Gregor Mendel’s Peas

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Analyzing Inheritance

Offspring resemble their parents. Offspring inherit genes for characteristics from their parents. To learn about inheritance, scientists have experimented with breeding various plants and animals. In each experiment shown in the table on the next slide, two pea plants with different characteristics were bred. Then, the offspring produced were bred to produce a second generation of offspring. Consider the data and answer the questions that follow.

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Interest Grabber

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1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics?

2. How do the characteristics of the second generation compare to the characteristics of the first generation?

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Parents

Long stems × short stems

Red flowers × white flowers

Green pods × yellow pods

Round seeds × wrinkled seeds

Yellow seeds × green seeds

First Generation

All long

All red

All green

All round

All yellow

Second Generation

787 long: 277 short

705 red: 224 white

428 green: 152 yellow

5474 round: 1850 wrinkled

6022 yellow: 2001 green

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11–1 The Work of Gregor Mendel A. Gregor Mendel’s Peas B. Genes and Dominance C. Segregation

1. The F1 Cross 2. Explaining the F1 Cross

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Section Outline

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Gregor Mendel’s Peas

• Mendel was one of the first scientists to study genetics, the scientific study of heredity. • Mendel carried out his research with ordinary garden pea plants. • Peas are self-pollinating , which means that sperm cells in pollen fertilize the egg cells in the same flower. • This process is called fertilization. • Mendel used true-breeding pea plants as the basis of his experiments.

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Mendel’s Experiment • Mendel wanted to produce seeds by joining the sperm and egg from different plants. • In order to prevent self-pollination, Mendel cut away the pollen bearing male parts and dusted pollen from another plant onto the flower. • This process is called cross-pollination. • Mendel studied seven different pea plant traits. • A trait is a specific characteristic such as seed color or plant height.

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Seed

Shape

Flower

Position

Seed Coat Color

Seed Color

Pod Color

Plant

Height

Pod Shape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

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Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants

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Parental (P) Cross

• Mendel crossed two plants with contrasting traits, a tall plant with a short plant. • This initial pair is called the Parental (P) generation. • The offspring produced are called the First Filial (F1) generation. • The offspring of crosses between parents with different traits are called hybrids. • The results of the cross produced all tall pea plants.

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F1 Cross •  Mendel drew 2 conclusions from the results.

1.  Biological inheritance is determined by factors that are passed from one generation to the next. –  These factors are genes.

2.  Mendel’s principle of dominance which states that some alleles are dominant and others are recessive.

•  Mendel wanted to find out if the short gene disappeared or was it hidden.

•  Mendel self-pollinated the F1 generation to produce a F2 generation.

•  The results produced 3 tall plants for every 1 short plant.

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P Generation F1 Generation F2 Generation

Tall Short Tall Tall Tall Tall Tall Short

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Principles of Dominance

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Explanation of the F1 Cross • Mendel assumed the dominant allele masked the recessive allele. • The reappearance of the recessive allele in the F2 generation indicated the allele for shortness separated, or segregated, from the tall allele. • Mendel assumed this happened during the formation of the gametes. • The alleles segregate and each gamete contains only one allele.

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Tossing Coins

If you toss a coin, what is the probability of getting heads? Tails? If you toss a coin 10 times, how many heads and how many tails would you expect to get? Working with a partner, have one person toss a coin ten times while the other person tallies the results on a sheet of paper. Then, switch tasks to produce a separate tally of the second set of 10 tosses.

Section 11-2

Interest Grabber

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1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how do the results of your tosses compare? How about the results of your partner’s tosses? How close was each set of results to what was expected?

2. Add your results to those of your partner to produce a total of 20 tosses. Assuming that you expect 10 heads and 10 tails in 20 tosses, how close are these results to what was expected?

3. If you compiled the results for the whole class, what results would you expect?

4. How do the expected results differ from the observed results?

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11–2 Probability and Punnett Squares A. Genetics and Probability B. Punnett Squares C. Probability and Segregation D. Probabilities Predict Averages

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Section Outline

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Genetics and Probability • The likelihood that a particular event will occur is called probability. • Take a coin flip of example, what are the chances of getting heads? • Your chances are 1 in 2 flips or 1 : 2 or 50 percent. • What are your chances to get 3 heads in a row?

–  1/2 X 1/2 X 1/2 = 1/8 • The way in which alleles segregate is completely random. • The principle of probability can be used to predict the outcomes of genetic crosses.

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Punnett Squares

• A diagram that can show the gene combinations from a genetic cross. • Organisms that have identical alleles for a trait are called homozygous. • Organisms that have two different alleles for the same trait are heterozygous. • Physical characteristics are called phenotypes. • Genetic characteristics are called genotypes. • Dominant traits are represented by capital letters, while recessive traits are represented by lower case letters.

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Section 11-2

Tt X Tt Cross

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Section 11-2

Tt X Tt Cross

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Probabilities Predict Averages

• Probability can not predict the precise outcome of an individual event. • However, they can predict the average outcome of a large event. • The same holds true for genetics. • The larger the number of offspring, the closer the resulting numbers will get to the predicted values.

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Height in Humans

Height in pea plants is controlled by one of two alleles; the allele for a tall plant is the dominant allele, while the allele for a short plant is the recessive one. What about people? Are the factors that determine height more complicated in humans?

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Interest Grabber

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1. Make a list of 10 adults whom you know. Next to the name of each adult, write his or her approximate height in feet and inches.

