratio in the F2 generation.

The fact that Mendel had come up with the basic laws of genetics after studying pea plants, which are also eukaryotic like the maize or corn plant, should verify why the theoretical results should hold validity for the corn kernels. Also it is a known fact that each kernel on the cob has a different genotype and all the kernels on the ear of the corn are the result of crossing over of the parents. Therefore, studying the variation and their number gives us sufficient opportunity to study and support or contradict Mendelian principle of independent assortment. However, as corn is a eukoryote like Pisum sativum, the Mendelian dihybrid ratio is most likely to hold good and thus the result is expected to show a 9331 ratio.

This experiment was conducted with the purpose of studying corn genetics and comparing the theoretical and practical values from the results of this experiment using statistical methods like Chi-Square Test (X2)

The following tables record my observations and our group results. We identified and studied the corn kernels exhibiting two varying characteristics in each kernel from the given corn samples. The parent corn plants used for the dihybrid cross showed two contrasting  characters within their kernels - Purple Starchy and Yellow Sweet. The purple color is due to a layer of pigments in the aleurone represented by C and R alleles. If the layer is colorless, the yellow color of the endosperm shows through and the kernel appears yellow. Sweet corn wrinkle on drying whereas the starchy grains due to the presence of amylopectin starch remain smooth. Kernels, therefore, are purple starchy, purple sweet, yellow starchy or yellow sweet. These are counted and recorded.

Chi-square distribution, a statistical tool, is then used to  analyze the results and verify whether our hypothesis was confirmed or refuted.
Table 1  C 2  Values

 Probability (p)d. f. . of freedom  1 for monohybrid cross and 3 for dihybrid cross                                                                                                                        (APBiology, 2010)
Note This is standard format for this table and may be found in other websites and science journals.

The results of the experiment carried out in class can be explained by consulting the data as observed. Our findings during lab have been recorded in the following tables.

Table 2 Summary of Data and Calculations for Dihybrid F2 in Maize Individual data
PhenotypeObserved number (O)Expected number (E)Deviation (O-E)Purple starchy5251.750.25Purple sweet2017.252.75Yellow starchy1317.25-4.25Yellow sweet75.751.25Totals92920

Table 3 Summary of Data and Calculations for Dihybrid F2 in Maize Class data
PhenotypeObserved number (O)Expected number (E)Deviation (O-E)Purple starchy406412.88-6.88Purple sweet138137.630.38Yellow starchy132137.63-5.63Yellow sweet5845.8812.13Totals7347340

Hence, we see that the observed values deviate slightly from the expected values. But, ignoring the minor discrepancies, due to random error, or mistakes in calculation by individuals, the values have retained the expected 9331 ratio.

Table 4 Chi-Square Analysis of Dihybrid Cross Group Data
PhenotypeObs.ExpO-E(O-E)2(O-E)2EPurple starchy5251.750.250.06250.0012Purple sweet2017.252.757.56250.4436Yellow starchy1317.25-4.2518.06251.0471Yellow sweet75.751.251.56250.2717Total929201.4640

Table 5 Chi-Square Analysis of Dihybrid Cross Class Data
PhenotypeObs.ExpO-E(O-E)2(O-E)2EPurple starchy406412.875-6.87547.2700.1145Purple sweet138137.6250.3750.1410.0012Yellow starchy132137.625-5.62531.6410.2008Yellow sweet5845.87512.125147.0153.2047Total734734.0000.0003.5212

We calculated the degrees of freedom in the results obtained from the Chi-Square result. It is almost  3 which is the degree of freedom known to be for dihybrid crosses which supports our hypothesis that corn follows Mendelian genetics.

We had originally predicted that the results of the experiment conducted in the lab would confirm the hypothesis that Mendelian ratio for dihybrid cross would hold true for corn. It meant that the results obtained would show the expected phenotypic ratio of 9331. After conducting the experiment, we see that the expected ratio is achieved. When we look at table 2 and 3, we find that the phenotypic ratio of 9331 is validated by the experiment conducting by our class.

Also the data from tables 4 and 5, when compared with tables 1 shows the degree of freedom to be 3, in both cases. Degree of freedom is known to be 3 for dihybrid crosses alone. The probability, p, is non-significant varying between 0.7 and 0.3 for group and class data, respectively.

Thus, we see that the results of the experiment conducted by our class support the Mendelian ratio of 9331 for phenotypes in corn kernels from the ear of a corn.

The reasons for deviation from exact theoretical values could be due to presence of some environmental factors, some random errors due to miscalculation on the part of the individuals who were noting and calculating the results or even the fact that exact parental phenotype was not known could have played a significant role in the deviation.

Some other work done on corn is with purple color and shrunken or smooth, red color wrinkled or yellow smooth and what can be said with conviction is that the purple (CR) is a dominant allele while starchy aleurone (Su) is the dominant character as opposed to the yellow color and wrinkled or sweet kernels.


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