Houghton mifflin boston 2 Program Authors William Badders



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Conclusion

1. Use Numbers How did the data change as you counted more people in the class? Why is it important that scientists use a large number of samples when doing research, such as you did in this activity?

2. Evaluate Of the five traits you studied, which could change as a person grows older? Which always stay the same?

3. Analyze Data For each trait, which form did you find to be the most common?

STEP 2

Letter and Number:

Questions:

Answers:

1. What color are your eyes?




2. Are your earlobes attached or detached?




3. Do you write with your left hand, right hand, or both?




4. Do you know how to ride a skateboard?




5. Can you recite the alphabet backwards very quickly?




[Image: STEP 3.]

***


Investigate More! Design an Experiment

With your teacher's permission, survey students in other classes about the five traits. Is the additional data consistent with the data from your class? What conclusions can you draw from your results?

***

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* VOCABULARY

acquired trait p. A84

Chromosome p. A86

DNA p. A86

Gene p. A87

Heredity p. A84

Mutation p. A89

Nucleotide p. A87

* READING SKILL

Sequence Use a chart like the one below to show the order of events in DNA replication.

[Image: A chart.]



Traits

MAIN IDEA Living things inherit many traits from their parents. They acquire others from their environment.

Traits of Organisms

Do you look like anyone in your family? People tend to look like their parents and grandparents because of heredity. Heredity is the process through which traits are passed from parents to offspring.

Human traits that are passed by heredity--or inherited traits--include face shape, hair color, and blood type. The color of an animal's fur, the shape of its ears, and the arrangement of its teeth are also examples of inherited traits. The colors of a flower or the shape of a fruit are examples among plants. An inherited trait can also be a behavior, such as the way a spider spins a web.

Not every trait that you can observe is passed down by heredity. Some traits are acquired. An acquired trait is one that an organism develops after it is born. Some acquired traits come from the environment, others are learned, and still others are brought about purposely.

[Image: Inherited traits are passed on from generation to generation through chromosomes.]

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Acquired Traits

Flamingos are known for their beautiful pink color, which sets them apart from other large, long-legged water birds. The color comes from pigments in shrimp and certain algae that flamingos eat.

The pink color of flamingos is an example of an acquired trait. This kind of trait comes from interactions with the environment. In a similar way, organisms can acquire traits from food, soil, water, and other elements in their environment.

Some acquired traits are learned. You were not born knowing how to ride a bicycle or a skateboard--you learned how to ride. All types of animals learn behaviors. Dogs can learn to follow commands, lions can learn to hunt, and birds can learn how to sing songs. An acquired trait of this kind is called a learned trait.

Manipulated traits are traits that people deliberately change. Plants and animals have been manipulated for thousands of years. Breeders mate animals or cross plants with the most desirable traits. Gardeners also control the shape and design of a plant to keep some plants small or to give others interesting shapes.

Some traits arise from combinations of inherited and acquired features. For example, a person may inherit the capacity to be tall. However, the trait will show itself only if combined with proper nutrition and exercise.

***

SEQUENCE

How .night a plan, develop an acquired trait?

[Image: Learned Organisms can acquire traits, like the ability to ride a skateboard, by learning and remembering skills and information .]

[Image: Environmental Organisms can acquire traits from their environments. A flamingo is pink because it eats pink food .]

[Image: Manipulated Manipulated traits, such as coiled bamboo, are created by human design. People can change both inherited and acquired traits .]



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Chromosomes and Genes

What determines the inherited traits of an organism? The information is stored within the cell nucleus in a molecule called deoxyribonucleic acid, DNA. A DNA molecule consists of two, long parallel strands. The strands coil around each other like edges of a twisted ladder.

Molecules of DNA are passed from one generation to the next during reproduction. Recall that reproduction is the process through which organisms make offspring. For DNA to be passed to offspring, a copy of the molecules must first be made.

In cells that are not dividing, DNA and protein are found in a loose form that is spread out within the nucleus. This form of DNA, called chromatin, is difficult to see even under a light microscope.

