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Discover the Possibilities!
Introduction
The natural world is filled with many kinds of organisms. Where did they all come from? If an organism appears to be a smaller version of a larger, similar-looking organism, is it reasonable to believe it may be the larger organism’s “baby” (offspring)? Do organisms always produce similar-looking offspring?
Background
For centuries, people have grown and harvested crops, such as corn. Growing crops has always involved planting seeds in soil, which germinate and develop into plants and seeds that look like the original plants and seeds. Of course, they also observed the same thing about animals, sheep always gave birth to sheep, chickens hatched baby chickens, etc. However, it wasn’t until the middle of the 19th century (1856) that someone began to carefully and systematically try and answer the question: Why do offspring look similar to their parents?
Gregor Mendel, (1822–1884), an Austrian monk, grew pea plants in a greenhouse located in the garden of the monastery where he lived. Over a period of eight years, he carefully documented the results of the breeding experiments he carried out on about 28,000 plants. From his exhaustive research and results, three basic laws of inheritance were discovered that provide the foundation for modern genetics today. In fact, Mendel’s findings were so significant that he has been called “The Father of Genetics.”
Mendel’s work with pea plants involved studying seven different traits that in his own words “stand out clearly and definitely in these plants.” These seven traits, like height and flower position, and their two forms—tall/short, axial/terminal—are shown in Figure 1. Note: The first trait listed is dominant.
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Probability or chance is used by many people to predict the likelihood that a particular event will occur. For example, if you buy a raffle ticket, the odds or chance of winning depend on the number of people who also buy a ticket. As the number of ticket holders increases, the chance of winning decreases. In a similar manner, chance is also used by geneticists to predict the likelihood of someone inheriting a particular genetic defect. However, unlike games of chance, the odds of inheriting a genetic defect and/or being affected by the defect increase if many members of one’s family already have the defect.
The physical characteristics that an organism possesses is used to help identify its species. The physical appearance of any organism is referred to as its phenotype. The phenotype results from the expression of at least two forms (alleles) of the same gene. For example, the color of pea seeds is determined by a gene that has two forms, yellow and green. The genes that are inherited by the offspring from the parents, referred to as genotype, determine the physical appearance. In all sexually reproducing organisms, every individual inherits at least one allele for a trait from each parent.
A simple diagram commonly used to help make inheritance predictions is called a “Punnet square,” after its inventor—Reginald Punnet (1875–1967) (see Figure 2). This tool allows geneticists to predict how the genes located on chromosomes may combine during fertilization to create offspring that are similar to the parents, yet uniquely different.
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Experiment Overview
The purpose of this activity is to investigate several of the basic principles of inheritance including incomplete dominance and Mendel’s Law of Dominance. Some of the specialized vocabulary associated with genetics will also be introduced, such as phenotype/genotype and homozygous/heterozygous.
Procedure
Part 1. Law of Dominance
Answer all the questions in this part to the best of your ability. Make corrections as needed during the class discussion that will follow. Work individually or with a partner to collect any data.
- What is the chance (probability) that when a coin is flipped, it will land “heads up”? (Write the answer as a percent.) Why? (Explain your answer.)
- Based on your answer to Question 1, if the coin is flipped 20 times, how many times would you expect it to land “tails up”?
- Get one plastic coin and one plastic cup.
- Use the cup to flip/shake the coin twenty times. Record in the spaces below the number of times the coin lands “heads up” versus “tails up.”
{10683_Procedure_Table_a}
- Did the results above match the written prediction from Question 2? If not, why not?
- Complete the Punnet square below. (H) represents that “heads” side of a coin and (h) represents the “tails” side. Write two letters inside each box, following the pattern shown in Figure 2.
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- After writing the letter combinations inside each box, use those letter combinations to answer the questions that follow:
- If two coins were flipped together 100 times, what percent of 100 would you expect both coins to land “heads up”?
- If two coins were flipped together 100 times, what percent of 100 would you expect one coin to land “heads up” and the other coin to land “tails up”?
- If two coins were flipped together 100 times, what percent of 100 would you expect both coins to land “tails up”?
- Write the percentages, as written in 7a, 7b and 7c, in the form of a ratio in the spaces below:
{10683_Procedure_Figure_4}
[This ratio is referred to as the genotype ratio or genotypic ratio.]
Application
If the seeds from pea plants with axial flowers or pea plants with terminal flowers (see Figure 1) always produce pea plants with axial flowers or terminal flowers, respectively; geneticists call such plants homozygous or purebred. If the axial flower form (allele) is more common than the terminal flower form, then the axial position is dominant (A) and the terminal position is recessive (a). However, if seeds from an axial plant are planted and most of the offspring plants have axial flowers but some have terminal flowers, geneticists call the original axial flower plants heterozygous or hybrid.
- Obtain two bingo chips—one yellow and one green—and a cup. Apply one sticker with a capital “A” to one side of one chip and apply a sticker with little “a” to the other side. Do the same to the second chip. Place both chips inside the plastic cup. Shake them out 20 times and record the results in the appropriate column below. (Put tick marks under the letter combination that matches the two chips after each toss. When finished, write the total number of marks for each combination.
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Total:
- Calculate the percent of each genotype (letter combination) by dividing the total number of each letter combination by 20. Record the percentage of each genotype under the correct column heading below.
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- Did the actual results—Question 9—match the predicted results in 7d? Write one reasonable explanation for any observed differences.
