Name: __________________________
Lesson 7.2: Gregor Mendel and Genetics
Mendelian Inheritance
Period: _____
Key Concept
Gregor Mendel experimented with pea plants to study heredity. Based on his research, he developed his
Laws of segregation and independent assortment. Mendel’s laws were rediscovered in 1900 and are
now understood in terms of genes and alleles.
Lesson Objectives
• Explain why and how Mendel studied pea plants.
• Describe the results of Mendel’s experiments.
• State Mendel’s laws of segregation and independent assortment.
• Outline the genetics of inheritance.
• Define probability.
• Explain how probability is related to inheritance.
• Describe how to use a Punnett square.
• Explain how Mendel interpreted the results of his experiments.
• Describe complex patterns of inheritance.
Introduction
1. What is genetics? ________________________________________________________________
2. Who is known as the ‘Father of Genetics’? ____________________________________________
3. What three things contributed to the importance of Gregor Mendel’s work?
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Mendel and His Pea Plants
1. What type of plants did Mendel use in his studies? _____________________________________
Blending Theory of Inheritance
1. What was the blending theory of inheritance?
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2. Go to the following website: http://www.dnalc.org/view/16170-Animation-3-Gene-s-don-tblend-.html; scroll down the page and click on topic ID 16002: Gregor Mendel and pea plants to
learn why Mendel chose to study inheritance through pea plants. Summarizing his three reasons
for using the pea plants below.
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Why Study Pea Plants?
1. How many different characteristics of pea plants did Mendel study? _______________________
2. List the characteristics he studied in pea plants.
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Controlling Pollination
1. Define pollination.
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2. What is pollen?
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3. Where is pollen produced?
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4. How does pollination occur?
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5. What is the stigma, what does it do?
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6. State the difference between self-pollination versus cross-pollination in pea plants.
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7. What are hybrids?
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Mendel’s First Set of Experiments
1. What letter represents the parental generation? ___________
2. Describe Mendel’s first experiment, what did he cross?
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F1 and F2 Generations
1. What does F1 mean? ____________________________________________________________
2. What were the results in the F1 generation of Mendel’s first experiments?
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3. Did Mendel cross-pollinate or self-pollinate the F1 generation? ___________________________
4. What does F2 mean? _____________________________________________________________
5. What were the results in the F2 generation of Mendel’s first experiments?
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6. After studying hundreds of F2 generation plants, what same result kept appearing?
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Law of Segregation
1. What law did Mendel establish after his first set of experiments? What does it state?
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Mendel’s Second Set of Experiments
1. What information did Mendel want to study in his second set of experiments?
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2. Describe Mendel’s second experiment, what did he cross?
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F1 and F2 Generations
1. What were the results in the F1 generation? __________________________________________
2. What were the results in the F2 generation after the F1 generation was self-pollinated?
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Law of Independent Assortment
1. What law did Mendel establish after his second set of experiments? What does it state?
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Mendel’s Laws and Genetics
1. Did Mendel’s discoveries make a big impact on science? ________
Why or why not? ________________________________________________________________
Rediscovering Mendel’s Work
1. When was Mendel’s work rediscovered? _______________
2. Name the three scientists whose work led to the rediscovery of Mendel’s ideas.
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Genetics of Inheritance
1. What controls the inheritance of characteristics in organisms? ___________________________
2. Define locus. ___________________________________________________________________
3. What are alleles? ________________________________________________________________
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Genotype and Phenotype
1. Differentiate between genotype and phenotype.
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2. Differentiate between homozygous and heterozygous.
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3. Differentiate between dominant alleles and recessive alleles.
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Principle of Dominance. _ states that when two alleles are segregated from each other, each gamete
carries only a single copy of each gene. Thus some alleles are dominant and others recessive.
