Our PBL Project
Introduction
Our project will investigate the how different genetic crosses can produce different genotypes and phenotypes. We will research genotypes and phenotypes, flip coins to simulate the passing of parents' genes on to their offspring, and create diagrams displaying and explaining the inheritance of genetic disorders. The driving question is "How do genetic crosses provide information about inheritance?" Our hypothesis is "If two parents produce an offspring and pass on their genetic material through meiosis, then the phenotype of the offspring depends on which genes are dominant or recessive and whether or not the trait is sex-linked."
Materials
2 coins (to flip and simulate the random passing of alleles from each parent to the offspring)
Writing materials (to label the coins and to illustrate pedigrees and principles of heredity)
Computer with printer (to research important concepts, type them up, print out the guide, and find real-life examples of the inheritance of genetic disorders)
Procedure
Thoroughly research (deeper than what was done in the background research) Gregor Mendel, his experiments, the Punnett Square, the laws and principles of heredity, genotype and phenotype, dominant and recessive traits, generations of offspring, and meiosis.
Type up all research.
Print out the handout from biologycorner.com to use as a guide and carefully read it.
Make changes to the handout as needed, including changing the phenotype (printed as short/long toe) to purple/non-purple plants.
Recognize/Acknowledge that flipping two coins represents meiosis. It signifies crossing-over, random assortment, and random fertilization. T represents the dominant purple stem allele, and t is the recessive non-purple stem allele.
Complete the Punnett square in order to see the theoretical results of the offspring's genotypes. Determine the ratio of purple stems to non-purple stems.
Each team member obtains one coin.
Both of team members flip their coins at the same time.
Record the data as a tally mark in the table similar to the one provided in the handout. Heads counts as T, while tails is t.
Repeat the previous 2 steps 49 more times for a total of 100 coin flips and the genotypes of 50 offspring as the result.
Multiply each of the results in the 3 categories by 2 to see the percentage.
Add up the number TT and Tt genotypes, since T is dominant, and any offspring with this genetic makeup will have purple stems.
Calculate the ratio of purple stems to non-purple stems.
Compare the experimental ratio to the theoretical ratio.
Connect the results of the experiment back to the research, including the parts about cell division and the accuracy of the Punnett square.
Repeat steps 7-15 for genetic outcomes for Tt and tt. Only one coin needs to be flipped, since the tt coin will have the same outcome no matter which side of the coin lands facing upward.
Use the Punnett square to predict the genetic outcomes for TT and Tt.
Research 3 different genetic conditions (PTC non-tasters -- autosomal recessive, hypercholesterolemia -- autosomal dominant, hemophilia -- x-linked recessive) and describe them, along with their causes and patterns of inheritance (including whether or not they are sex-linked).
Find case studies of each conditions and record all findings.
Draw out pedigrees for each genetic condition that show how each one was passed down through at least 3 generations.
Safety
Maintain a clean and neat environment while flipping the coins.
Scientific Principle
Our project is tied to the work of Gregor Mendel, which illustrates the laws of inheritance. When we flip a coin, we are stimulating the process of meiosis, when each parent passes on half of his or her chromosomes to the offspring during the process of sexual reproduction.
Phenotype/Genotype:
TT - Purple Stem
Tt - Purple Stem
tt - Non-Purple Stem
Data Tables
First Experiment (Tt and Tt)
Phenotypes
Purple (TT+Tt)
Non-Purple (tt)
Total
34
16
Second Experiment (Tt and tt)
Genotypes/Phenotypes
Tt (Purple)
tt (Non-Purple)
Number of Flips
24
26
Graphs
Observations
For the first experiment, the simulated cross was between parent plants with genotypes Tt and Tt. When the Punnett Square was drawn, it was clear that out of the four squares, one was TT, two were Tt, and one was tt. This meant that, in theory, 75% of the offspring would have purple stems and the other 25% would have non-purple stems, despite both parents' stems being purple. When the two pennies were flipped fifty times each, however, there were 11 cases of both heads, 23 cases of a head and a tail, and 16 cases of both tails. This translated to 68% purple stem and 32% non-purple stem. Although this ratio was not exactly the same as the predicted ones, it was close.
In the second experiment, the same process was repeated for parents whose genetic makeups were Tt and tt - one heterozygous purple stem and one non-purple stem (which must be homozygous since this allele is recessive). As seen in the Punnett Square, about 50% of the offspring would be heterozygous purple stem, and the other 50% would be non-purple stem. This time, there is no TT possibility. With the coin toss, 48% acquired purple stems and 52% had non-purple. Again, the actual ratios were similar, though not identical, to the predicted.
Pedigrees Created
Analysis and Discussion
Since it is very rare that anything can be exact, it is understandable that the predicted and actual ratios were close to each other, but not the exact same. The more offspring simulated, the closer the predicted and actual ratios will become. From the coin flip experiments and our research, we were able to see how dominant traits often control the phenotypes but are unable to completely wipe out the recessive alleles. If this experiment were to be conducted again, it could be improved by testing out more parents' genotypes and doing more trials to get a clearer picture of how close the experimental results were to the predicted results.
