Foundations of Modern Biology
Lesson Four, Theory: An Introduction to Mendelian Genetics

Determining What You Already Know About Mendelian Genetics

Experience of artificial fertilization, such as is effected with ornamental plants in order to obtain new variations in color, has led to the experiments which will here be discussed.

- Gregor Mendel, "Experiments in Plant Hybridization"

Preparing for the Lesson

In this lesson, we will be learning about Gregor Mendel's laws of heredity. The organism on which Mendel experimented was Pisum sativum, the pea plant. To better understand his experiments, you need to understand how flowering plants reproduce, meiosis, and the chromosomal basis of heredity. Below are some activities to check your background knowledge. If you do well, you may continue with the lesson. If one or two of the activities are not easy for you, you may want to explore the background links provided. Good luck!

Flowers

Flowers are the reproductive structures of angiosperms, the largest and most successful of the plant groups. The activities below review flower plant anatomy and physiology.

Would you like some more background on flowering plant reproduction? Try these links!

Flowers (see Botany Online -- The Internet Hypertextbook -- available on page 7 of the Mentor Guidelines Web site)

Angiosperm Reproduction and Life Cycles (courtesy of the University of California Musem of Paleontology)

Reproduction in Flowering Plants (courtesy of Professor Carrington of the University of the West Indies)

Determining What You Already Know About Mendelian Genetics, continued

Meiosis

Though the concept was unknown to Mendel himself, meiosis is a cellular process that helps to explain some of Mendel's findings. We will not be discussing cell biology or molecular genetics in this course, but you may wish to review meiosis in order to better understand the significance of Mendel's work.

After these questions and activities, would you like to review meiosis? Try these background links:

Meiosis Tutorial (see the University of Arizona The Biology Project Web site, found on page 7 of the Mentor Guidelines Web site)

"Meiosis" from JohnKyrk.com

Video of Meiosis (found at YouTube.com)

Chromosomes

Even though chromosomes were discovered and studied in the nineteenth century, the significant role of the chromosome in heredity was not known until decades after Mendel's research.

It was not until decades after Mendel's research when the central role of the chromosome in heredity was known. However, you need to understand about chromosomes before you tackle Mendel's findings. Want to see what you know?

Even if you did well on the chromosome questions, you may enjoy these chromosome background links!

Tour of the Basics (found at Learn.Genetics Web site of the University of Utah)

Chromosome animation video (found at YouTube.com)

 Ready to begin looking at Mendel's work? On to the next page!

How the Theory of Heredity Helped the Darwinian Revolution

In the decade following the publication of Origin of Species, the theory of evolution by natural selection became the primary working theory of biologists. By the turn of the century, however, natural selection had lost many of its first supporters. While Darwin was still held in high esteem by scientists, practicing researchers had begun to explore other theories of evolution that downplayed the importance of Darwin's mechanism of selection. The Darwinian Revolution, it seemed, had lost some of its steam.

Why had Darwinism lost so much of its initial strength? As well-argued as the theory of natural selection was, during the nineteenth century it lacked a model of heredity, a model of how characteristics get passed from generation to generation (from parents to offspring). In order for the theory of evolution via natural selection to work, organisms had to pass their traits on with some regularity. Otherwise, there would be no guarantee that those organisms with traits that promote survival would actually pass those traits on to the next generation. How did traits get passed from parents to offspring?

Scientists of the late nineteenth century recognized from general observations that some traits might be passed from generation to generation, but they did not possess a mature knowledge of genetics, or science of heredity, that could explain how and why this happened. Darwin recognized the difficulty that heredity posed to his theory, and after finishing Origin of Species he spent several years experimenting on heredity using the plants and animals around his estate. In 1868, he finished The Variation of Animals and Plants under Domestication (courtesy of Robert J. Robbins, Electronic Scholarly Publishing), a treatise intended largely to explain how heredity worked.

In Variation of Animals and Plants, Darwin proposed the hypothesis of pangenesis. According to this hypothesis, cells in an organism's body released gemmules when a particular action was performed. For example, the cells in an athlete's muscle tissue would release gemmules for increased muscle growth when the athlete lifted weights. These gemmules would collect in the organism's reproductive organs, and the parent organism could pass these gemmules on to its offspring. These gemmules would shape the next generation; the athlete's children might be stronger because of her regular exercise at the gym (Provine 9-10).

As you might notice, Darwin's hypothesis of pangenesis looks an awful lot like J.B. Lamarck's inheritance of acquired characteristics. Some scientists accepted Darwin's new model for heredity, but most did not. As a result, during the period from around 1870 to 1920, most biologists used some theory of evolution to guide their research, but no single theory of evolution united the discipline as a whole. Darwin had changed the field of biology, but there was a great deal of debate about how much of his theory should be accepted.

Genetics remained a vague and uncertain discipline until the work of an Austrian monk named Gregor Mendel was rediscovered in 1900, more than thirty years after it had first been published. In this lesson, we are going to explore Mendelian genetics, the theory of heredity that has become one of the foundations of modern biology. As you encounter Mendel's ideas, think about how his model of heredity might be brought together with Darwin's mechanism of selection to create a more robust theory of evolution.

