genetics: the science of heredity mendel’s...

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Name ____________________________ Date ____________________ Class ____________

Mendel’s WorkKey Concepts

! What were the results of Mendel’s experiments, or crosses?

! What controls the inheritance of traits in organisms?

Heredity is the passing of physical characteristics from parents to offspring. Gregor Mendel was curious about the different forms of characteristics, or traits, of pea plants. Mendel’s work was the foundation of genetics, the scientific study of heredity.

A new organism begins to form when egg and sperm join in the process called fertilization. Before fertilization can happen in pea plants, pollen must reach the pistil of a pea flower through pollination. Pea plants are usually self-pollinating, meaning pollen from a flower lands on the pistil of the same flower. Mendel developed a method by which he cross-pollinated, or “crossed,” pea plants.

Mendel crossed two pea plants that differed in height. He crossed purebred tall plants with purebred short plants. These parent plants, the P generation, were purebred because they always produced offspring with the same trait as the parent. In all of Mendel’s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants. From his results, Mendel reasoned that individual factors, one from each parent, control the inheritance of traits. Today, scientists call the factors that control traits genes. The different forms of a gene are called alleles.

An organism’s traits are controlled by the alleles it inherits from its parents. Some alleles are dominant, while other alleles are recessive. A dominant allele is one whose trait always shows up in the organism when the allele is present. A recessive allele is hidden whenever the dominant allele is present. A trait controlled by a recessive allele will only show up if the organism does not have the dominant allele.

In Mendel’s cross, the purebred tall plant has two alleles for tall stems. The purebred short plant has two alleles for short stems. The F1 plants are all hybrids: they have two different alleles for the trait—one allele for tall stems and one for short stems. Geneticists use a capital letter to represent a dominant allele and a lowercase version of the same letter for the recessive allele.

Mendel’s discovery was not recognized during his lifetime. In 1900, three different scientists rediscovered Mendel’s work. Because of his work, Mendel is often called the Father of Genetics.

Genetics: The Science of Heredity ! Section Summary

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Name ____________________________ Date ____________________ Class ____________G

enetics: The Science

of Heredity

This section describes how Gregor Mendel identified the method by which characteristics are passed from parents to their offspring.

Use Target Reading SkillsAs you read, complete the outline about Mendel’s work. Use the red headings for the main idea and the blue headings for the supporting ideas.

I. Mendel’s experimentsA. crossing pea plantsB.C.D.

II. A.B.C.

D.

Introduction1. Gregor Mendel experimented with hundreds of pea plants to

understand the process of ________________________.

Match the term with its definition.

Term Definition

Mendel’s Work

____ 2. heredity

____ 3. genetics

____ 4. traits

a. The scientific study of heredity

b. Physical characteristics

c. The passing of traits from parents to offspring

Mendel’s Experiments5. In a flower, the female sex cells, or eggs, are produced by the

________________________. Pollen, which contains the male sex cells, is produced by the ________________________.

6. What are purebred organisms?________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

Genetics: The Science of Heredity ! Guided Reading and Study

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Name ____________________________ Date ____________________ Class ____________

Mendel’s Work (continued)

7. Complete the flowchart below, which summarizes Mendel's first experiment with pea plants.

8. Circle the letter of other traits in garden peas that Mendel studied. a. seed size, seed shape, seed colorb. seed color, pod color, flower shapec. flower size, pod shape, seed coat colord. pod color, seed shape, flower position

9. Two forms of the trait of seed shape in pea plants are ________________________ and ________________________.

Dominant and Recessive Alleles10. Circle the letter of each sentence that is true about alleles.

a. Recessive alleles are never present when dominant alleles are present.b. Alleles are different forms of a gene.c. A trait controlled by a dominant allele always shows up in the organism when

the allele is present.d. Recessive alleles hide dominant alleles.

11. Is the following sentence true or false? Only pea plants that have two recessive alleles for short stems will be short. ________________________

Purebred tall plants are crossed with purebred

Mendel's Experiment

plants.

.and

F1 offspring are all

F1 offspring are allowed to self-pollinate.

F2 offspring are

.




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of Heredity

Match the pea plant with its combination of alleles.

Pea Plant Combination of Alleles

____ 12. purebred short

____ 13. purebred tall

____ 14. hybrid tall

a. Two alleles for tall stems

b. One allele for tall stems and one allele for short stems

c. Two alleles for short stems

15. A dominant allele is represented by a(n) ________________________ letter.

16. A recessive allele is represented by a(n) ________________________ letter.

17. How might a geneticist write the alleles to show that a tall pea plant has one allele for tall stems and one allele for short stems? ________________________

18. Is the following sentence true or false? Some scientists during Mendel's time thought Mendel should be called the Father of Genetics. ________________________




Name ____________________________ Date ____________________ Class ____________

Mendel’s Work

Understanding Main Ideas

Study the diagram. Then answer the following questions on a separate sheet of paper.

1.

What trait in pea plants is being studied in the cross above?

2.

What are the two alleles of this trait?

3.

Which allele is the dominant allele? Explain how you know.

4.

Which allele is the recessive allele? Explain.

5.

What alleles do the F

1

offspring have? Explain which allele was inherited from which parent.

Building Vocabulary

Match each term with its definition by writing the letter of the correct definition on the line beside the term.

