Exploring Mendelian Genetics in Corn Kernels: Lab Report

In the late 19th century, an Augustinian monk named Gregor Mendel embarked on experimental studies of inheritance, laying the foundation for the field of Genetics. Mendel, unaware of chromosomes and the basis of inheritance, formulated his observations into two fundamental laws: "The Law of Segregation" and "The Law of Independent Assortment."

Introduction

Mendelian traits are specific phenotypical characteristics. Mendel described the dominant trait, which appeared in the F1 generation and was more abundant in the F2 generation, as well as the recessive trait.

The physical appearance of an organism is referred to as its phenotype, while its genetic constitution is known as its genotype (Campbell Biology).

Mendel's first law states that "when any individual produces gametes, the two copies of a gene separate, ensuring that each gamete receives only one copy."

Mendel employed dihybrid crosses to study the inheritance of two characteristics, which he defined as "the independent assortment of different genes" (Campbell Biology).

Corn, a diploid plant, has been widely used to study Mendelian traits.

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In corn, the dominant gene "R" determines purple kernels, while the recessive gene "r" results in yellow kernels. The dominant gene "Su" represents smooth endosperm, resulting in kernels that appear hard and starchy, while the recessive phenotype produces wrinkled kernels that appear shrunken.

Materials and Methods

Monohybrid Cross and Chi-Square Test

In the monohybrid cross experiment, one pair of alleles was studied. Purple and yellow kernels on an ear of corn were counted, and the data were recorded in a table. The obtained ratio from the cross was validated using the chi-square statistical test.

Similarly, in the dihybrid cross, two pairs of alleles were studied by counting the purple/smooth, purple/wrinkled, yellow/smooth, and yellow/wrinkled kernels of the corn. Punnett squares were generated to find the F2 generation. The observations were recorded in tables, and the data were validated using the chi-square test.

Observation

Monohybrid Cross

In a monohybrid cross, a genetic cross involving a single trait, if both parents are homozygous, one will be homozygous dominant, and the other will be homozygous recessive. The first generation will be heterozygous, exhibiting the dominant parent trait. As Mendel demonstrated, the F2 offspring will follow a 3:1 ratio. This prediction was validated using the chi-square statistical test.

In corn (Zea mays), the monohybrid cross focused on the inheritance of a single gene. Purple color was determined by a dominant allele, while yellow color was recessive. Corn kernels' color pigments were found in the aleurone part of the endosperm, which could be colorless, purple, red, or yellow/white in the endosperm proper.

Monohybrid Cross - Punnett Square

The F2 phenotypic ratio should be 3:1 (Purple to Yellow), and the genotypic ratio should be 1:2:1 (RR:Rr:rr), adhering to Mendel's law.

Chi-square statistics for the monohybrid cross:

Dihybrid Cross

In a dihybrid cross, two pairs of alleles were studied. In corn kernels, purple color is produced by the presence of pigmented layers within the grains, while yellow color is visible when these layers are absent. Sweet corn grains become wrinkled upon drying, while starch grains remain smooth. The following types of corn kernels were observed in the experiment:

• Purple & Smooth (RSu)
• Purple & Shrunken (Rsu)
• Yellow & Smooth (rsu)
• Yellow & Shrunken (rsu)

Homozygous genotypes for both traits were crossed to produce offspring in the dihybrid cross:

• Purple smooth: 396
• Purple shrunken: 109
• Yellow smooth: 121
• Yellow shrunken: 41

Chi-square statistics for the dihybrid cross:

Discussion

In the monohybrid cross, the observed phenotypic ratio of 3 purple to 1 yellow seed matched Mendel's predicted ratio. The chi-square test supported this, and the null hypothesis of no difference between observed and expected values was accepted. Similarly, in the dihybrid cross, the observed phenotypic ratio of 9:3:3:1 was in line with Mendel's laws, and the chi-square value of 3.376 indicated consistency with expected outcomes.

Conclusion

This study confirms that Mendel's laws of inheritance, "The Law of Segregation" and "The Law of Independent Assortment," are applicable to the inheritance patterns of corn traits. Both monohybrid and dihybrid crosses in corn kernels exhibit the expected Mendelian ratios. Therefore, corn serves as a suitable model organism for teaching and understanding Mendelian genetics.

These findings contribute to our understanding of genetic inheritance and can be applied to various fields, including agriculture and breeding programs.