Focus on California Life Science 7th Grade Unit 2 - Chapter 5 Genetics The Science of Heredity

Chapter Genetics The Science of Heredity ( Standards Preview All living organisms are posed of cells . from just one to many trillions , whose details ly are visible only through a scope . As a basis for understanding this concept a . Students know cells function similarly in all living organisms . A typical cell of any organism contains genetic instructions that specify its traits . Those traits may be modified by environmental . As a basis for standing this concept . Students know sexual duction produces offspring that inherit half their genes from each parent . Students know plant and animal cells contain many thousands of different genes and typically have two copies of every gene . The two copies ( or alleles ) of the gene may or may not be identical , and one may be dominant in ing the phenotype while the other is recessive . These puppies and their mother resemble each other in many ways . 1489

, SCION Genetic The Science of Heredity on the BIG Idea How are traits passed from parents to offspring ?

Check What You Know In a litter of puppies , some of the puppies have a black and white coat , and others have a red and white coat . What can you infer about how their parents look ?

What can you infer about the parents DNA ?

Chapter 149 The images shown here represent some of the Key Terms in this chapter . You can use this vocabulary skill to help you understand the meaning of some Key Terms in this Chapter . Use A suffix is a word part that is added to the end of a word to change its meaning . For example , the suffix tion means process of . If you add the suffix to the verb produce , you get the noun production . Production means the process of making . The table below lists some common and their meanings . Suffix Meaning of Suffix Example and Meaning of Example , state of quality of importance State of being important , inclined to likely to dependent Likely to rely on something or someone else state of quality of simplicity State of being simple or easy Apply . What is the in the word dominant ?

if the verb dominate means to have . what do you think dominant ' means ?

whatdoes dominance probably mean ?

Chapter Vocabulary Section ( page 154 ) heredity trait genetics fertilization purebred gene dominant allele recessive allele hybrid ' page 162 ) probability square phenotype genotype homozygous heterozygous Section ( sexual reproduction diploid meiosis ( messenger RNA transfer RNA mutation ' Build Science Vocabulary Online Visit web Code Chapter 151

How to earl Se en Reading Skill ?

Take Notes When you take notes , you write the important ideas in the textbook in shortened form . Use a red or blue heading as the title of your notes . In the left column , write questions about the text that lows the heading . Write the answers in the right column . Write a summary statement that expresses the main idea . See the sample notes below , which are notes on part of Section . Questions Notes Chromosomes and Inheritance Where did Sutton evidence in grasshopper cells have 24 that chromosomes were chromosomes . sex cells only 12 . Important in inheritance ?

What happene to chromosomes Chromosomes form pairs . One chromosome after fertilization ?

in each pair comes from each parent . Where are genes ?

On chromosomes Statement The chromosome theory of inheritance says that parent page to their offspring on chromosomes . Apply It ! Review the notes in the right column . What is one important idea found in the notes ?

What question in the left column helps you recall the idea ?

Finish these notes as you read Section . Also take notes on all of Section .

1532 Standards Investigation All in the Family Did you ever wonder why some offspring resemble their parents while others do not ?

In this chapter , you learn how offspring come to have traits similar to those of their parents . You create a family of paper pets to explore how traits pass from parents to offspring . Your Goal To create a paper pet that will be crossed with a classmates pet , and to determine what traits the offspring will have To complete this investigation successfully , you must create your own unique paper pet with five different traits cross your pet with another pet to produce six offspring determine what traits the offspring will have , and explain how they came to have those traits follow the safety guidelines in Appendix A Plan It ! Cut out your pet from either blue or yellow construction paper . Choose other traits for your pet from this list square eyes or round eyes oval nose or triangular nose pointed teeth or square teeth Then create your pet using materials of your choice . Chapter 153

Standards Focus Students know plant and animal cells contain many sands of different genes and cally have two copies of every gene . The two copies ( or alleles ) of the gene may or may not be identical , and one may be dominant in mining the phenotype while the other is recessive . What were the results of experiments , or crosses ?

What controls the inheritance of traits in organisms ?

Key Terms heredity trait genetics I fertilization purebred gene alleles dominant allele recessive allele hybrid Gregor , Mendel Mendel Work Standards What Does the Father Look Like ?

Observe the colors of the kitten in the photo . Record the kitten coat colors and pattern . include as many details as you can . Observe the mother cat in the photo . Record her coat color and pattern . Think It Over Inferring Based on your observations , describe what you thinkthe kitten father might look like . Identify the evidence on which you based your inference . In the century , a priest named Gregor Mendel tended a garden in a central monastery . Mendel ' experiments in that peaceful garden would one day the study of heredity . Heredity is the passing of physical characteristics from parents to of . Mendel wondered why different pea plants had different characteristics . Some pea plants grew tall , while others were short . Some plants produced green seeds , while others had low seeds . Each different form of a characteristic , such as stem height or seed color , is called a trait . Mendel observed that the pea plants traits were often similar to those of their parents . Sometimes . however . the plants had different traits from those . of their parents . Mendel experimented with thousands of pea plants to understand the process of heredity , Today , discoveries form the foundation of genetics , the study of heredity .