As the cell gets ready to divide, however, the double strands of DNA coil tightly. These shorter, thicker coils of DNA form rod-shaped structures called chromosomes. Chromosomes are visible under a microscope.

As a cell is dividing, each chromosome consists of two identical halves called chromatids. The central region that holds the chromatids together is called the centromere.

In the bodies of most organisms, all cells have an even number of chromosomes. In fact, they form pairs. The chromosomes in each pair are similar, but not identical.

The number of chromosomes per cell is different in different species. Humans, for example, have 46 chromosomes in each cell. Dogs have 78, cats have 38, and fruit flies have 8 chromosomes.



Cells and DNA

[Image: Organism In the cells of a human, an onion plant, or any organism, a molecule called DNA controls all cell activity .]

[Image: Cell Just before the cell divides, its DNA coils tightly into rod-shaped chromosomes .]

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In every plant and animal, one type of cell is made with only half the chromosomes of other cells. These are the cells used for reproduction. They are called gametes, or egg and sperm cells. Human gametes, for example, contain only 23 chromosomes.

When two gametes combine, their chromosomes become part of the nucleus of a cell of a new individual. The cell has a complete set of chromosomes.

There are many more traits than there are chromosomes. The reason is that the information in one chromosome can determine many traits. Each trait of an organism is determined by a short segment of DNA known as a gene. One chromosome can have hundreds of genes on it.

The Structure of DNA

The basic units of DNA are called nucleotides. Each nucleotide is made up of a phosphate, a sugar, and a nitrogen base. There are four possible nitrogen bases.

Recall that a molecule of DNA resembles a ladder. The phosphate groups and sugar molecules make up the sides of the ladder. Pairs of nitrogen bases make up the steps. Pairs form only between specific bases.

The order of the nitrogen bases determines the genes of the organism. Because the bases can be arranged in a great number of ways, a great number of genes are possible.

***

SEQUENCE

Why do body cells have twice as many chromosomes as gametes?

[Image: Chromosome A chromosome is made of two identical strands, called chromatids, joined at the centromere. The DNA is organized into units called genes.

[Image: DNA The information in DNA is coded by its arrangement of nitrogen bases. A single DNA molecule may have billions of base pairs !]

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DNA Replication

[Image: During replication, a DNA molecule separates. Each strand is used to form a new DNA molecule.]

[Image: The two new molecules are identical to the original.]

DNA Replication

Before a cell divides, an exact copy of its DNA is made. This process is called DNA replication. To begin the process, the strands of the DNA molecule separate along their nitrogen bases. This part of the process resembles unzipping a zipper.

Quickly after, bases floating around the nucleus attach to the bases of unzipped strands of DNA. Remember that bases always pair up in the same combinations. That means that the same type of base that just separated from one strand attaches to take its place.

This process happens to both of the unzipped strands. When all the bases are in place, a new strand has formed on each of the original strands. In this way, two new DNA molecules are formed, each identical to the original.



Protein Synthesis

What is so important about the order of bases in the DNA molecule? DNA directs the production of substances called proteins. Proteins control most of the life processes in cells. They are also necessary for building and maintaining cells. It is the proteins that cause certain traits to be expressed in an organism.

Proteins are made up of smaller units known as amino acids. There are 20 different amino acids that can be arranged in many different combinations. The specific arrangement of amino acids determines the nature of the protein.

Recall from Chapter 1 that proteins are formed on cell organelles called ribosomes. By means of intermediate molecules, DNA directs the order in which amino acids are arranged to form these proteins. Through this action, DNA controls all cell activities!



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Mutations

[Image: Illustration.]



Can you find the mutation?

Mutations

Occasionally, an error occurs during the process of DNA replication. This kind of change is called a mutation.

A mutation may cause a change in the proteins formed in a cell. Many mutations are harmful because they decrease an organism's chances for survival. In rare cases, mutations are helpful because they result in desirable traits.

Some mutations are neither harmful nor helpful. Even if they result in changes to the organism's proteins, the changes are less obvious and do not directly affect the survival of the organism.

Many mutations occur by chance. Other mutations are caused by environmental factors called mutagens. Ultraviolet radiation from the Sun and certain chemicals, such as some pesticides, can act as mutagens.