P Generation F1 Generation F2 Generation

!!

____ 6.

genetics

____ 7.

alleles

____ 8.

traits

____ 9.

recessive allele

____ 10.

genes

____ 11.

hybrid

____ 12.

heredity

____ 13.

dominant allele

a.

the passing of traits from parents to offspring

b.

an organism with two different alleles for a trait

c.

factors that control traits

d.

physical characteristics of organisms

e.

an allele whose trait always shows up in the organism

f.

the different forms of a gene

g.

the scientific study of heredity

h.

an allele whose trait is masked in the presence of a dominant allele

Genetics: The Science of Heredity

!

Review and Reinforce

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of Heredity

The Test CrossWhen an organism has a trait controlled by a dominant allele, it can either be a hybrid or a purebred. To find out which, geneticists can use a test cross. In a test cross, the organism with the trait controlled by a dominant allele is crossed with an organism with a trait controlled by a recessive allele. If all offspring have the trait controlled by the dominant allele, then the parent is probably a purebred. If any offspring has the recessive trait, then the dominant parent is a hybrid. Study the test cross below, then answer the questions.

Answer the following questions on a separate sheet of paper.

1. Is the long-haired cat in the P generation a hybrid or a purebred? Explain your answer.

2. Is the short-haired cat in the P generation a hybrid or a purebred? Explain your answer.

3. If the short-haired cat in the P generation were purebred, what would you expect the offspring to look like?

4. In horses, the allele for a black coat (B) is dominant over the allele for a brown coat (b). A cross between a black horse and a brown horse produces a brown foal. Is the black horse a hybrid or a purebred? Explain.

5. In guinea pigs, the allele for a smooth coat (S) is dominant over the allele for a rough coat (s). Explain how you could find out whether a guinea pig with a smooth coat is a hybrid or a purebred.

P Generation

Test Cross

F1 Generation

H ?hh

H ? hh

H ?

H = short hairh = long hair

H ?

!

Test Cross

H = Short hairh = Long hair

P Generation

F1 Generation

Genetics: The Science of Heredity ! Enrich



Name ____________________________ Date ____________________ Class ____________

Take a Class SurveyProblemAre traits controlled by dominant alleles more common than traits controlled by recessive alleles?

Skill Focusdeveloping hypotheses, interpreting data

Materialsmirror (optional)

Procedure

PART 1 Dominant and Recessive Alleles1. Write a hypothesis reflecting your ideas about the problem question.

________________________________________________________________________

________________________________________________________________________

2. For each of the traits listed in the data table on the next page, work with a partner to determine which trait you have. Circle that trait in your data table.

3. Count the number of students in your class who have each trait. Record that number in your data table. Also record the total number of students.

PART 2 Are Your Traits Unique?4. Look at the circle of traits in your text. All the traits in your data table

appear in the circle. Place the eraser end of your pencil on the trait in the small central circle that applies to you—either free ear lobes or attached ear lobes.

5. Look at the two traits touching the space your eraser is on. Move your eraser onto the next description that applies to you. Continue using your eraser to trace your traits until you reach a number on the outside rim of the circle. Share that number with your classmates.

Genetics: The Science of Heredity ! Skills Lab



Name ____________________________ Date ____________________ Class ____________G


of Heredity

Analyze and ConcludeWrite your answers in the spaces provided.

1. Observing The traits listed under Trait 1 in the data table are controlled by dominant alleles. The traits listed under Trait 2 are controlled by recessive alleles. Which traits controlled by dominant alleles were shown by a majority of students? Which traits controlled by recessive alleles were shown by a majority of students? ________________________________________________________________________

________________________________________________________________________

2. Interpreting Data How many students ended up on the same number on the circle of traits? How many students were the only ones to have their number? What do the results suggest about each person’s combination of traits? ________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

3. Developing Hypotheses Do your data support the hypothesis you proposed in Step 1? Write an answer with examples.________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

Data Table

Total Number or Students ________________________

Trait 1 Number Trait 2 Number

A Free ear lobes Attached ear lobes

B Hair on fingers No hair on fingers

C Widow’s peak No widow’s peak

D Curly hair Straight hair

E Cleft chin Smooth chin

F Smile dimples No smile dimples




Name ____________________________ Date ____________________ Class ____________

Take a Class Survey (continued)

Design an ExperimentDo people who are related to each other show more genetic similarity than unrelated people? Write a hypothesis. Then design an experiment to test your hypothesis. Obtain your teacher’s permission before carrying out your investigation.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________




Name ____________________________ Date ____________________ Class ____________

Probability and HeredityKey Concepts

! What is probability and how does it help explain the results of genetic crosses?

! What is meant by genotype and phenotype?

! What is codominance?

Probability is a number that describes how likely it is that an event will occur. The principles of probability predict what is likely to occur, not necessarily what will occur. For example, in a coin toss, the coin will land either heads up or tails up. Each of these two events is equally likely to happen. In other words, there is a 1 in 2 chance that a tossed coin will land heads up, and a 1 in 2 chance that it will land tails up. A 1 in 2 chance can be expressed as a fraction, 1/2, or as a percent, 50 percent. The result of one coin toss does not affect the result of the next toss. Each event is independent of another.

When Gregor Mendel analyzed the results of his crosses in peas, he carefully counted all the offspring. Over time, he realized that he could apply the principles of probability to his crosses. Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses.