Experiments shows a pea . The petals round the pistil and the stamens . The pistil produces female sex cells , or eggs . The stamens produce pollen , which contains the male sex cells , or sperm . 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 . This process is called pollination . Pea plants are usually . In , pollen from a lands on the pistil of the same flower . Mendel developed a method by which he . or pea plants . To cross two plants , he removed pollen from a on one plant . He then brushed the pollen onto a on a second plant . Crossing Pea Plants What could you do to study the of traits in pea plants ?

Mendel decided to cross plants with contrasting example , tall plants and short plants . He started his experiments with . or purebred plants . A purebred organism is the offspring of many generations that have the same trait . For example , purebred short pea plants always come from short parent plants . To prevent pollination , Mendel removed the producing structures from a pink . on another plant . He brushed this pollen onto the pink . FIGURE Crossing Pea Plants Gregor Mendel crossed pea plants that had different traits . The illustrations show how he did this . Interpreting Diagrams How did Mendel prevent pollination ?

The egg cells in the pink were then fertilized by sperm from the white flower . After a time , peas formed in the pod .

Generation The Offspring In one experiment , Mendel crossed bred tall plants with purebred short plants . Scientists today call these parent plants the parental generation , or generation . The offspring from this cross are the ( FIL ee ul ) or the , generation . The word comes from and , the Latin words for daughter and In Figure , notice that all the offspring in the tion were tall . Even though one of the parent plants was short , none of the offspring were short . The shortness trait seemed to disappear ! The Offspring When the plants in the generation were , Mendel allowed them to . the plants in the ( second ) generation were a mix of tall and short plants . The shortness trait had reappeared , even though none of the parent plants were short . Mendel counted the tall and short plants . About three fourths of the plants were tall , while one fourth were short . Experiments With Other Traits Mendel also crossed pea plants with other contrasting traits . Compare the two forms of each trait in Figure . In all of Mendel crosses , only one form of the trait appeared in the generation . However , in the generation , the lost form of the trait always reappeared in about one fourth of the plants . What did Mendel observe about the , Results of a Cross When Mendel crossed purebred plants with purebred plants , the offspring all had tall stems . Then he allowed the plants to pollinate . About 75 percent of the offspring had tall stems , and about 25 percent had short stems . i , a i ' Tall Tall Tall Short

Genetics of Pea Plants by Dominant i Allele by Recessive Allele wrinkled Green Dominant and Recessive Alleles Mendel reached several conclusions on the basis of his mental results . He reasoned that individual factors , or sets of genetic information , must control the inheritance of traits in peas . The factors that control each trait exist in pairs . The female parent contributes one factor , while the male parent contributes the other factor . Finally , one factor in a pair can mask , or hide , the other factor . The tallness factor , for example . masked the shortness factor . Genes and Alleles Today , scientists use the word gene for the factors that control a trait . Alleles ( uh ) are the different forms of a gene . The gene that controls stem height in peas , for example , has one allele for tall stems and one allele for short stems . Each pea plant inherits two alleles from its allele from the egg and the other from the sperm . A pea plant may inherit two alleles for tall stems , two alleles for short stems , or one of each . An organism traits are determined 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 , on the other hand , is hidden ever 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 . Figure shows dominant and recessive alleles in Mendel crosses . Mendel studied several traits in pea plants . Interpreting Diagrams Is yellow seed color controlled by a dominant allele or a recessive allele ?

a 157 Skills Activity Predicting in fruit flies , long wings are dominant over short wings . A scientist crossed a purebred male fruit fly with a purebred female . Predict the wing length of the , offspring . If the scientist crossed a hybrid male , fruit fly with a hybrid , female , what would their offspring probably be like ?

