This is the first in a series of units dealing with genetics. In this unit we study "Mendelian" or "Classical" genetics. Classical genetics involves the study of the patterns of inheritance of characteristics from one generation to the next; the Laws of Genetics which Gregor Mendel established in the mid 1800's. Although Mendel knew nothing about genes, chromosomes, or meiosis when he conducted his experiments in the garden of his monastery, we can now relate his results and conclusions to the movement of genes and chromosomes in meiosis as gametes are produced. Geneticists have made many important advances in our understanding of the inheritance of characteristics and the role of chromosomes and genes, but Mendel's fundamental laws still form the foundation of modern genetics. In subsequent units we will study the molecular underpinning of classical genetics (DNA, RNA, protein synthesis) and the technological advances in molecular biology that have so changed our lives and have even greater potential in the near future (recombinant DNA, PCR, and gene sequencing technologies).
Learning Objectives: The successful student will be able to ...
- outline the basic experiments of Gregor Mendel and explain his results in terms of the fundamental laws of genetics; Laws of dominant/recessive alleles, segregation, and assortment.
- relate the patterns of inheritance seen in Mendel's experiments to the events of meiosis.
- use the Punnett Square technique to design, interpret, and make predictions of monohybrid, dihybrid, and X linked crosses.
- interpret the results of a simple test cross to identify individuals with unknown genotypes.
- apply the Chi Square test to evaluate hypotheses generated in various genetic crosses (AP students only).
- recognize the occurrence of crossing over and identify the cross over "products" in a dihybrid cross and explain the significance of these events in terms of the linkage and arrangement of genes on the chromosomes and events during meiosis.
- recognize more complex patterns of inheritance such as codominance, partial dominance, and epistasis and be able to set up Punnett squares for these situations.
Lesson One: Gregor Mendel and the Monohybrid Cross.
There are very few examples in biology of individuals who, working in isolation, have been responsible for major, long lasting contributions to science. Even Charles Darwin, probably the individual who has made the greatest contribution to biology, worked in an environment where he received the encouragement, support, and help from many colleagues. Mendel is clearly the exception. Largely through circumstances of history and his vocation, Mendel worked for years in obscurity, published is work, went on to other pursuits, and was largely forgotten for decades before his work was rediscovered and credit given to his foundational achievements in genetics.
Go to the reading from Villanova University to get an appreciation for the wide range of scientific work Mendel did and the difficulties he had becoming recognized for his contributions.
Now go to the Kimball page that describe some of the first simple genetic crosses done by Mendel. There is a good deal of vocabulary in these reading. Go to the class notes for a list and be sure you know these terms; they can be confusing and we use them frequently.
Image of pea plant flowers from Fotothings.com
Homework, Due. Classical genetics is a lot like math and physics; you can only learn this material thoroughly by practicing the problems yourself. Your test on this unit will be mostly problems such as these.
- Problem Set One: Go to this University of Arizona tutorial and do the first 8 problems. Note that for each question there is a tutorial that helps explain the concepts involved in the question. Use these tutorials if you have questions; they are helpful!.
- Problem Set Two: Go to this problem set and do all the questions in Parts 1, 3, and 4.
- Problem Set Three: Go to the same problem set and do all the questions in Part 5.
Lesson Two: Dihybrid Cross.
In the previous lesson, you worked on problems that involved a single gene with two alternative alleles. Now we add the complication of a second independent gene. Independent genes are genes located on separate chromosomes. We deal with genes on the same chromosome (linked genes) in a latter lesson.
Keep in mind that since the two genes are independent each pair of alleles for each of the genes is still inherited in the same fashion as the monohybrid cross. Mendel observed this fact and formulated his Law of Independent Assortment as a result.
Go to the Kimball page on dihybrid crosses and read up through "independent assortment." This second reading gives a good example of Mendel's dihybrid experiments and the interpretation of the results of these experiments.
Homework, Due. With these dihybrid problems it becomes more important to follow the steps of the Punnett Square technique. You will make far fewer mistakes if you do!
- Problem Set Five: Return to the Biology Project and do questions 1 to 9.
- Problem Set Six: Go to the problem set and do the questions in Part 6.
Homework, Due. Go to this online tutorial to solve a few simple problems and construct family pedigree diagrams.
Lesson Three (AP Students Only): Chi Square Test.
In a previous unit you learned how to use the Student's T Test to evaluate a hypothesis that the means of two populations (or samples) were statistically different. Genetic cross such as the ones you have been solving do not produce data for which you can calculate mean and variance statistics and therefore the T test cannot be used. The Chi Square (X2) is often used in cases where the data are represented by counts of individuals as in a Punnett Square.
The key to this lesson is understanding that a Punnett Square is a prediction or hypothesis of the expected results of a genetic cross. For example, if you believe that you have crossed two individuals hetrozygous for a trait with a simple dominant/recessive relationship between the alleles and this cross generates 200 F1 progeny, then your Punnett Square predicts that 150 of the progeny will have the same phenotype as their parents and 50 will have the recessive phenotype (a 3:1 ratio). The Chi Square test compares these expected results with your actual observed results.
The Chi Square test generates a number (the Chi Square), and using a selected confidence level (e.g. 95%) and a calculated degrees of freedom, you compare your calculated Chi Square to the table value of Chi Squares to determine if your results deviate from the expected too far to be accounted for by random variation in the data. If your calculated Chi Square is too large, then you must reject your hypothesis.
Go to this page from City University of New York for a simple demonstration of the Chi Square in a monohybrid cross. You will have to scroll down past some introductory material and problems to get to the Chi Square section.
This PDF document gives a nice explanation of a simple example. Unfortunately, some of the genetics examples used are not as clear.
And here is an example of the Chi Square with both a monohybrid and dihybrid cross.
Homework, Due. Go to this tutorial from the University of Manitoba that deals with the genetics of corn. Complete the tutorial online. Email your calculated Chi Square for the data and the tabulated Chi Square at 95% probability level and the correct degrees of freedom. Briefly explain the conclusion you made using these statistics.
Lesson Four: Genetic Testing.
Throughout this unit you will be learning about various genetic conditions that can afflict humans; diseases (e.g. hemophilia and Tay-Sach's disease) and other anomalies (e.g. polydactyla). Recent advances in technology have made it possible to assess the genetic profile of an individual with varying degrees of confidence. These test can reveal a genetic profile that essentially guarantees that a person will suffer from a serious disease or simply indicate a slightly higher risk factor than the general population. These genetic test, and even just the ability to do these tests, have raised a number of important medical, ethical, political, and personal questions that go well beyond the science itself.
Homework, Due 25 February. Read the Discover magazine article that chronicles one person's experience with genetic testing and answer the following questions. Email your answers using your FBA account.
- If you had to provide the author with a brief summary of her genetic profile, what would you tell her? Keep in mind that she took three different test, is of mixed ethnic background, and is a young person with most of her adult life ahead of her. Explain your answer with some degree of specificity.
- At this point in time there is very little government regulation of these genetic testing companies similar to the agencies that protect our auto, air travel, and food safety. Do you think that there should be governmental regulation of these companies? Explain your answer.
- Do you think that health insurance companies and prospective employers should have access to the results of these test when you consider the fact that they are responsible for a major share of health care costs? Explain your answer.
- Would you want to know your genetic profile (assuming that the results are confidential)? Explain your answer.