A tool that applies the laws of probability to genetics is a Punnett square. A Punnett square is a chart that shows all the possible combinations of alleles that can result from a genetic cross. Geneticists use Punnett squares to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome. In a Punnett square, all the possible alleles from one parent are written across the top. All the possible alleles from the other parent are written down the left side. The combined alleles in the boxes of the Punnett square represent all the possible combinations in the offspring. In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

Two useful terms that geneticists use to describe organisms are genotype and phenotype. An organism’s phenotype is its physical appearance, or visible traits. An organism’s genotype is its genetic makeup, or allele combinations. When an organism has two identical alleles for a trait, the organism is said to be homozygous for that trait. An organism that has two different alleles for a trait is said to be heterozygous for that trait.

For all of the traits in peas that Mendel studied, one allele was dominant while the other was recessive. This is not always the case. In an inheritance pattern called codominance, the alleles are neither dominant nor recessive. As a result, both alleles are expressed in the offspring. Codominant alleles are written as capital letters with superscripts to show that neither is recessive.




Name ____________________________ Date ____________________ Class ____________G


of Heredity

Probability and HeredityThis section explains what probability is and how the laws of probability can be used in the study of genetics.

Use Target Reading SkillsAfter you read the section, reread the paragraphs that contain definitions of Key Terms. Use all the information you have learned to write a definition of each Key Term in your own words.

Principles of Probability1. A number that describes how likely it is that an event will occur is called

________________________.

2. Circle the letter of each answer that equals the probability that a tossed coin will land heads up.a. 1 in 2b. 1/2c. 50 percentd. 20 percent

3. Is the following sentence true or false? When you toss a coin 20 times, you will always get 10 heads and 10 tails. ________________________

4. If you toss a coin five times and it lands heads up each time, can you expect the coin to land heads up on the sixth toss? Explain. ________________________________________________________________________

________________________________________________________________________




Name ____________________________ Date ____________________ Class ____________

Probability and Heredity

(continued)

Probability and Genetics

5.

When Mendel crossed two hybrid plants for stem height (

Tt

), what results did he always get? ________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

6.

Mendel realized that the principles of probability could be used to ________________________ the results of genetic crosses.

7.

A chart that shows all the possible combinations of alleles that can result from a genetic cross is called a(n) ________________________.

8.

Write in the alleles of the parents and the possible allele combinations of the offspring in the Punnett square below. (Note that both parents are tall. Three of the offspring are tall and one is short.)


!

Guided Reading and Study

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of Heredity

Phenotypes and GenotypesMatch the term with its definition.

Term Definition____ 9. phenotype

____ 10. genotype

____ 11. homozygous

____ 12. heterozygous

a. Describes an organism with two identical alleles for a trait

b. An organism’s physical appearance, or visible traits

c. An organism’s genetic makeup, or allele combinations

d. Describes an organism that has two different alleles for a trait

13. Mendel used the term ________________________ to describe heterozygous pea plants.

Codominance14. Is the following sentence true or false? In codominance, the alleles are

neither dominant nor recessive. ________________________

15. In cattle, red hair and white hair are codominant. Cattle with both white hair and red hair are ________________________.




Name ____________________________ Date ____________________ Class ____________

Probability and HeredityUnderstanding Main IdeasComplete the two Punnett squares below, and then answer the questions on a separate sheet of paper.

3. In the cross between two black guinea pigs shown in Punnett Square A, what is the probability that an offspring will be black? White?

4. Is it possible that the cross between two black guinea pigs in Punnett Square A would not produce a white guinea pig? Explain.

5. What color are the guinea pig parents in the cross shown in Punnett Square B?6. Which guinea pig parent(s) in Punnett Square B is homozygous? Which

is heterozygous? Explain how you know.7. Calculate the probability that an offspring will be black in the cross in

Punnett Square B. What is the probability that an offspring will be white?

Building VocabularyMatch each term with its definition by writing the letter of the correct definition on the line beside the term.

1. Punnett Square A: 2. Punnett Square B:

B

bB

b

Bb bb

Bb bb

____ 8. heterozygous

____ 9. Punnett square

____ 10. genotype

____ 11. codominance

____ 12. probability

____ 13. homozygous

____ 14. phenotype

a. a chart that shows all the possible combinations of alleles that can result from a genetic cross

b. a number that describes how likely it is that an event will occur

c. an organism that has two identical alleles for a trait

d. an organism’s physical appearance

e. an organism’s genetic makeup, or allele combinations

f. an organism that has two different alleles for a trait

g. inheritance pattern in which the alleles are neither dominant nor recessive

Genetics: The Science of Heredity ! Review and Reinforce



Name ____________________________ Date ____________________ Class ____________G


of Heredity

Genetic Crosses With Two TraitsIn his work with garden peas, Mendel also set up crosses in which he studied the inheritance of two traits at one time. For example, he crossed tall plants having green pods (TTGG) with short plants having yellow pods (ttgg). The F1 offspring showed both traits controlled by dominant alleles, tall and green. Mendel allowed the F1 offspring to self-pollinate. The F2 offspring had four different phenotypes: tall plants with green pods, tall plants with yellow pods, short plants with green pods, and short plants with yellow pods. These results led Mendel to formulate the Law of Independent Assortment, which states that alleles of one gene separate or assort independently of alleles of another gene. In other words, the distribution of alleles of one gene does not affect the distribution of alleles for another gene.