Who Was Gregor Mendel?

Who was Gregor Mendel? What did he do to lay the intellectual foundation of modern genetics? In the following passage, you will be introduced to the basics of Mendelian genetics. As you read the passage, take some notes on the terms and concepts that were central to Mendel's work. After you have finished the reading, complete the crossword puzzle below in order to test your knowledge.

Reading Assignment 7: Mendel's Principles of Inheritance

Read pages 194-196 in the Foundations of Modern Biology Textbook, Solomon, Berg, and Martin's Biology.

Ready for a Review?

Your Personal Journal (Entry 15)

After you have finished reading the excerpt, answer each of the questions below in your journal. Use the TCQC Short-Answer Response Format.

Questions to Open Discussion:

1. What is a true-breeding organism? What is a hybrid? What happens when two hybrids mate with each other?
2. Why were pea plants such good experimental subjects for Mendel? What seven characters did he study in pea plants?
3. What does the term phenotype mean? Use the concept of phenotype to define what a true-breeding organism is.
4. What is a gene? What is an allele? Stem height in pea plants may be either tall or short. When a true-breeding tall plant is mated to a true-breeding short plant, all the offspring are tall. Describe this situation using the terms trait, gene, phenotype, dominant allele, and recessive allele.
5. For a given trait, an organism has two alleles. During sexual reproduction, the organism passes one of those two alleles on to its offspring (there's a 50% chance that it could be either). The offspring receives one allele from each parent. This is the principle of segregation. Therefore, how might probability have a role in heredity?
6. When did Mendel begin his experiments? When did he publish them? When did Darwin publish Origin of Species? Could Darwin have known about Mendel's experiments? How might Darwin have reacted to Mendel's findings?

Now, with your mentor, discuss each question. After your discussion, add any comments to your answers, so that you have fully answered each question. Make sure you feel comfortable with the topics and the vocabulary of this section before you continue.

Want some additional practice? Visit the Cave Spring Middle School Web site "Genetics: Practice Quiz" found on page 7 of the Mentor Guidelines Web site.

Your Personal Journal (Entry 16)

While much of the early work in evolution was inspired by studying plants and animals in the wild, genetics has its roots in the study of domesticated organisms, or organisms that have adapted to living within human society. We are surrounded by domesticated organisms. Think about what you did during the past week.

1. How many domesticated plants and animals did you encounter? Describe as many of them as you can.
2. What are some differences between domesticated organisms and wild organisms?

Investigating Monohybrid and Dihybrid Crosses

Once you understand Mendel's ideas, it becomes possible to make effective predictions about patterns of inheritance for some traits in some organisms. In the following passage, you will be introduced to monohybrid and dihybrid crosses, two types of matings that are important to geneticists. You will also learn how to use a Punnett square to analyze matings.

As you read the passage, think about the new terms and concepts you are learning. After you have finished the reading, complete the crossword puzzle below in order to test your knowledge.

Reading Assignment 8: Monohybrid and Dihybrid Crosses

Read pages 197-200 in the Foundations of Modern Biology Textbook, Solomon, Berg, and Martin's Biology.

Ready for a Review?

Your Personal Journal (Entry 17)

After you have finished reading, the following questions will help you to focus on the most important concepts in the passage. In your journal, answer each of these questions. Use the TCQC Short-Answer Response Format. Then read each question with your mentor, and use it to open a discussion.

Questions to Open Discussion:

• What does it mean to say that an organism is homozygous for a given trait? What does it mean to say that an organism is heterozygous for a given trait?
• What is a genotype? Give an example of a situation in which two organisms might have identical phenotypes but different genotypes.
• Each parent contributes the same amount of genetic material to a child, yet the child may resemble one parent more than another. How is this possible?
• What is a Punnett square? Explain what the Punnett square shows.
• What is a test cross? When would a breeder need to perform a test cross?
• How does a dihybrid cross differ from a monohybrid cross? Can a dihybrid cross be modeled using a Punnett square? How would the Punnett square for a dihybrid cross differ from one for a monohybrid cross?
• What is the principle of independent assortment? Why is the principle of independent assortment important when we consider dihybrid crosses? Could basic Punnett squares be used to make predictions about dihybrid crosses if the principle of independent assortment did not hold true?

Do you understand everything so far? Soon you will be solving genetic problems. If you feel a little unsure about the terminology, you may want to review some more with your mentor before proceeding!

A Mendelian Review

The following questions will help you to test and extend your knowledge from this lesson. A Mendelian Review will remind you of some of the basic ideas from this lesson, while Problem Set 1 will challenge you to apply these ideas in new and more challenging ways.

A Mendelian Review

Assume that in pea plants, yellow pod color is dominant over green pod color.

1. Draw a Punnett square to illustrate the expected outcomes if a plant with two dominant alleles is crossed with a plant with two recessive alleles. (Need a little help getting started on drawing Punnett squares? Use this Web page by Ruth Rogers of FeatherSong Aviary)
2. If we plant seeds from this cross and 400 plants grow, how many of the plants would we expect to be homozygous dominant? Hint! Homozygous recessive? Heterozygous? How many would we expect to have green pods? Yellow pods?
3. Draw a Punnett square to illustrate the expected outcomes if two plants that are heterozygous for pod color are crossed together.
4. If we plant seeds from this cross and 400 plants grow, how many of the plants would we expect to be homozygous dominant? Homozygous recessive? Heterozygous? How many would we expect to have green pods? Yellow pods?