Conclusion
Our hypothesis was correct because it is usually possible to predict the phenotype of the offspring using the parents' genetic information. The knowledge of which genes are dominant or recessive and whether or not the trait is sex-linked is also required. From the two experiments that we conducted and the three pedigrees that we drew, we learned about the patterns of inheritance and how they affect the genotype of an organism. Dominant genes tend to cover up recessive ones, but once crossed with the right genes, the recessive phenotype will be show again. The Punnett Square can be used to find the approximate ratio of phenotypes and genotypes, and the type of genetic disorder can be determined using a pedigree. Even if the genotype is unknown, it is possible to figure it out by using careful genetic crosses.
Real Life Connection/Further Applications
There are numerous ways that people use genetics to make their lives better. Genetic testing is an increasingly popular method of getting to know what diseases or conditions a person may contract within his or her lifetime. It used to cost thousands of dollars, but now simple genetic tests can be conducted for less than $100. Scientists and researchers use patients' chromosomes or genes to try to find variations or mutations of a normal person's DNA. This can lead to the cure of some genetic disorders and the prevention of many others. In the future, we will hopefully be able to do more with genetic mapping than only giving someone a percentage as to how likely he or she is to acquire a certain disease.
Although genetic mapping is expensive and, in some cases, doesn't solve anything, a much easier way for people to determine what physical traits their children might have is through the Punnett Square. Complex Punnett Squares can predict many different combinations of traits through many different grids. Anyone can learn how to manipulate them, and after a few attempts, it becomes a simple and fun method of finding out genotypes through phenotypes. Blood typing, which was often used before DNA testing was possible, is also done through this method. All a person needs to know is which traits are dominant and which traits are recessive, along with the genotypes of both parents.
Reflection Questions
In genetics experiments, what is the "F1" generation? What is the "F2" generation?
The F1 generation is the children of the original crosses. The F2 generation is the grandchildren, or the children of the F1 generation. This F2 generation often displays the 3:1 ratio.
How can the dominant form of a trait be determined?
Experiment with several crosses between two different phenotypes for the same trait and see which phenotype appears the most in the next generation. That phenotype probably contains the dominant genotype.
Was stem color the phenotype or genotype of the plant? What is the relationship between phenotype and genotype?
The stem color was the phenotype of the plant, since it was the visible outcome. The genotype determines the phenotype, but the genotype cannot always be determined through phenotype because of dominant and recessive alleles.
In Mendel's monohybrid crosses, he often found the phenotypic ratio in the F2 generation to be 3:1, three plants with the dominant genotype to every one plant displaying the recessive phenotype. Why did the F2 generation show a 3:1 ratio?
The two plants had homozygous alleles, but one had two dominant alleles and the other had two recessive ones. Because of this, all four offspring in the F1 generation (according to the Punnett Square) would be heterozygous, with a phenotype of the dominant trait. If one of these offspring were to be crossed with another heterozygous-alleled plant, the result would be similar to the Punnett Square that we created for our first coin flip experiment (see above). 25% of these offspring would be homozygous dominant, 50% would be heterozygous dominant, and the remaining 25% would contain the recessive alleles (which must be homozygous). These are only the genotypes, however. 75% of the F2 generation would display the dominant trait, while the recessive trait would be seen in 25%. This forms a phenotypic ratio of 3:1.
A key to Gregor Mendel's success in discovering aspects of inheritance was his careful control of pollination, so he was always certain of the parentage of the offspring plants. How did Mendel control pollination?
Pea plants are usually self-pollinating, which means that the pollen from the anthers (male part) of one plant are transferred to the stigma (female portion) of the same plant. Mendel, however, wanted to see what the offspring of two phenotypically different plants would look like. He had to prevent the pea plants from pollinating the natural way. The scientist removed the anthers from the flowers of the plants that he was experimenting with and pollinated each one by hand with pollen from other parent plants that he chose.
Suppose you have a purple stem plant, but you don't know the plant's genotype. How can a test cross reveal the plant's genotype?
The easiest method would be to cross the plant with a non-purple stem plant, preferably many times. If all the offspring always have purple stems, then the original plant was homozygous purple. However, if approximately half of the offspring contain non-purple stems, it can be concluded that the original plant was heterozygous purple.
At the DNA level, what is the difference between the purple stem allele and the non-purple stem allele? How does DNA determine whether a color (pigment) is present in a plant?
The very few genome variations at the DNA level account for the differences between certain traits in plants. These mutations occur during sexual reproduction and determine what each organism's phenotype will be.
Do all traits show "Mendelian inheritance"? If yes, explain why. If no, describe some examples and explain why the inheritance is not Mendelian.
No, not all traits show "Mendelian inheritance" because some are sex-linked, such as the hemophilia that we illustrated in our pedigrees. These conditions are more likely to be found in one gender than in the other.