Probability
1. Define probability. _______________________________________________________________
2. Each time you toss a coin what is the probability of it landing on heads? ____________________
3. Each time you toss a coin what is the probability of it landing on tails? _____________________
4. What percentage of the time approximately will heads land up if you toss a coin 1000 times?
_________
Probability and Inheritance
1. What two main events determine the genotypes of offspring?
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Probability and Gamete Formation
1. How is gamete formation like tossing a coin?
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Probability and Fertilization
Which gametes joins in fertilization, is a matter of chance, like tossing a coin. Thus, we can
assume that either type of gamete has an equal chance of uniting with any of the gametes produced by
the other parent. To answer questions of genetic probability, geneticists use a simple tool called a
Punnett square.
Using a Punnett Square
1. Define Punnett square.
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2. Where on the Punnett square do you place the gametes produced by the male parent?
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3. Where on the Punnett square do you place the gametes produced by the female parent?
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4. How are the different possible combinations of alleles for the offspring determined?
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5. Who invented the Punnett square? _________________________________________________
6. At this link: http://www.dnalc.org/view/16192-Animation-5-Genetic-inheritance-follows-rules.html, you can watch an animation on how to use Punnett squares and answer the questions
below. Stop at the end of the one trait segment; do not go on to the two trait segment of the
animation.
a. What is the ratio of dominant traits (alleles) to recessive traits (alleles) in a hybrid cross?
_________
b. Fill in the Punnett square below with the information viewed on the website link:
c. What will be the genotypes of the offspring?
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d. What are the phenotypes of these offspring?
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Predicting Offspring Genotypes
In single trait crosses, you can expect that one out of four offspring (25 percent) will have
homozygous dominant genotypes (example: BB), one out of four (25 percent) will have homozygous
recessive genotypes (example: bb), and two out of four (50 percent) will have heterozygous dominant
genotype (example: Bb). These percent of genotypes are what you would expect in any cross between
two heterozygous parents.
Predicting Offspring Phenotypes
You can predict the percentages of phenotypes in the offspring from their genotypes. Example: B
is dominant to b, so offspring with have either the BB, Bb, or bb genotype. BB and Bb produces purpleflower phenotypes and bb produces white-flower phenotypes. Therefore, in a cross where both parents
are heterozygous (Bb), you would expect three out of four (75 percent) of the offspring to have purple
flowers (BB or Bb) and one out of four (25 percent) to have white flowers (bb).
Determining Missing Genotypes
A Punnett square can also be used to determine a missing genotype based on the other genotypes
involved in a cross. Suppose you have a parent plant with purple flowers and a parent plant with white
flowers. Because the b allele is recessive, you know that the white-flowered parent must have the
genotype bb. The purple-flowered parent, on the other hand, could have either the BB or the Bb
genotype. The Punnett square below shows this cross. The blank boxes in the chart could be either B or
b alleles.
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1. Fill in the blank boxes below with their missing alleles…
b
b
B
Bb
__b
__
Bb
__b
a. What are the phenotypes of
the parents?
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b. What are the phenotypes of
the offspring, and how many
of each?
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Punnett Square for Two Characteristics
Using a Punnett square for two characteristics is more complicated because many more combinations of
alleles are possible. For example, with two genes each having two alleles, an individual has four alleles,
and these four alleles can occur in 16 different combinations, see the Punnett square below. In this cross,
both parents are heterozygous for pod color (Gg) and seed color (Yy).
How Mendel Worked Backward to Get Ahead
1. Define Laws of probability (you may need to do some research).
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2. Mendel also was lucky when he did his second set of experiments. He happened to pick
characteristics that are inherited independently of one another. We now know that these
characteristics are controlled by genes on non-homologous chromosomes. What if Mendel had
studied characteristics controlled by genes on homologous chromosomes? Would they be
inherited together (think back to meiosis and Prophase I), explain your answer?
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Non-Mendelian Inheritance
Inheritance that can involve genes found on homologous chromosomes.
Codominance and Incomplete Dominance
Codominance
1. Define codominance.
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Example:
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Incomplete Dominance
1. Define incomplete dominance.
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Example:
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Multiple Alleles, Polygenic Characteristics, and environment’s effect covered in next
chapter…
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