Problem Set 1: An Introduction to Mendelian Genetics

In the distant past, an imaginary ship has washed up on an imaginary island in the not-so-imaginary Pacific Ocean. The imaginary crew includes an imaginary naturalist who has discovered a new species of imaginary reptiles that he, not so imaginatively, has named lizardos. The lizardos are tiny and ugly. They have two distinct colors: some are bright red, while others are bright yellow. Some lizardos have long tails, while others have stubby short tails. Our imaginary naturalist friend has managed to capture a small population of lizardos and has brought them on board the ship in order to learn more about their genetics. Answer the following questions.

1. After performing a series of breeding experiments, the naturalist determines that yellow coloration in lizardos is dominant to red coloration. Draw a Punnett square to illustrate the expected outcomes if a red lizardo were mated with a lizardo that was heterozygous for coloration. If the clutch produced includes 16 eggs, how many yellow lizardos would you expect? Hint!
2. A yellow female lizardo and a red male lizardo have become particularly close. However, over many matings that have produced over 15 living offspring, the pair has never produced a red lizardo. What is the male lizardo's genotype? What is the female lizardo's most likely genotype? Hint! Can you be 100% sure about the female's genotype? Why or why not?
3. Two red lizardos mate. If they have four offspring of their own, how many of these offspring would you expect to be red?
4. Assume that tail length in lizardos operates on a system of complete dominance like Mendel described. Design a breeding experiment that the naturalist could undertake in order to determine which allele, long or short, is recessive. What are the possible outcomes of this breeding experiment? What would each outcome reveal to you? While designing your experiment, you may also assume that the naturalist has spent enough time with his reptilian buddies to determine which individuals are true-breeding and which are not.
5. After following your instructions, the naturalist performs the experiment and finds that short tail length is dominant over long tail length in lizardos. The naturalist now mates a yellow female with a short tail with a yellow male with a short tail. He knows from previous experiments that both lizardos are heterozygous for both traits. Draw a Punnett square to illustrate this dihybrid cross (for a model of what this might look like, refer to Figure 10-7 on page 200 of Reading Assignment 8).
6. If the lizardo couple in Question 5 had 16 offspring, how many of them would you expect to be red? How many would you expect to be yellow? How many would you expect to be orange? How many would you expect to have long tails? How many would you expect to have short tails? How many would you expect to be red with short tails? How many would you expect to be red with long tails? How many would you expect to be yellow with short tails? How many would you expect to be yellow with long tails?

Design an Imaginary Organism

Each of us who now looks at his own patch of work sees Mendel's clue running through it: whither that clue will lead, we dare not yet surmise.

- William Bateson in 1902 (for a biography of Bateson, visit Cold Harbor Laboratory's DNA from the Beginning Web site, found on page 7 of the Mentor Guidelines Web site)

Final Assessment (Personal Journal Entry 18)

While the imaginary naturalist in Problem Set 1 experimented on the lizardos, one of his colleagues set off into the interior of the island to look for other new species. What new imaginary species did she find?

Get out your journal! We have some science to do!

1. The answer is up to you. Design an imaginary organism that a naturalist might discover on a tropical island. (Do you feel like a mad scientist designing fantastical critters? Look ahead: you can't be too Frankenstein-ish about this creation!) Provide a name and a general description. Use the Sensory Description Rubric from The Writer's Journey, Volume 1, and adapt as needed with your mentor so that it suits your purposes here. Remember, your creature has never been seen before, so you want to render it visually in words. Looks, sounds, smells, textures, and other sensory aspects should be noted.
2. If you have some artistic skills, draw a sketch of what the organism might look like. While the species is imaginary, it should not be physically impossible; use your knowledge of Darwinian evolution to create an organism that could realistically have evolved on a tropical island. You may wish to do some research on real species in order to envisage your new one.
3. Now that you have a sense of what this organism is like, think about what variation there might be in its population. Record three traits the organism has that exhibit some variation. For each of these traits, list two alleles. Once again, try to imagine alleles that might actually provide an organism with some survival advantage in a tropical setting. Decide which of the two alleles you have sketched is the recessive and which is the dominant. Confused? Need an example?
4. Draw five different individuals of the species, with each phenotype exhibiting different combinations of traits. For each individual, choose agenotype that can match the individual's phenotype (each of these genotypes will include six alleles, two alleles for each of the three traits).
5. The naturalist captures two of the five individuals you have drawn (why don't you let your mentor "capture" (pick) two of the five individuals?). She carries them back to the ship and crosses them in order to begin analyzing their hereditary systems. Draw a Punnett square that will predict the result of this trihybrid cross. Need a hint on how to do this?

Share your work with your mentor.  

Looking for guidance? Consult the work of Melina, 2008 TIP Tester, found here.