Study the Punnett square of a genetic cross between two pea plants with two different traits. Then answer the questions that follow.


1. What are all the possible combinations of alleles that each F1 parent can pass on to the offspring?

2. What are the possible genotypes of the F2 offspring? What are the possible phenotypes of the F2 offspring?

3. What is the probability that an F2 offspring will be tall with green pods? What is the probability that an F2 offspring will be short with yellow pods?


P Generation

F1 Generation

!

F2 Generation

TTGG

TTGG

TtGg

ttgg

TTGg TtGG TtGg

TG Tg tG tg

TG

TTGg TTgg TtGg TtggTg

TtGG TtGg ttGG ttGgtG

TtGg Ttgg ttGg ttggtg



Name ____________________________ Date ____________________ Class ____________

Make the Right Call!ProblemHow can you predict the possible results of genetic crosses?

Skills Focusmaking models, interpreting data

Materials2 small paper bags3 blue marbles

marking pen3 white marbles

Procedure1. Label one bag “Bag 1, Female Parent.” Label the other bag “Bag 2, Male Parent.”

Then read over Part 1, Part 2, and Part 3 of this lab. Write a prediction on another sheet of paper about the kinds of offspring you expect from each cross.

PART 1 Crossing Two Homozygous Parents2. Place two blue marbles in Bag 1. This pair of marbles represents the female parent’s

alleles. Use the letter B to represent the dominant allele for blue color.3. Place two white marbles in Bag 2. Use the letter b to represent the recessive allele for

white color.4. For Trial 1, remove one marble from Bag 1 without looking in the bag. Record the

result in your data table. Return the marble to the bag. Again, without looking in the bag, remove one marble from Bag 2. Record the result in your data table. Return the marble to the bag.

5. In the column labeled Offspring’s Alleles, write BB if you removed two blue marbles, bb if you removed two white marbles, or Bb if you removed one blue marble and one white marble.

6. Repeat Steps 4 and 5 nine more times.

PART 2 Crossing Homozygous and Heterozygous Parents7. Place two blue marbles in Bag 1. Place one white marble and one blue marble

in Bag 2.8. Repeat Steps 4 and 5 ten times, and record your data in the data table for Part 2.

PART 3 Crossing Two Heterozygous Parents9. Place one blue marble and one white marble in Bag 1. Place one blue marble and one

white marble in Bag 2.10. Repeat Steps 4 and 5 ten times, and record your data in the data table for Part 3.




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of Heredity

Data Table: Part 1

Data Table: Part 2

Data Table: Part 3

Trial Allele From Bag 1 (Female Parent)

Allele From Bag 2 (Male Parent)

Offspring’s Alleles

123456789

10




123456789

10




123456789

10




Name ____________________________ Date ____________________ Class ____________

Make the Right Call! (continued)

Analyze and ConcludeWrite your answers on a separate sheet of paper.

1. Making Models Make a Punnett square for each of the crosses you modeled in Part 1, Part 2, and Part 3.

2. Interpreting Data According to your results in Part 1, how many different kinds of offspring are possible when the homozygous parents (BB and bb) are crossed? Do the results you obtained using the marble model agree with the results shown by a Punnett square?

3. Predicting According to your results in Part 2, what percentage of offspring are likely to be homozygous when a homozygous parent (BB) and a heterozygous parent (Bb) are crossed? What percentage of offspring are likely to be heterozygous (Bb)? Does the model agree with the results shown by a Punnett square?

4. Communicating According to your results in Part 3, what different kinds of offspring are possible when two heterozygous parents (Bb ! Bb) are crossed? What percentages of each type of offspring are likely to be produced? Does the model agree with the results of a Punnett square?

5. Inferring For Part 3, if you did 100 trials instead of 10 trials, would your results be closer to the results shown in a Punnett square? Explain.

6. Communicating In a paragraph, explain how the marble model compares with a Punnett square. How are the two methods alike? How are they different?

More to ExploreIn peas, the allele for yellow seeds (Y) is dominant over the allele for green seeds (y). What possible crosses do you think could produce a heterozygous plant with yellow seeds (Yy)? Use the marble model and Punnett squares to test your predictions.




Name ____________________________ Date ____________________ Class ____________

The Cell and Inheritance Key Concepts

! What role do chromosomes play in inheritance?

! What events occur during meiosis?

! What is the relationship between chromosomes and genes?

In the early 1900s, scientists were working to identify the cell structures that carried Mendel’s hereditary factors, or genes. In 1903, Walter Sutton observed that sex cells in grasshoppers had half the number of chromosomes as the body cells. He also noticed that each grasshopper offspring had exactly the same number of chromosomes in its body cells as each of the parents. He reasoned that the chromosomes in body cells actually occurred in pairs, with one chromosome in each pair coming from the male and the other coming from the female.

From his observations, Sutton concluded that genes are located on chromosomes. He proposed the chromosome theory of inheritance. According to the chromosome theory of inheritance, genes are carried from parents to their offspring on chromosomes.

