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Mendel’s Pea Plants
Why Do You Look Like Your Family?
For a long time people understood that traits , the qualities or characteristics of an organism, are passed down through families. The rules of how this worked were unclear. The work of Gregor Mendel was crucial in explaining heredity , the passage of traits from one generation to the next.
Mendel’s Experiments
What does the word “inherit” mean? To inherit is to receive a characteristic through the transmission of hereditary material, also known as DNA. You can inherit a parent’s eye color, hair color, or even the shape of your nose and ears! Genetics is the study of the process of inheritance. The field of genetics seeks to explain how traits are passed on from one generation to the next. In the late 1850s, an Austrian monk named Gregor Mendel (pictured to the right) performed the first genetics experiment, which is why we consider him the “Father of Genetics.” To study genetics, Mendel chose to work with pea plants for three reasons: 1) they have easily identifiable traits, 2) they grow quickly, and 3) they can self-pollinate or be cross- pollinated. Self-pollination means that only one flower is involved; the flower’s pollen lands on its own reproductive organs. Cross-pollination is done by hand, by moving pollen from one flower to the stigma of another. As a result, one plant’s DNA combines with another plant’s DNA to produce offspring. This is called a “cross.” Since Mendel could move pollen between plants, he could carefully control and then observe the results of crosses between two different types of plants. He studied the inheritance patterns for many different traits in peas, including round seeds versus wrinkled seeds, white versus purple flowers, and tall versus short plants.
Mendel’s First Experiment
In Mendel’s first experiment, he crossed a short plant and a tall plant. Most people would assume the offspring would be medium-sized plants, but Mendel saw something unexpected: the offspring were all tall!
Mendel’s Second Experiment
In Mendel’s second experiment, he allowed the offspring from the first experiment to pollinate each other. He found that 75% of the offspring were tall and 25% of the offspring were short.
The Pattern of Inheritance
In Mendel’s experiments, he saw that the short trait skipped a generation. By analyzing almost 3 0,000 plants, Mendel discovered that three out of four plants would have one version of the trait and one out of every four would have the other version. This discovery did not just apply to the height of the plant; Mendel studied numerous other pea plant traits, such as seed shape, seed color, flower color, pod shape, pod color, and the position of the pods. When Mendel studied the color of the flowers on the pea plants (purple or white) he saw the same effect. The color of the flowers did not blend together – purple showed itself 75% of the time and white showed itself 25% of the time. A summary of his results is shown in the image to the right.
Review of “Mendel’s Pea Plants” Reading
- Where does your appearance come from?
- Describe Gregor Mendel’s significance in the study of genetics.
Mitosis Meiosis
Type of Cell
# Chromosomes per Cell
# of Cells Made
Cell Similarity
Beginning End Number of chromosomes per cell.
- I have detached earlobes. Yes No
- I can roll my tongue. Yes No
- I have dimples. Yes No
- I am right-handed. Yes No
- I have freckles. Yes No
- I have naturally curly hair. Yes No
- I have a cleft chin. Yes No
- I have allergies. Yes No
- I cross my left thumb over my right when I clasp my hands. Yes No
- I can see the colors red and green. (I am not colorblind.) Yes No
- The hairline on my forehead is straight. Yes No
- I am a: Male Female
Modern Genetics
Did Mendel Know About DNA?
No, people did not understand that DNA is our hereditary material until long after Mendel’s time. Our modern understanding of DNA and chromosomes helped to explain why Mendel’s rules worked.
Modern Genetics
Mendel laid the foundation for modern genetics, but there were still a lot of questions he left unanswered. What exactly are the dominant and recessive factors that determine how all organisms look? And how do these factors work? Since Mendel’s time, scientists have discovered the answers to these questions. Genetic material is made out of DNA. It is the DNA that makes up the hereditary factors that Mendel identified. By applying our modern knowledge of DNA and chromosomes, we can explain Mendel’s findings and build on them.
Traits, Genes, and Alleles
Recall that our DNA is wound into chromosomes. Each of our chromosomes contains a long chain of DNA that encodes hundreds, if not thousands, of genes (segments of DNA that control a particular trait). Each of these genes can have slightly different versions from individual to individual. These versions of genes are called alleles. For example, remember that for the height gene in pea plants there are two possible factors. These factors are alleles. There is a dominant allele for tallness (T) and a recessive allele for shortness (t).