2. What can you observe about the heights of the ten people?

3. Do you think height in humans is controlled by 2 alleles, as it is in pea plants? Explain your answer.

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11–3 Exploring Mendelian Genetics A. Independent Assortment

1. The Two-Factor Cross: F1 2. The Two-Factor Cross: F2

B. A Summary of Mendel’s Principles C. Beyond Dominant and Recessive Alleles

1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Polygenic Traits

D. Applying Mendel’s Principles E. Genetics and the Environment

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Section Outline

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Two Factor Crosses

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Figure 11-10 Independent Assortment in Peas

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concluded that

which is called the

which is called the

Gregor Mendel

Law of Dominance

Law of Segregation

Pea plants

“Factors” determine

traits Some alleles are dominant,

and some alleles are recessive

Alleles are separated during gamete formation

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Concept Map

experimented with

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Incomplete Dominance

• A scenario where neither gene is dominant over the other. • Example:

–  Red snapdragon crossed with a white snapdragon produces a pink snapdragon.

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Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

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Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

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Codominance

• A situation where both alleles contribute to the phenotype. • Example:

–  Certain chickens with black feathers crossed with a chicken with white feathers produces an offspring with both black and white feathers.

X =

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Multiple Alleles • Genes that have more than two alleles for a gene. • Examples:

–  Blood types in humans: A, B, AB, or O –  Human hair or eye color –  Fur color in rabbits

Full Color Chinchilla Himalayan Albino

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Polygenic Traits

• Traits that are controlled by two or more genes. • Example:

–  Human skin colors

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How Many Chromosomes?

Normal human body cells each contain 46 chromosomes. The cell division process that body cells undergo is called mitosis and produces daughter cells that are virtually identical to the parent cell. Working with a partner, discuss and answer the questions that follow.

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Interest Grabber

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1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis?

2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo?

3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

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11–4 Meiosis A. Chromosome Number B. Phases of Meiosis

1. Meiosis I 2. Meiosis II

C. Gamete Formation D. Comparing Mitosis and Meiosis

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Section Outline

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Section 11-4

Crossing-Over

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Crossing-Over

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Crossing-Over

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Meiosis I

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Figure 11-15 Meiosis

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Meiosis I

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Meiosis I

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Meiosis I

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Meiosis I

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Meiosis I

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Meiosis I

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Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase II The chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

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Figure 11-17 Meiosis II

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Meiosis II





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Meiosis II





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Meiosis II





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Meiosis II





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Gamete Formation

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Forever Linked?

Some genes appear to be inherited together, or “linked.” If two genes are found on the same chromosome, does it mean they are linked forever? Study the diagram, which shows four genes labeled A–E and a–e, and then answer the questions on the next slide.

Section 11-5

Interest Grabber

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1. In how many places can crossing over result in genes A and b being on the same chromosome?

2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e?

3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes?

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11–5 Linkage and Gene Maps A. Gene Linkage B. Gene Maps

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Section Outline

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Earth

Country

State

City

People

Cell

Chromosome

Chromosome fragment

Gene

Nucleotide base pairs

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Comparative Scale of a Gene Map

Mapping of Earth’s Features

Mapping of Cells, Chromosomes, and Genes

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Exact location on chromosomes Chromosome 2

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Figure 11-19 Gene Map of the Fruit Fly

Videos

Click a hyperlink to choose a video. Meiosis Overview Animal Cell Meiosis, Part 1 Animal Cell Meiosis, Part 2 Segregation of Chromosomes Crossing Over

Click the image to play the video segment.

Video 1

Meiosis Overview


Video 2

Animal Cell Meiosis, Part 1


Video 3

Animal Cell Meiosis, Part 2

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Video 4

Segregation of Chromosomes


Video 5

Crossing Over

The latest discoveries in genetics Interactive test

Articles on genetics

For links on Punnett squares, go to www.SciLinks.org and enter the Web Code as follows: cbn-4112. For links on Mendelian genetics, go to www.SciLinks.org and enter the Web Code as follows: cbn-4113. For links on meiosis, go to www.SciLinks.org and enter the Web Code as follows: cbn-4114.

Go Online Interest Grabber Answers

1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics? All offspring had the same characteristic, which was like one of the parents’. The other characteristic seemed to have disappeared.

2. How do the characteristics of the second generation compare to the characteristics of the first generation? Both characteristics appeared in this generation. The characteristic that had “disappeared” in the first generation did not appear as often as the other characteristic. (It appears about 25 percent of the time.)

Interest Grabber Answers

1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how do the results of your tosses compare? How about the results of your partner’s tosses? How close was each set of results to what was expected? Results will vary, but should be close to 5 heads and 5 tails.

2. Add your results to those of your partner to produce a total of 20 tosses. Assuming that you expect 10 heads and 10 tails in 20 tosses, how close are these results to what was expected? The results for 20 tosses may be closer to the predicted 10 heads and 10 tails.

3. If you compiled the results for the whole class, what results would you expect? The results for the entire class should be even closer to the number predicted by the rules of probability.

4. How do the expected results differ from the observed results? The observed results are usually slightly different from the expected results.


1. Make a list of 10 adults whom you know. Next to the name of each adult, write his or her approximate height in feet and inches. Check students’ answers to make sure they are realistic.

2. What can you observe about the heights of the ten people? Students should notice that there is a range of heights in humans.

3. Do you think height in humans is controlled by 2 alleles, as it is in pea plants? Explain your answer. No, height does not seem to be controlled by two alleles, as it is in pea plants. Height in humans can vary greatly and is not just found in tall and short phenotypes.

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1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? 46 chromosomes

2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo? 46 + 46 = 92; a developing embryo would not survive if it contained 92 chromosomes.

3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have? Sperm and egg should each have 23 chromosomes.


1. In how many places can crossing over result in genes A and b being on the same chromosome? One (between A and B)

2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e? Two (between A and B and A and C); Four (between A and B, A and C, A and D, and A and E)

3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes? The farther apart the genes are, the more likely they are to be recombined through crossing over.

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Gregor Mendel’s Peas