Organisms produce sex cells during meiosis. Meiosis is the process by which the number of chromosomes is reduced by half to form sex cells—sperm and eggs. During meiosis, the chromosome pairs separate and are distributed to two different cells. The resulting sex cells have only half as many chromosomes as the other cells in the organism. When they combine, each sex cell contributes half the number of chromosomes to produce offspring with the correct number of chromosomes.

Punnett squares show the results of meiosis. When chromosome pairs separate, so do the alleles carried on the chromosomes. One allele from each pair goes to each sex cell.

Chromosomes are made up of many genes joined together like beads on a string. Each chromosome contains a large number of genes, each gene controlling a particular trait. Each chromosome pair has the same genes. The genes are lined up in the same order on both chromosomes. However, the alleles for some of the genes might differ from each other, making the organism heterozygous for some traits. If the alleles are the same, the organism is homozygous for those traits.




Name ____________________________ Date ____________________ Class ____________G


of Heredity

The Cell and InheritanceThis section describes how one set of chromosomes from each parent is passed on to the offspring.

Use Target Reading SkillsAs you read, identify the evidence that supports the hypothesis that genes are found on chromosomes. Write the evidence in the graphic organizer below.

Chromosomes and Inheritance1. Circle the letter of each sentence that is true about what Sutton observed

about chromosome number.a. Grasshopper sex cells have half the number of chromosomes as body cells.b. Grasshopper body cells have half the number of chromosomes as sex cells.c. Grasshopper body cells and sex cells have the same number of chromosomes.d. When grasshopper sex cells join, the fertilized egg has the same number of

chromosomes as the body cells of the parents.

2. What is the chromosome theory of inheritance?________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

Meiosis3. What is meiosis?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

Hypothesis

Evidence

Chromosomes are importantin inheritance.

Grasshoppers: 24 chromosomesin body cells, 12 in sex cells




Name ____________________________ Date ____________________ Class ____________

The Cell and Inheritance (continued)

4. Complete the cycle diagram, which describes the events that occur during meiosis.

5. A Punnett square is a shorthand way to show the events that occur during ________________________.

6. Is the following sentence true or false? During meiosis, the two alleles for each gene stay together. ________________________

7. If the male parent cell is heterozygous for a trait, Tt, what alleles could the sperm cells possibly have?________________________________________________________________________

________________________________________________________________________

A Lineup of Genes8. How many pairs of chromosomes do human body cells contain?

________________________

9. How are the genes lined up in a pair of chromosomes?________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

Sex cells combine to produce offspring.

Each offspring has

chromosomes, one pair from each parent.

Parent cell has four chromosomes

arranged in pairs.

Chromosome pairs and are

chromosomes.

distributed to sex cells. Each sex cell has




Name ____________________________ Date ____________________ Class ____________G


of Heredity

The Cell and InheritanceUnderstanding Main IdeasComplete the table below by filling in the spaces with the correct stage of meiosis—Beginning, Meiosis I, Meiosis II, End.

Answer the following questions in the spaces provided.

5. What is the chromosome theory of inheritance?________________________________________________________________________

________________________________________________________________________

6. Why is it important that sex cells have half the number of chromosomes as body cells?________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

Building VocabularyFill in the blank to complete the statement.

7. The process by which the number of chromosomes is reduced by half to form sex cells is called ________________________.

Event Stage in Meiosis

The double-stranded chromosomes move to the center of the cell. The centromeres separate.

1. ________________________

Two cells form, each with half the number of chromosomes. Each chromosome still has two chromatids.

2. ________________________

Four sex cells form with half the number of chromosomes as the parental cells.

3. ________________________

The chromosomes are copied. 4. ________________________




Name ____________________________ Date ____________________ Class ____________

A Model of MeiosisFollow the procedure below to make a model of meiosis.

Materialsdifferent colors of pipe cleaners or yarnbeadsmacaronistringgluescissorsmarkerconstruction paper or poster board

Procedure1. Study the diagram of the stages of meiosis in your textbook.2. Decide how you can use the materials listed above, or other materials of

your choice, to make a model of meiosis. Your model should include the beginning of meiosis, meiosis I, meiosis II, and the end of meiosis.

3. Create your model. Begin with at least six copied pairs of chromosomes. Label the stages of meiosis and all the important structures. On a separate sheet of paper, write a description in your own words of what happens in each stage of meiosis.

Analyze and ConcludeAnswer the following questions on a separate sheet of paper.

1. What is meiosis?2. What must happen before meiosis can begin?3. What happens to chromosomes during meiosis I?4. What happens to chromosomes during meiosis II?5. Compare the sex cells produced by meiosis to the parent cell. Why is the

difference between the sex cells and parent cell important?6. Why are chromosomes important to heredity?




Name ____________________________ Date ____________________ Class ____________

The DNA ConnectionKey Concepts

! What forms the genetic code?

! How does a cell produce proteins?

! How can mutations affect an organism?

Today, scientists know that genes control the production of proteins in the cells of an organism. Proteins determine the size, shape, and other traits of organisms. Recall that chromosomes are composed mostly of DNA. A DNA molecule is made up of four nitrogen bases—adenine (A), thymine (T), guanine (G), and cytosine (C). The order of the nitrogen bases along a gene forms a genetic code that specifies what type of protein will be produced. In the genetic code, a group of three DNA bases codes for one specific amino acid.