Genotype and Phenotype
A genotype is a way to describe the combination of alleles that an individual has for a certain gene (see the table below). For each gene, an organism has two alleles, one on each chromosome inherited from parents. The genotype is represented by letter combinations, such as TT, Tt, and tt. When an organism has two of the same allele for a specific gene, it is called homozygous , or purebred. An organism can either be homozygous dominant (TT) or homozygous recessive (tt). If an organism has two different alleles for the gene (Tt), it is known as heterozygous , or a hybrid. Genotype Definition Example Homozygous (or “purebred”) Two of the same allele. TT or tt Heterozygous (or “hybrid”) One dominant allele and one recessive. Tt Homozygous Dominant Two dominant alleles. TT Homozygous Recessive Two recessive alleles. tt
Review of “Modern Genetics” Reading
- Make a diagram showing how DNA, chromosomes, genes, and alleles are related.
- What is the difference between a genotype and a phenotype?
- What is the difference between someone who is homozygous for a trait and someone who is heterozygous for a trait?
Modern Genetics Practice
------------ Whole Class ------------
Use the information provided and your knowledge of genetics to answer each question.
- For each genotype below, indicate whether it is a heterozygous (He) OR homozygous (Ho). TT _____ Bb _____ DD _____ Ff _____ tt _____ dd _____ Dd _____ ff _____ Tt _____ bb _____ BB _____ FF _____ Which of the genotypes in #1 would be considered purebred? ____________________________________ Which of the genotypes in #1 would be hybrids? _______________________________________________
- Determine the phenotype for each genotype using the information provided about SpongeBob. Yellow body color is dominant to blue. BB ________________ Bb ________________ bb ________________ Square shape is dominant to round. RR ________________ Rr ________________ rr ________________
- For each phenotype, give the genotypes that are possible for Patrick. A tall head (H) is dominant to short (h). Tall = _______________ Short = _______________ Pink body color (B) is dominant to yellow (b). Pink body = _____________ Yellow body = _________________
------------ Individual ------------
Complete each definition.
- Organisms that are purebred, also called __________________________, have gene pairs with genes that are the ___________.
- Organisms that are hybrids, also called __________________________, have gene pairs with genes that are _______________________.
- Genotypes are two-___________ codes that represent the actual _________ inherited.
- Phenotypes are the ______________________ of an organism and are determined by the _________________.
- Classify each of the following gene pairs as heterozygous (He) or homozygous (Ho). TT ______ Bb _____ dd _____ Ff _____ Rr _____
- Give the possible genotypes for each trait based on the information provided in the chart. A. Purebred Squarepants – _______________ B. Hybrid Round Eyes – __________________ C. Heterozygous Squarepants – ____________ D. Blue Body – __________________________ E. Purebred Roundpants – ________________ F. Homozygous Yellow Body – _____________
- Give the phenotypes for each genotype based on the information provided in the chart.