Can a mutation be passed to an organism's offspring? This depends on the cell in which the mutation occurs. If the mutation affects a gamete--a sperm cell or an egg cell--the mutation may be passed on to the next generation. Mutations that affect body cells only are not passed along.

***

SEQUENCE

How does a mutation affect the traits of an organism?

[Image: Do you see the different streaks and colors on the corn kernels? They came from mutations--changes to the normal replication of genes .]



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Genes and Health

Some human diseases are inherited because they result from mutations in one or more genes. Sickle cell anemia is a disorder that results when a person inherits a mutated gene for the production of hemoglobin. In blood, hemoglobin is the protein that carries oxygen.

In this disorder, one of the nitrogen bases in the gene for hemoglobin is changed. This error results in the production of protein that causes red blood cells to be shaped like sickles instead of the normal donut shape. The sickle shape prevents the cells from working properly.

Cystic fibrosis is another inherited disorder. This is a disease that results from genes that produce a defect in proteins that control the flow of certain materials into and out of cells.

Some other inherited diseases include hemophilia and muscular dystrophy. Hemophilia is a disease in which the blood does not clot properly. Muscular dystrophy causes the muscles to break down over time.

Another type of genetic disorder occurs when chromosomes do not separate properly during reproduction. Recall that chromosomes are copied before a cell divides. The copies split apart, and one copy goes to each new cell. Down syndrome occurs when a specific pair of chromosomes does not separate properly. The extra chromosome results in physical problems and some mental retardation.

***

SEQUENCE

How does sickle cell anemia affect human health?

Inherited Disease

Disorder

Description

Cystic fibrosis

Makes mucus extra thick and sticky, clogging lungs and digestive track.

Hemophilia

Prevents body from producing clotting factors, resulting in uncontrolled bleeding

Muscular dystrophy

Prevents body from making protein for muscle cells, resulting in muscle weakness.

Sickle cell anemia

Creates red blood cells shaped like sickles. Sickle cells block other cells in the bloodstream and do not carry as much oxygen.

[Image: Sickle cell anemia produces blood cells in an abnormal sickle shape. Sickle cells block other blood cells.]

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Lesson Wrap-Up Visual Summary

Traits can be inherited from parents or acquired from the environment.

DNA in chromosomes contains genes that determine the traits of an organism.

Mutations occur when the bases in a genetic sequence change.

***

LINKS for Home and School

MATH Make a Bar Graph Research the number of chromosome pairs in five different species. Create a bar graph that compares them. Does the number of chromosomes per cell relate to the complexity of the organism? Draw a conclusion from your data.

TECHNOLOGY Write a Documentary

Scientists continue to develop new technology to study and manipulate DNA. Research the Human Genome Project, genetic engineering, or other scientific advances. Present your findings in the style of a television documentary.



Review

1 MAIN IDEA What material carries the information that determines genetic traits?

2 VOCABULARY Write a sentence or short paragraph using the terms genes and DNA.

3 READING SKILL: Sequence

What steps take place during DNA replication?



4 CRITICAL THINKING: Apply

How do mutations affect an organism? Why can some mutations have no effect?



5 INQUIRY SKILL: Analyze Data

What information would a scientist need to discover whether a disorder had a genetic cause, an environmental cause, or a combination of the two?

***

TEST PREP

Human traits are most accurately described as



A. almost entirely genetic in origin.

B. both inherited and acquired.

C. almost entirely learned from other humans.

D. like those of plants.

***


Technology

Visit www.eduplace.com/scp/ to find out more about heredity.



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Biography Rosalind Franklin (1920-1958)

In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize for discovering the structure ©f DNA. Yet their achievement relied greatly on a woman who was not so honored. Alas, Rosalind Franklin had died four years earlier.

As a young girl, Franklin was energetic and talkative, often arguing politics with her father. She excelled in school, especially at science. She became the first woman in her family to graduate from college with a science degree.

Her most famous works are her x-ray pictures of DNA, such as the one shown here. Other scientists had tried to take pictures like this. However, Franklin recognized that DNA changed its shape in water. She carefully and precisely added just the right amount of water to her specimens. From her pictures and other data, she determined much about the shape and composition of the DNA molecule.

