The basic foundation of modern genetics was led by Gregor Mendel (Corcos, 1993). Mendel was not the first to experiment with heredity, and our Lyman Briggs biology class will not be the last to deal with genetics. Genetics is the science of heredity. In our lab, we had three main objectives. First, we evaluated our data on monohybrid and dihybrid corn cross seed counts against Mendel's theoretical expectations of independent assortment and the segregation of alleles. Next, we used the Hardy-Weinberg Theorem to provide a theoretically expected value for allele frequencies for single human gene traits. Lastly, we dealt with Drosophila melanogaster and we examined red and white eye alleles to determine if this gene is sex-linked or autosomal.
During the mid 1800's Mendel bred garden peas to study inheritance. He choose these plants because of their well defined characteristics and the ability to be grown and crossed (Campbell, 1996). Mendel wanted to know the genetic basis for variation among individuals and what accounted for the transmission of traits from generation to generation. Mendel followed traits for the P generation, F1 generation, and F2 generation. The P generation is the original true-breeding parents. Their hybrid offspring is the F1 generation, the first filial. The F2 generation is the second filial and is the self- pollination of the F1 hybrids. It was predominantly his research on the F2 generation that led to Mendel's Law of Segregation and Law of Independent Assortment (Campbell, 1996).
Mendel's Law of Segregation states that alleles sort into separate gametes. He formed this through performing monohybrid crosses. The F2 generation will have a 3:1 phenotypic ratio. By considering more than one trait Mendel formed his Law of Independent Assortment. He questioned whether traits were inherited independently or dependently. By performing dihybrid crosses he found that genes are independent and will form all possible combinations . Crossing two different traits resulted in a 9:3:3:1 phenotypic ratio (Campbell, 1996).
Thomas Hunt Morgan also had a major contribution in the study of inheritance. He was the first to associate a specific gene with a specific chromosome. Morgan used Drosophila melanogaster, which are commonly known as fruit flies. These were a good choose because they are prolific breeders, and they only have four pairs of chromosomes (Davis, 1996). Morgan linked a fly's eye color to its sex. He found that females carry two copies of this gene, while the male only carries one . Morgan's work also led to a new, more wildly used way for symbolizing alleles (Campbell, 1996).
Materials and Methods
Materials and methods were as per Davis (1996). For the corn cross lab, corn was counted off of the ears of the corn, rather than through jars. For the human characteristics between 143 to 149 students were observed. Seven different single human gene traits were considered for this lab. The fruit fly cross was set up on September 24, 1996. The parental (P) generation begun with ten red-eyed males and six white-eyed females. The parent flies were removed on October 3, 1996. Data collection was stopped on October 10, 1996.
A punnett square was used for the monohybrid corn cross to find the genotypes of the potential offspring. The gamete combinations were Su=smooth seeds with an observed value of 497, and an expected value of 451.5; and su= wrinkled seeds with an observed value of 105, and an expected value of 150.5. The chi-squared value was 18.35, this value didn't correspond with any of the given probability values. The Null hypothesis with a 3:1 phenotypic ratio was rejected (see figure 1). Hence, the observed number of smooth seeds versus wrinkled seeds is not different from the expected 3:1 ratio for a monohybrid cross (see table 1).
A dihybrid cross was used to find the genotypes of potential offspring regarding two traits. The offspring possibilities were SuP=smooth, purple seeds with an observed value of 577 and an expected value of 617.6; Sup=smooth, yellow seeds with an observed value of 229 and an expected value of 205.9; suP=wrinkled, purple seeds with an observed value of 210 and an expected value of 205.9; and sup=wrinkled, yellow seeds with an observed value of 68.6. The chi-squared value was 7.96, and the Null hypothesis was again rejected. The Null hypothesis stated that the observed number of smooth, purple seeds versus smooth, yellow seeds versus wrinkled, purple seeds versus wrinkled, yellow seeds is not different than the expected ratio of 9:3:3:1 for a dihybrid cross (see table 2 and figure 2).
For the single gene human traits, between 143 to 149 students were observed. The amount of data for each of the seven different traits varied. One trend was that six out of the seven traits had a higher frequency for the recessive allele (see table 3).
In the fruit fly cross, a total of 99 fruit flies were collected. Forty-four white-eyed males with a phenotype of X^wY, and forty-five red-eyed females with a phenotype of X^+X^w. The phenotypes of the flies were the cross between a red-eyed male, X^+Y, and a white-eyed females, X^wX^w (see table 4).