During protein synthesis, the cell uses information from a gene on a chromosome to produce a specific protein. Protein synthesis occurs on the ribosomes in the cytoplasm of the cell. DNA, however, is located in the cell nucleus. Before protein synthesis occurs, a genetic “messenger,” called ribonucleic acid or RNA, is made based on a code in the DNA. RNA is similar to DNA, except RNA has only one strand and it has uracil instead of thymine.

In the first step of protein synthesis, the DNA molecule “unzips” and directs the production of messenger RNA. There are several types of RNA involved in protein synthesis. Messenger RNA copies the coded message from the DNA in the nucleus, and carries it to the ribosomes in the cytoplasm. Transfer RNA carries amino acids and adds them to the growing protein.

Sometimes changes called mutations occur in a gene or chromosome. Mutations can cause a cell to produce an incorrect protein during protein synthesis. As a result, the organism’s trait, or phenotype, may be different from what it normally would have been. If a mutation occurs in a body cell, the mutation affects only the cell that carries it. However, if a mutation occurs in a sex cell, the mutation can be passed on to an offspring and affect the offspring’s phenotype. Some mutations are the result of small changes in an organism’s hereditary material. Others occur when chromosomes don’t separate correctly during meiosis.

Some of the changes brought about by mutations are harmful to an organism. A few mutations, however, are helpful, and still others are neither harmful nor helpful. A mutation is harmful if it reduces an organism’s chance for survival and reproduction. Whether or not a mutation is harmful depends partly on the organism’s environment. For example, a white lemur may not survive in the wild, but the mutation has no effect on its ability to survive in a zoo.




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of Heredity

The DNA ConnectionThis section tells how the DNA molecule is related to genes, chromosomes, and the inheritance of traits.

Use Target Reading SkillsAs you read, complete the flowchart below to show protein synthesis. Put the steps of the process in separate boxes in the flowchart in the order in which they occur.

The Genetic Code1. Circle the letter of each sentence that is true about genes, chromosomes,

and proteins.a. Genes control the production of proteins in an organism’s

cells.b. Proteins help determine the size, shape, and other traits of an

organism.c. Chromosomes are made up mostly of proteins.d. A single gene on a chromosome contains only one pair of

nitrogen bases.

2. What are the four nitrogen bases that make up a DNA molecule?________________________________________________________________________

________________________________________________________________________

Protein Synthesis

DNA provides code to form messenger RNA.

Messenger RNA attaches to ribosome.




Name ____________________________ Date ____________________ Class ____________

The DNA Connection (continued)

3. What is the genetic code?________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

4. One group of three nitrogen bases codes for one ________________________.

How Cells Make Proteins5. During protein synthesis, the cell uses information from a

________________________ on a chromosome to produce a specific ________________________.

6. Proteins are made on ________________________ in the cytoplasm of the cell.

7. Complete this Venn diagram to show some of the similarities and differences between DNA and RNA. Tell where each nucleic acid is located and what bases it contains.

DNA RNA

AdenineGuanine

Stays insidethe nucleus




Name ____________________________ Date ____________________ Class ____________G


of Heredity

8. List the two kinds of RNA and describe their jobs.

a. _____________________________________________________________________

________________________________________________________________________

________________________________________________________________________

b. _____________________________________________________________________

________________________________________________________________________

________________________________________________________________________

9. Circle the letter of the first step in protein synthesis.a. Transfer RNA carries amino acids to the ribosome.b. The ribosome releases the completed protein chain.c. Messenger RNA enters the cytoplasm and attaches to a ribosome.d. DNA “unzips” to direct the production of a strand of messenger RNA.

10. Circle the letter of the last step in protein synthesis.a. Transfer RNA carries amino acids to the ribosome.b. The protein chain grows longer as each transfer RNA molecule

adds an amino acid.c. Messenger RNA enters the cytoplasm and attaches to a ribosome.d. DNA “unzips” to direct the production of a strand of messenger RNA.

Mutations11. What is a mutation?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

12. How can mutations affect protein synthesis in cells?________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________


sxtr05_bkc_ch3_ls4.fm 3 Wednesday, December 22, 2004 4:03 PM


Name ____________________________ Date ____________________ Class ____________

The DNA Connection (continued)

13. Circle the letter of each sentence that is true about mutations.a. Cells with mutations will always make normal proteins.b. Some mutations occur when one nitrogen base is substituted for another.c. Some mutations occur when chromosomes don’t separate correctly during

meiosis.d. Mutations that occur in a body cell can be passed on to an offspring.

14. Mutations can be a source of genetic ________________________.

15. Is the following sentence true or false? All mutations are harmful. ________________________

16. Mutations that are ________________________ improve an organism’s chances for survival and reproduction.

17. Whether a mutation is harmful or helpful depends partly on an organism’s ________________________.




Name ____________________________ Date ____________________ Class ____________G


of Heredity

The DNA ConnectionUnderstanding Main IdeasComplete the table below by stating whether each mutation is helpful, harmful, or neutral to the organism.

Answer the following questions on the lines provided.

4. Describe what occurs during protein synthesis._______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

5. What is the genetic code?_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

Building VocabularyFill in the blank to complete each statement.

6. A ________________________ is any change in a gene or chromosome.7. A type of RNA that carries amino acids and adds them to the growing

protein is called ________________________ .8. ________________________ is RNA that copies the coded message from

the DNA in the nucleus and carries the message into the cytoplasm.