Demonstration
- Show the overall effect of a dominant trait. Each partner obtain another green and yellow bingo chip. Put the two chips together, one on top of the other, and hold them up to the light.
- Which color is visible as you look through the chips at the light?
- Which color would be considered the dominant color? Why?
- In your own words, describe the Law of Dominance.
Part 2. Incomplete Dominance
Answer all the questions in this part to the best of your ability. Make corrections as needed during the class discussion that will follow. Work individually or with a partner to collect any data.
A mating between two organisms which results in a phenotype that is a mixture of the two parent phenotypes is called Incomplete Dominance. This occurs when neither allele in the genotype is fully expressed, resulting in a blended phenotype. For example, certain types of flowers are pink because of the incomplete dominance of the color red over the color white. In horses, the palomino color—creamy-gold with pale mane and tail—is a result of incomplete dominance.
- Complete the Punnet square below showing the results of a mating between a chestnut-colored (dark brown) stallion (BB) and a white-colored mare (bb).
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- What would be the expected genotype ratio of all the offspring of this mating?
- What would be the expected phenotype (coat color) ratio of all the offspring?
Demonstration
- Each student obtain a yellow and blue bingo chip without a sticker. Put the two chips together, one on top of the other, and hold them up to the light.
- Which color is visible as you look through them at the light?
- In this demonstration, is there a dominant or recessive color? Explain your answer.
- In your own words, define Incomplete Dominance.
Co-Dominance
Sometimes a mating between two organisms results in a phentotype that is a combination of both parent phenotypes. This is called co-dominance because both alleles are fully expressed. This means that the proteins each allele codes for get made and the offspring contain both proteins.
One of the best known examples is the human blood type AB. This blood type occurs as a result of a mating between a parent with blood type A and a parent with blood type B. The offspring end up with blood cells containing both A proteins and B proteins. A simple diagram can be used to illustrate what happens:
{10683_Procedure_Figure_8}
- In your own words, define co-dominance.
Part 3. Dihybrid Crosses (Law of Independent Assortment)
Answer all the questions in this part to the best of your ability. Make corrections as needed during the class discussion that will follow. Work individually or with a partner to collect any data.
Parts 1 and 2, which involved inheriting only one trait—such as round seeds versus wrinkled, axial flowers versus terminal—describe monohybrid crosses. This section involves the inheritance of two traits at the same time. These studies are referred to as dihybrid crosses. Although chromosomes had not been discovered in Mendel’s day, he found that the “factors” (genes) that determine an organism’s phenotype are located in different areas of a plant. His work also showed that an organism’s “factors” (its genes) separate independently from one another. This means that when the male sex cells (pollen) and the female sex cells (eggs) of a plant are formed, the gametes (sex cells) may contain genes that cause a plant to produce axial flowers (a dominant trait) but may also contain genes that will cause the plant to produce wrinkled seeds, a recessive trait.
Application—Predicted results
Two 4-sided dice will be used to represent all the possible allele combinations that may be found within female gametes (eggs) and male gametes (pollen). Shaking the two dice together will simulate a mating and fertilization between two plants with round seeds and axial flowers. The following chart is the key to interpreting the numbers on the dice as they are shaken.
If one die’s # = 1, then the allele combination will = RA
If one die’s # = 2, then the allele combination will = Ra
If one die’s # = 3, then the allele combination will = rA
If one die’s # = 4, then the allele combination will = ra
- Complete the Punnet square below showing a cross between two parent plants that are heterozygous for round seeds and axial flowers. The Punnet square for this type of cross has 16 squares—four rows and four columns.
- Combine the two-letter combinations on the outside into four-letter combinations inside each square. Note: The same letters are always written together (i.e., RrAA).
{10683_Procedure_Figure_9}
- Use the completed Punnet square above to fill in the spaces below. List all the possible genotypes and phenotypes that may result from this cross. Note: To complete the third column, count the number of genotypes in the Punnet square that result in each phenotype. This number is the “expected/predicted” number.
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- Which genotype(s) from this cross would be homozygous (purebred) for their traits?
- Which phenotype(s) would be the easiest to identify as homozygous? Why?
Application—Actual results
- Put two four-sided dice into a cup, shake and let them fall on the desk or table.
- In the following table, use tick marks to record how many times a specific number combination is “right side up” on each die. After 24 rounds, use the key chart to determine the four-letter genotype that each number combination represents. Record the genotype in the table. Example: If one die shows #1 and the other die shows #3, the genotype would be RrAA. Note: Write genotypes with capital letters first and the same letters together.
{10683_Procedure_Table_1}
- Complete the phenotype column of the table by using the recorded genotypes to determine the phenotypes present in the offspring. Record the phenotypes in the table. Hint: Refer back to Question 3.
- Use the table in Question 6 to fill in each phenotype and the number of each phenotype in the blanks below:
{10683_Procedure_Table_3}
- Are the actual results above the same as the expected results from Question 3? Briefly explain why the actual results may be different than the expected results.
- If you were a farmer and planted 5,000 seeds from hybrid pea plants with the same genotype—RrAa—barring any weather-related problems, how many of each phenotype would you expect (see Question 3) when the plants reached maturity? Write each phenotype and the calculated number in the space provided:
- After the plants reached maturity, if you decided to only plant homozygous (purebred) pea plants in your fields, from how many plants would you have to collect seeds? Why?
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