158 pea plants , the allele for tall stems is dominant over the allele for short stems . Pea plants with one allele for tall stems and one allele for short stems will be tall . The allele for tall stems masks the allele for short stems . Only pea plants that inherit two recessive alleles for short stems will be short . Alleles in Crosses In Mendel cross for stem height , the purebred tall plants in the generation had two alleles for tall stems . The purebred short plants had two alleles for short stems . The plants each inherited an allele for tall stems from the tall parent and an allele for short stems from the short parent . Therefore . each , plant had one allele for tall stems and one for short stems . The , plants are called hybrids . A hybrid ( HY brid ) organism has two different alleles for a trait . All the , plants are tall because the dominant allele for tall stems masks the recessive allele for short stems . When Mendel crossed the , plants , some of the offspring in the generation inherited two dominant for tall stems . These plants were tall . Other plants inherited one dominant allele for tall stems and one recessive allele for short stems . These plants were also tall . The rest of the plants inherited two recessive alleles for short stems . These plants were short . Symbols for Alleles Geneticists use letters to represent alleles . A dominant allele is represented by a capital letter . For example , the allele for tall stems is represented by I A recessive allele is represented by the lowercase version of the letter . So , the allele for short stems would be represented by When a plant inherits two dominant alleles for tall stems , its alleles are written as . When a plant inherits two recessive alleles for short stems , its alleles are written as . When a plant inherits one allele for tall stems and one allele for short stems , its alleles are written as . FIGURE Black Fur , White Fur In rabbits , the allele for black fur is dominant over the allele for white fur . Inferring What combination of alleles must the white rabbit have ?

Significance of Mendel Contribution Mendel work eventually changed scientists ideas about heredity . Before Mendel , most people thought that genetic information could be blended to produce new traits . They thought that traits could be blended to form a combined version , the same way red and white paint can be mixed to make pink paint . ing to this incorrect model , if a tall plant and a short plant were crossed , the offspring would all have medium height . However . when Mendel crossed purebred tall and purebred short pea plants , the offspring were all tall . Mendel ments demonstrated that parents traits do not simply blend in the offspring . Instead , traits are determined by individual , separate alleles inherited from each parent . Some of these les are recessive . If a trait is determined by a recessive allele . the trait can seem to disappear in the offspring . Unfortunately , the importance of Mendel discovery was not recognized during his lifetime . Then , in 1900 , three mu ent scientists rediscovered Mendel work These scientists The Mendel Medal quickly recognized the importance of Mendel ideas . Because Every yea , to honor the memo , of his work , Mendel is often called the Father of Genetics . of Gregor Mendel . an outstanding scientist is awarded the Mendel If an allele is represented by a capital letter , what ) does this indicate ?

Section i Assessment Vocabulary Skill In the key term . Relating Cause and Effect Explain how fertilization , what does the mean ?

dominant and recessive alleles for the trait of . stem height determine whether a pea plant ( Reviewing ( ey Concepts be or Short . Identifying In Mendel cross for stem Applying concepts can a short Pea plan , ever be a hybrid for the trait of stem height ?

Pea be Why or why not ?

As part of your explanation , all alts did the write the letters that represent the alleles for plants in the , generation exhibit ?

When stem height of a she pea pram you think of the traits of the parent plants . why is this result surprising ?

Comparing and Contrasting Contrast A At Hom Activity the offspring in the generation to the ' the . Gardens and Heredity Some gardeners save the and the seeds produced by flowers and plant them ' in the spring . If there are gardeners in your What is a dominant allele ?

ask them how the plants that , What is a recessive allele ?

grow from these seeds resemble the parent plants . Are the offspring traits ever different from those of the parents ?

Chapters 159 For Data sharing Visit Ta a Web Code Are Your Traits Unique ?

Are traits controlled by dominant alleles more . I . common than traits controlled by recessive alleles ?

at ( am on page . All the traits In your data table appear in the circle . Place the eraser end of your Focus cil on the trait in the small central circle that . in data applies to free ear lobes or attached ear lobes . Look at the two traits touching the space your ' Optimal ) eraser is on . Move your eraser onto the next description that applies to you . Continue using Procedure your eraser to trace your traits until you reach a number on the outside rim of the circle . PART Dominant and Recessive Alleles Share that number with your classmates . Write a hypothesis reflecting your ideas about the problem . Then copy the data table . Analyze and . For each of the traits listed in the data table , I . Observing The traits listed under Trait in work with a partner to determine which trait the data table are controlled by dominant you have . Circle that trait in your data table . alleles . The traits listed under Trait are trolled by recessive alleles . which traits trolled by dominant alleles were shown by a majority of students ?

Which traits controlled . Count the number of students in your class who have each trait . Record that number in your data table . Also record the total ber of student by recessive alleles were shown by a majority of students ?

Free ear lobe widow peak Cleft chin Attached ear lobe No widow peak No cleft chin 160 . 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 tion of traits ?

Developing Hypotheses Do your data port the hypothesis you proposed in Step ?

Write an answer with examples . Communicating Based on your observation of traits in this lab , write a paragraph explaining why people look so different from one another . Design an Experiment Do 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 sion before carrying out your investigation . Data Table Total Number of Student Trait I Hair on Attached ear lobe No hair on fingers Curly No ' Straight hair Smooth No smile dimples 151

Section ' A Standards Focus Students know plant and animal cells contain many sands of different genes and cally have two copies of every gene . The two copies ( or alleles ) of the gene may or may not be identical , and one may be dominant in determining the phenotype while the other is recessive . what is probability and how does it help explain the results of genetic crosses ?