Mutation Effect

White lemur (in a zoo) 1. ________________________

White lemur (in the wild) 2. ________________________

Antibiotic resistance in bacteria 3. ________________________




Name ____________________________ Date____________________ Class ____________

The Genetic Code

The genetic code is made up of groups of three nitrogen bases in the messenger RNA. Each three-base group, called a

codon,

codes for one amino acid. The table below shows the genetic code. To find the amino acid that is coded for by the codon UGG in messenger RNA, look in the row of the first base in the codon—U. Then move to the box that is specified by the second base in the codon—G. Finally, look down the list of amino acids in the box until you find the one in row “G,” the third base in the codon. You should find that UGG is the codon for tryptophan.

Transfer RNA matches up with the messenger RNA at the ribosome to deliver the correct amino acid to the growing protein chain. Transfer RNA has a three-base code called an

anticodon

that matches up with the codon in the messenger RNA.


1.

If the DNA sequence of a gene was TACTTACCGAGCTAGACT, then what is the sequence of the messenger RNA?

2.

Use the genetic code to identify the sequence of amino acids encoded by the messenger RNA that you identified in Question 1.

3.

What are the sequences of the anticodons for the transfer RNA molecules that carry each of the amino acids in the protein sequence that you identified in Question 2?

4.

How would the protein change if a mutation caused a base to be added, making the mutated DNA sequence TACGTTACCGAGCTAGACT? How is the protein affected by this mutation? (

Hint:

How does the extra letter change the series of bases?)

5.

How would the protein change if a mutation caused one base to replace another, making the mutated DNA sequence TACTTACCTAGCTAGACT? How does this mutation affect protein function?


!

Enrich

The Genetic Code (messenger RNA)

Second Base in Codon

Firs

t Ba

se in

Cod

on

Thir

d Ba

se in

Cod

onGlutamineGlutamineHistidineHistidine

ArginineArginineArginineArginine

A

C

AGUC

G

LeucineLeucineLeucineLeucine

ProlineProlineProlineProline

"Stop" codon"Stop" codonTyrosineTyrosine

"Stop" codonTryptophanCysteineCysteine

U

AGUC

LeucineLeucinePhenylanalinePhenylanaline

SerineSerineSerineSerine

Glutamic acidGlutamic acidAspartic acidAspartic acid

GlycineGlycineGlycineGlycine

G

AGUC

ValineValineValineValine

AlanineAlanineAlanineAlanine

LysineLysineAsparagineAsparagine

ArginineArginineSerineSerine

A

AGUC

IsoleucineMethionineIsoleucineIsoleucine

ThreonineThreonineThreonineThreonine

U C

sxtr05_bkc_ch3_ls4.fm Page 276 Thursday, January 20, 2005 11:14 AM


Name ____________________________ Date ____________________ Class ____________G


of Heredity

1. The process by which the number of chromosomes is reduced by half in sex cells

2. A chart that shows all possible allele combinations resulting from a genetic cross

3. An organism’s physical appearance4. RNA that copies the coded message

in DNA5. Describes an organism that has two

different alleles for a trait

6. Number that describes the likelihood that a certain event will occur

7. An allele whose trait always shows up in the organism when the allele is present

8. Physical characteristic of an organism9. A factor that controls a trait

10. The scientific study of heredity11. Offspring of many generations that

have the same trait

Key TermsUse the clues below to identify Key Terms from the chapter. Write the terms below, putting one letter in each blank. When you finish, the word enclosed in the diagonal lines will reveal what Mendel studied.

Clues

1. ___ ___ ___ ___ ___ ___ ___

2. ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___

3. ___ ___ ___ ___ ___ ___ ___ ___ ___

4. ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___

5. ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___

6. ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___

7. ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___

8. ___ ___ ___ ___ ___

9. ___ ___ ___ ___

10. ___ ___ ___ ___ ___ ___ ___ ___

11. ___ ___ ___ ___ ___ ___ ___ ___

Genetics: The Science of Heredity ! Key Terms



Name ____________________________ Date ____________________ Class ____________

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Connecting ConceptsDevelop a concept map that uses the Key Concepts and Key Terms from this chapter. Keep in mind the big ideas of this chapter. The concept map shown is one way to organize how the information in this chapter is related. You may use an extra sheet of paper.

Genetics: The Science of Heredity ! Connecting Concepts



Genetics: The S

cience of H

eredityTEACHER NOTES

Genetics: The Science of HeredityLaboratory Investigation

Chromosomes and InheritanceKey ConceptThe traits of offspring are determined by the alleles of the parents.

Skills Focusobserving, making models

Time40 minutes

Materials (per group)markercraft stickAlternate Materials: If craft sticks are unavail-able, strips of cardboard or acetate could be used. Using clear, colorless acetate strips for recessive alleles and colored strips for dominant alleles can be helpful.

Advance PreparationGet enough craft sticks so that you have four for each group of students (eight if students do the More to Explore).

Teaching Tips! (Step 2) Remind students that scientists

generally represent a dominant allele with a capital letter, often the initial of the word that best describes the trait, and the recessive allele with the corresponding lowercase letter.

! (More to Explore) Remind students that Punnett square results show only probabili-ties. The traits of offspring from actual mat-ings may not match the predictions.