One of Franklin's colleagues described her with these words: "...Miss Franklin was distinguished by extreme clarity and perfection in everything she undertook. Her photographs are among the most beautiful of any substances ever taken."

[Image: Rosalind Fran.]

X-ray Picture of DNA

[Image: Compare Franklin's X-ray picture to the illustration of DNA on page A87.]



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Lynn Margulis (1938- )

In the 1980s, scientists studying cells discovered something they did not expect.

Mitochondria, the cell parts that produce energy, have their own DNA! This DNA controls only the mitochondria. It works apart from the DNA in the nucleus.

Yet, the discovery did not surprise everyone. Years earlier, biologist Lynn Margulis predicted just such a finding. The prediction was part of a larger idea she proposed about cell history. It is called the endosymbiont hypothesis.

According to this hypothesis, mitochondria and chloroplasts were once small, free-living cells, much like bacteria. At some point, a larger, amoeba-like cell engulfed some of them. Yet, instead of breaking apart, the small cells stayed alive inside the larger cell. After many cycles of reproduction, they became cell parts.

The endosymbiont hypothesis explains much about mitochondria. As for Margulis, she now is recognized as an insightful and dedicated scientist.

[Image: Lynn Margulis.]

***

Animal Cell

[Image: Animal cell.]



Unique features of mitochondria

* Able to use oxygen

* Surrounded by a double membrane

* Divides separately from the rest of the cell

* Has its own DNA that resembles bacterial DNA

***


Sharing Ideas

1. READING CHECK What did Franklin and Margulis show about DNA?

2. WRITE ABOUT IT

What does Margulis' hypothesis explain about cells?



3. TALK ABOUT IT Which science skills did Franklin and Margulis practice?

***


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Lesson 2 Why Are Some Traits Very Common? Why It Matters...

For a species to continue, its members must reproduce. Sometimes, new individuals are exact copies of one parent. Other times, they are similar to two parents, but not exactly like them. When the two young giraffes become adults, they may pass traits to young of their own.



PREPARE TO INVESTIGATE Inquiry Skill

Use Models When you use models, you make an object that helps you understand a concept or process.



Materials

* 4 each of 3 shapes of beads or buttons, with each shape in 2 colors (12 beads total)

* chenille stems

***


Science and Math Toolbox

For step 2, review Making a Chart to Organize Data on page H11.

***

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Chromosome Combinations Procedure

1 Use Models Model genes on chromosomes by stringing three beads on each chenille stem. Refer to the chart to find the meaning of each type of bead. Thread the ear shape gene first, then the eye color gene, and then the gene for hair color. Separate the chromosomes into two pairs to represent two parents.

2 Record Data In your Science Notebook, draw the bead shapes and colors on the chromosomes for each parent. Create a chart to organize your data.

3 Experiment Take one chromosome from the first parent to form a gamete chromosome. Repeat for the second parent. Draw the gamete chromosomes in the chart. Then combine them to form the chromosomes for a new cell called zygote. Draw the chromosomes of the zygote in the chart.

Conclusion

1. Analyze Data Is it possible to predict the offspring's chances of having a certain hair color, eye color, and ear shape? Why or why not?

2. Use Models Make a model of two parents whose traits are exactly alike and model the offspring's chromosomes. Are the traits of this offspring more predictable than those of the first offspring you modeled?

STEP 1

Trait

A

B

ear shape

Attached

Not Attached

eye color

Blue

Brown

hair color

Brown

Black

[Image: STEP 2.]

[Image: STEP 3.]

***

Investigate More! Design an Experiment

Find out what happens when you add more choices of traits. Make models with several choices for eye color, hair color, and one other trait.

***

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* VOCABULARY

asexual p. A9

reproduction

dominant p. A99

hybrid p. A99

recessive p. A99

selective breeding p. A100

sexual reproduction p. A98

* READING SKILL

Cause and Effect Use a chart like the one below to show the effects of selective breeding.

[Image: A chart.]



How Traits Are Passed
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