Mendelian Genetics: Its Behavior on Two Diffeent Genes for Chlorophyll Production in Tobacco(nicotiana Tabacum) Seeds

Abstract

According to MendelÐŽ¦s Law of Segregation, phenotypic ratios may be influenced by dominance of one allele compared to another. When an organism produces its gametes by meiosis, the alleles separate. This is MendelÐŽ¦s First Law-the Law of Segregation. This experiment investigated the effects of complete and incomplete dominance on the behavior of two different genes for chlorophyll production in tobacco. Two sets of approximately 50-100 tobacco seeds were planted for one week. The plants were the F2 result of two sets of monohybrid F1 crosses Gg x Gg and CyCg x CyCg. A chromosome for these plants has two sets of genes. The gene at one locus has two alleles that code for normal chlorophyll production. Dominant allele represented as G produce the green phenotype. The mutant allele, g, produces no chlorophyll producing an albino phenotype. Therefore genotypes GG and Gg phenotypically are green. When mutant allele g is homozygous for it, gg, no chlorophyll is present and the plants phenotypically are albino. Therefore an expected phenotypic ratio would be 3:1. Another gene at a different locus also affects chlorophyll production. This gene also has two sets of alleles, but in this case the mutant of this allele, Cy, exhibits incomplete dominance with the other. Genotype CgCg is phenotypically green. Genotype CyCy is phenotypically yellow and genotype CyCg is phenotypically green-yellow. This would suggest a phenotypic ratio of 1:2:1. In the F2 generation of the first set, Gg x Gg, green plant color was observed more than the albino color. In the F2 generation of the second set, CyCg x CyCg, green and green-yellow were noticed more than yellow. The observations made suggest that in the first set of F1generation the null hypothesis cannot be rejected. As for observations for the second set of F1 generation, the null hypothesis can be rejected.

Introduction

Gregor Mendel pioneered modern genetics. His most famous analyses were based on clear-cut traits with simple dominance (Campbell & Reece, 2005). Genes contain information about a specific characteristic or trait and can either be dominant or recessive (Winshell, 2004). Not all copies of a gene are identical and alternative forms of a gene, called alleles, lead to the alternative form of a trait.(Winshell,2004). Alleles are a way of identifying the two members of a gene, which produce opposite, distinct phenotypes (Dolphin, 2005). When the alleles are identical, the individual is a homozygous for that trait; if the pair is made of two different alleles the individual is heterozygous. A homozygous pair can be either dominant (GG) or recessive (gg). Heterozygous pairs are made up of one dominant and one recessive allele (Gg). In heterozygous individuals, only one allele, the dominant, gain expression while the other allele, the recessive, is hidden but still present. Using a punnett square, in a F1 generation of the cross of a homozygous dominant allele with a homozygous recessive allele, 100% heterozygous genotype with 100% dominant phenotype would appear (Dolphin, 2005). According to Mendel and the punnett square, when the F1 generation is allowed to self pollinate, there would be a 3:1 phenotypical ratio with genotypes GG, Gg,gg (Winshell,2004). In simple or complete dominance, the heterozygote, even though genetically different, has the exact same phenotype as the dominant homozygote.

A form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in combined phenotype, is called incomplete dominance (Dolphin, 2005). The heterozygote shows a phenotype which is in between the homozygous recessive and homozygous dominant phenotypes. If the F1 generation is allowed to self-pollinate in this case three genotypes would appear CyCy, CyCg and CgCg (Winshell, 2004). The results would produce three phenotypes, yellow, green-yellow and green with a 1:2:1 phenotypical ratio.

The purpose of this study was to see the behavior of two different genes for chlorophyll production in tobacco seedlings. This experiment investigated the F2 generation results from two monohybrid crosses, Gg x Gg and CyCg x CyCg. A null hypothesis can be proposed, Ho, the phenotypical results of the F2 generation of the crosses will adhere to MendelÐŽ¦s Law of Segregation. There will be a phenotypical ratio of 3:1, green to albino phenotypes with the cross Gg x Gg. There will be a phenotypical ratio of 1:2:1, green to green-yellow to yellow with cross CyCg x CyCg. As an alternative hypothesis Ha, the phenotypical ratio will be far from the predicted ratios of 3:1 and 1:2:1.

Material and Methods

As outlined in Introduction to Mendelian Genetics Using Tobacco Seddlings, General Biology 101 Laboratory/Discussion Group Supplement, Winshell 2004 one week prior to observations 50-100 seeds were planted from a F1 generation Gg x Gg, resulting in a F2 generation. Another 50-100 seeds of a different F1 generation CyCg x CyCg were planted resulting in its F2 generation. All were planted on moistened filter paper in two separate Petri dishes and germinated in the light at a temperature between 15ÑžX-22ÑžX C. Using a dissecting microscope the seedlings were classified by color. They then were removed by forceps to prevent counting the same seedling twice. A chi-square test was then calculated for each individual cross observation to evaluate a null hypothesis.

Results

Table 1 illustrates the observations made from a sample of 26 plants. A visible difference can be

seen between the phenotypes green and albino. Nearly a 3:1 phenotype ratio can be observed

with this size sample. According to Table 1A, the chi-square in the single group observation is less than the critical value at 95% within 1 degree of freedom. Table 2 was comprised of 59 observations from the cross CyCg x CyCg that produced the three expected colors, green, yellow and green-yellow. Green and green-yellow phenotypes were equal and greater than the yellow phenotype. Thus, from Table 2A, the expected critical value was less than the calculated critical value in the experiment. An expected ratio of 1:2:1 does not seem to fit the findings. Table 3 and table 4 are class totals of a 58-plant sample and a 143-plant sample. The results are of the same crosses, Gg x Gg and CyCg x CyCg, with their respective phenotypic results and chi-square calculations.

The seeds planted last week are the offspring of two monohybrid crosses.

Cross 1

...