! (More to Explore) Based on the principles of probability, two of four, or 50%, of the young unimonsters would have curly hair (Cc), and two of four, or 50%, of the young unimon-sters would have straight hair (cc).



Name ____________________________ Date ____________________ Class ____________

Chromosomes and InheritancePre-Lab DiscussionHow are traits inherited? You can investigate this question by considering an imaginary animal called the unimonster. Suppose this animal has only one pair of chromosomes. Chromosomes carry genes, which control different genetic traits, such as hair color, height, and other physical characteristics. Different forms of a gene are called alleles. The presence of different alleles on the chromosomes of unimonsters determines whether they have one horn or two horns. During reproduction, parent unimonsters pass on alleles to their offspring.In this investigation, you will determine the different allele combinations for the offspring of two unimonsters and figure out the number of horns the young unimonsters will have.1. What are dominant and recessive alleles?

________________________________________________________________________

________________________________________________________________________

2. Define genotype and phenotype.________________________________________________________________________

________________________________________________________________________

3. What does it mean to say that an organism is homozygous for a trait? Heterozygous for a trait?________________________________________________________________________

________________________________________________________________________

4. How do the numbers of chromosomes in cells compare with the number of chromosomes in sex cells? During reproduction, what fraction of chromosomes does each parent contribute to its offspring?________________________________________________________________________

________________________________________________________________________

ProblemHow can you determine the traits of a unimonster ’s offspring?

Materials (per group)markercraft sticks

Genetics: The Science of Heredity ! Laboratory Investigation



Name ____________________________ Date ____________________ Class ____________G


of Heredity

Mother Unimonster Father Unimonster

Figure 1

Figure 2

Procedure1. Figure 1 shows a mother and a father

unimonster, each with different genetic traits. The allele for two horns is dominant over the allele for one horn. Look at the drawing and answer question 1 in Observations.

2. The mother unimonster is heterozygous. This means that she has one allele for two horns and one allele for one horn. Each of her sex cells will have either a chromosome with the two-horn allele or a chromosome with the one-horn allele. Follow Figure 2 and steps 3 and 4 to make a model of the mother unimonster ’s sex chromosomes.

3. One of the mother unimonster ’s chromosomes will carry the two-horn allele. Write "M1" (for mother) at one end of a craft stick. At the other end of the stick, write H for the dominant two-horn allele.

4. The mother unimonster’s other chromosome will carry the one-horn allele. Write "M2" at the end of a second stick. At the other end, write h for the recessive one-horn allele.

5. The father unimonster is homozygous (hh). Follow Figure 2 to make models of the father ’s chromosomes: F1 and F2.

6. During reproduction, the sex cells produced by the mother and father unimonsters combine to form a fertilized egg. The fertilized egg will grow into a young unimonster. Whether the young unimonster has one or two horns depends on the alleles on the chromosome contributed by each parent during reproduction. In Observations, use your chromosome models to answer questions 2–5. Remember that the allele for two horns is dominant. Anytime the dominant allele (H) is present, the unimonster will have two horns.




Name ____________________________ Date ____________________ Class ____________

Chromosomes and Inheritance (continued)

Observations1. Which unimonster parent has the dominant allele for number of horns?

How do you know?________________________________________________________________________

________________________________________________________________________

2. During reproduction, the sex cells containing the chromosomes M1 and F1 combine to form a fertilized egg.a. Which alleles are on each of the chromosomes?________________________________________________________________________

b. Will the young unimonster have one horn or two horns? Draw the appropriate number of horns on young unimonster 1 in Figure 3.

3. During reproduction, the sex cells containing the chromosomes M1 and F2 combine to form a fertilized egg.a. Which alleles are on each of the chromosomes? ________________________________________________________________________






Young Unimonster 1 Young Unimonster 2 Young Unimonster 3 Young Unimonster 4

Figure 3




Name ____________________________ Date ____________________ Class ____________G


of Heredity

Analyze and Conclude1. Which young unimonster(s) are homozygous and have one horn?

________________________________________________________________________

________________________________________________________________________

2. Which young unimonster(s) are heterozygous?________________________________________________________________________

________________________________________________________________________

3. Are any young unimonster(s) homozygous with two horns? Explain.________________________________________________________________________

________________________________________________________________________

Critical Thinking and Applications1. If a mother unimonster is homozygous and has two horns, and a father

unimonster is homozygous and has one horn, what are the phenotypes and genotypes of the possible offspring? Remember that the two-horn allele is dominant.________________________________________________________________________

________________________________________________________________________

2. Predict the phenotypes and genotypes of the offspring of a mother unimonster and a father unimonster that are both heterozygous. ________________________________________________________________________

________________________________________________________________________

More to ExploreRepeat the lab for the traits of curly hair versus straight hair. Assume that the curly-hair allele is dominant and the straight-hair allele is recessive. The mother is homozygous and has straight hair, while the father is heterozygous. Get four more craft sticks. Make all the combinations of different alleles. Determine all of the possible genotypes and the resulting phenotypes of the offspring. You may wish to use the Punnett square below to record the genotypes.

Father’s Alleles for Curly Hair

Mot

her ’s

Alle

les

for

Curly

Hai

r c

c

C c



genetics: the science of heredity mendel’s...

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