Data for the monohybrid cross did not correspond with the expected values. The monohybrid phenotypic ratio of 3 smooth seeds versus 1 wrinkled seed is derived from the punnett square (see table 1). My observed values were 497 smooth seeds and 105 wrinkled seeds. Their expected values were 451.5 smooth seeds and 150.5 wrinkled seeds (see figure 1). The chi-squared value was used to interpret data, and the value for chi-squared was too high. Therefore, it was rejected (see figure 1). This test can be used to see how well data fits a theoretical exception. The expected frequency can be found by multiplying the punnett square phenotypic ratios by the amount of corn counted. The chi-squared number was found to be 18.35 and was then compared to the probability chart (Davis, 1996). The probability value must be greater than 0.05 to accept the Null hypothesis. The Null hypothesis was rejected since there was not a 3:1 ratio.
The dihybrid cross also rejected the Null hypothesis. The observed and expected values differed for smooth & purple seeds, smooth & yellow seeds, wrinkled & purple seeds, and wrinkled & yellow seeds (see figure 2). The chi- squared value was calculated the same way as the monohybrid cross, with four different traits rather than just two. The Null hypothesis expected a 9:3:3:1 ratio, which it did not have.
Both the monohybrid and dihybrid crosses had chi-squaered values that were too high. Therefore, both Null hypothesis' were rejected. This may have been due to an observational error. The kernels may have been miscounted or interpreted wrong.
The monohybrid corn cross illustrated Mendel's Law of Segregation, and Mendel's Law of Independent Assortment was demonstrated by the dihybrid corn cross. The understanding of meiosis is central to an understanding in genetics. Meiosis is a process consisting of two consecutive cell divisions, called meiosis I and meiosis II, resulting in four daughter cells, each with only half as many chromosomes as the parent (Campbell, 1996). Genetics is the science of heredity. Therefore, in understanding the process of meiosis, we learn how it is that we acquire our own unique set of genes.
By conducting the single gene human trait experiment, phenotypes of a given list of traits were determined by individual students. A set of possible genotypes for these given traits were also found (see table 3).
The allele frequency is a numerical value of the dominant or recessive allele appears. We find these values by using the Hardy-Weinberg equation. One significant item was the frequency of alleles of the students in our class. The recessive allele had a higher frequency for six out of the seven traits considered. The traits that followed this trend were: the bent pinky, eye color, widow's peak, thumb crossing, ear lobe, and the hitchhhiker's thumb. Allele frequency is found by using the Hardy-Weinberg equation (see figure 3). It is a number that tells you how often that allele, dominant or recessive appears. The tendency for the recessive allele to have a higher frequency may have to do with natural selection. On the other hand, the phenotype frequency differs from the allele frequency because the phenotype is made of two alleles and the heterozygous genes consist of both alleles.
I could only definitely determine two of my genotypes for this experiment. They are not having a bent pinky, bb,and not having a widow's peak, ww. This can be determined because both of these traits are homozygous recessive (see table 3). I don't know whether or not the rest of my single human gene traits are homozygous dominant or heterozygous.
To decipher the genotype of the fruit flies a punnett square was used (see table 4). The fruit fly cross started with ten red-eyed males and six white-eyed females. Their offspring resulted with red-eyed females and white- eyed males. The experiment resulted in the way we expected. Red eyes are the dominant allele, with forty-five counted female offspring and the recessive allele, white eyes had forty-four males counted. These traits appeared in the opposite sex then in the P generation. Therefore, the red/white eye allele is sex-linked among fruit flies. Hence, red eyes is the dominant allele, the female receives this from the male parent, the male then picks up the white eyes (see table 4).
Through the genetics lab we observed many different concepts. Through our evaluation of the monohybrid and dihybrid corn cross seed counts we connected them to Mendel's Laws of Independent Assortment and the Segregation of Alleles. Using the monohybrid cross, Mendel's Law of Segregation was described by portraying the 3:1 phenotypic ratio in the first filial, F1 generation. While using the dihybrid corn cross, Mendel's Law of Independent Assortment illustrated that four possible phenotypes form a 9:3:3:1 phenotypic ratio. For the single gene human traits experiment, we used the Hardy-Weinberg Theorem and equation to find the allele frequencies. For the experiment with Drosophila melanogaster we examined a fruit fly cross between red-eyed males and white-eyed females. We determined that this trait is sex-linked when the offspring were red-eyed females and white-eyed males. Throughout the genetics lab each purpose was determined and explained. A lot was learned about Mendel, genetics, and the hereditary process that makes us who we are today.
Campbell, N.A. 1996. Biology. The Benjamin Cummings Publishing Co., New York, pp. 238-279.
Corcos, Alain F. and Floyd V. Monaghan. 1993. Gregor Mendel's Experiments on Plant Hybrids. Rutgers University Press, New Jersey, pp. 45-46, 76, 105-112, 133.
Davis, M. 1996. Genetics. LBS 144 Laboratory Manual. The Lyman Briggs School, Michigan State University, East Lansing, pp. 25-36.