rr – _________________________ Bb – _________________________ ee – _________________________ Rr – _________________________ EE – _________________________ BB – _________________________
Punnett Squares
What’s the Chance of a Coin Landing on Heads? There is always a 50-50 chance that a coin will land on heads. Half the time it will land on heads and half the time it will land on tails. What is the chance of it landing on heads twice in a row? What about three times? These rules of probability , or the chance that an event will occur, also apply to genetics. If a parent has one dominant and one recessive factor for a trait, then, on average, half the time the dominant factor will be passed on, and half the time the recessive factor will be passed on. Probability and Punnett Squares A Punnett square is a special tool used to predict the offspring from a cross, or a mating between two parents. This example of a Punnett square shows the results of a cross between two purple flowers that each have one dominant factor and one recessive factor (Bb). Results: Top left box: BB (or purple flowers). Top right box: Bb (or purple flowers). Lower left box: Bb (or purple flowers). Lower right box: bb (or white flowers). A Little Help With Math Conversions Number of Offspring Fraction Percentage Ratio 0 out of 4 0/4 0% 0 to 4 1 out of 4 1/4 25% 1 to 3 2 out of 4 2/4 50% 2 to 2 3 out of 4 3/4 75% 3 to 1 4 out of 4 4 /4 100% 4 to 0
Punnett Squares Practice
------------ Whole Class ------------
- SpongeBob SquarePants recently met SpongeSusie Roundpants at a dance. SpongeBob is heterozygous for his square shape, but SpongeSusie is round. Create a Punnett square to show the possibilities that would result if SpongeBob and SpongeSusie had children. Square = R, round = r. A. List the possible genotypes and phenotypes for their children. B. What are the chances of a child with a square shape? ______ out of ______ or ______% C. What are the chances of a child with a round shape? ______ out of ______ or ______%
- Patrick met Patti at the dance. Both of them are heterozygous for their pink body color, which is dominant over a yellow body color. Create a Punnett square to show the possibilities that would result if Patrick and Patti had children. Pink = B, yellow = b. A. List the possible genotypes and phenotypes for their children. B. What are the chances of a child with a pink body? ______ out of ______ or ______% C. What are the chances of a child with a yellow body? ______ out of ______ or ______%
- Everyone in Squidward’s family has light blue skin, which is the dominant trait for body color in his hometown of Squid Valley. His family brags that they are a “purebred” line. He recently married a nice girl who has light green skin, which is a recessive trait. Create a Punnett square to show the possibilities that would result if Squidward and his new bride had children. Use B to represent the dominant gene and b to represent the recessive gene.
A. List the possible genotypes and phenotypes for their children. B. What are the chances of a child with light blue skin? ______% C. What are the chances of a child with light green skin? ______% D. Would Squidward’s children still be considered purebreds? Explain!
- Assume that one of Squidward’s sons, who is heterozygous for the light blue body color, married a girl that was also heterozygous. Create a Punnett square to show the possibilities that would result if they had children. A. List the possible genotypes and phenotypes for their children. B. What are the chances of a child with light blue skin? ______% C. What are the chances of a child with light green skin? ______%
- Mr. Krabbs and his wife recently had a Lil’ Krabby, but it has not been a happy occasion for them. Mrs. Krabbs has been upset since she first saw her new baby who had short eyeballs. She claims that the hospital goofed and mixed up her baby with someone else’s baby. Mr. Krabbs is homozygous for his tall eyeballs, while his wife is heterozygous for her tall eyeballs. Create a Punnett square using T for the dominant gene and t for the recessive one. A. List the possible genotypes and phenotypes for their children. B. Did the hospital make a mistake? Explain your answer.
C. List the possible genotypes and phenotypes for the kids. D. What is the probability that the kids would have round eyes? ______ % E. What is the probability that the kids would be oval eyes? ______ %
- SpongeBob’s mother is so proud of her son and his new wife, SpongeSusie, as they are expecting a little sponge. She knows that they have a 50% chance of having a little roundpants, but is also hoping the new arrival will be blue (a recessive trait) like SpongeSusie and many members of her family. If SpongeBob is heterozygous for his yellow body color, what are the chances that the baby sponge will be blue? Create a Punnett square to help you answer this question.
- SpongeBob’s aunt is famous around town for her itty, bitty stubby nose! She recently met a cute squarepants fellow who also has a stubby nose, which is a recessive trait. Would it be possible for them to have a child with a regular long nose? Why or why not? Create a Punnett square to help you answer this question.
------------ Individual ------------
- Spongebob's cousin, SpongeJimBob, is a heterozygous yellow sponge. He recently married a blue sponge gal. Yellow = B, Blue = b. A. Create a Punnett square to help you answer the questions. B. What are the possible genotypes and phenotypes for the offspring? C. What percentage would be yellow? ______ % D. What percentage would be blue? ______ %
- SpongeJimBob has oval eyes, while his bride is believed to be homozygous for her round eye shape. Create a Punnett square to help you answer the questions. Round Eyes = E, Oval Eyes = e. A. What are the possible genotypes and phenotypes for the offspring? B. What percentage would have round eyes? ______ % C. What percentage would have oval eyes? ______ %
- In pea plants, purple flowers are dominant to white flowers. If a homozygous dominant plant is crossed with a homozygous recessive plant, what percentage of offspring will be white? (Use the letter R for the genotypes.)