What is meant by genotype and phenotype ?

What is ?

Key Terms probability I square phenotype genotype homozygous ' heterozygous , For Links on probability and genetics Visit Web Code 162 Integrating Mathematics Probability and Heredity What the Chance ?

Suppose you were to toss a coin 20 times . Predict how many times the coin would land with heads up and how many times it would land with tails up . Now test your prediction by tossing a coin 20 times . Record the number of times the coin lands with heads up and the number of times it lands with tails up . Combine the data from the entire class . Record the total number of tosses , the number of heads , and the number of tails . Think It Over Predicting How did your results in Step compare to your prediction ?

How can you account for any differences between your results and the class results ?

On a brisk fall afternoon , the stands are packed with football fans . Today is the big game between North Shore and South Shore high schools , and it almost time for the kickoff . Suddenly , the crowd becomes silent , as the referee is about to toss a coin . The outcome of the coin toss will decide which team kicks the ball and which receives it . The captain of the visiting North Shore team says . If the coin lands with heads up , North Shore High wins the toss and the right to decide whether to kick or receive the ball . What is the chance that North Shore High will win the coin toss ?

To answer this question , you need to understand the principles of probability . Principles of Probability If you did the Standards activity , you used the of probability to predict the results of a particular event . In this case , the event was the toss of a coin . Probability is a number that describes how likely it is that a certain event will occur .

Mathematics of Probability Each time you toss a coin , there are two possible ways that the coin can up or tails up . Each of these two events is equally likely to occur . In mathematical terms , you can say that the probability that a tossed coin will land with heads up is in . There is also a in probability that the coin will land with tails up . A in probability can also be expressed as the fraction or as a percent . The laws of probability predict what is likely to occur , not necessarily what will occur . If you tossed a coin 20 times , you might expect it to land with heads up 10 times and with tails up 10 times . However , you might not get these results . You might get 11 heads and tails , or heads and 12 tails . The more tosses you make , the closer your actual results will be to the results predicted by probability . Independence of Events When you toss a coin more than once , the results of one toss do not affect the results of the next toss . Each event occurs independently . For example , suppose you toss a coin times and it lands with heads up each time . What is the probability that it will land with heads up on the next toss ?

Because the coin landed heads up on the previous tosses , you might think that it would be likely to land heads up on the next toss . However , this is not the case . The ity of the coin landing heads up on the next toss is still in , or 50 percent . The results of the first tosses do not affect the result of the sixth ) what is probability ?

Percentage One way you can express a probability is as a percentage . A percentage ( 96 ) is a number compared to 100 . For example , 50 means 50 out of 100 . Suppose that out of tossed coins landed with heads up . Here how you can calculate what percent of the coins landed with heads up . Write the comparison as a fraction . Multiply the fraction by 100 to express it as a percentage . 100 60 Practice Problem Suppose out of 12 coins landed with tails up . How can you express this as a percent ?

FIGURE A Coin Toss The result of a coin toss can be explained by probability . 6153 FIGURE How to Make a Square The diagrams show how to make a square . In this cross , both parents are heterozygous for the trait of seed shape . it represents the dominant round allele , and represents the recessive wrinkled allele . A Coin Crosses Here how you can use coins to model cross between two pea plants . Place a small piece of masking tape on each side of two coins . Write a ( for tall ) on one side of each coin and a ( for short ) on the other . Toss both coins together 20 times . Record the letter combinations that you obtain from each toss . Interpreting Data How many of the offspring would plants ?

Hint What different letter combinations would result in a tall plant ?

How many would be short ?

Convert your results to percentages . Then compare your results to . Try This Activity a Start by drawing Write the male parent alleles along I box and dividing the top of the square and the female it into four squares . parent alleles along the left side . Probability and Genetics How is probability related to genetics ?

To answer this question , think back to experiments with peas . Remember that Mendel carefully counted the offspring from every cross that he carried out . When Mendel crossed two plants that were hybrid for stem height ( three fourths of the , plants had tall stems . One fourth of the plants had short stems . Each time Mendel repeated the cross , he obtained similar results . Mendel realized that the mathematical principles of probability applied to his work . He could say that the ity of such a cross producing atall plant was in . The of producing a short plant was in . Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses . Squares A tool that can help you understand how the laws of probability apply to genetics is called a square . A square is a chart that shows all the possible combinations of alleles that can result from a genetic cross . Geneticists use squares to show all the possible comes of a genetic cross , and to determine the probability of a particular outcome . Figure shows how to construct a square . In this case , the square shows a cross between two hybrid pea plants with round seeds ( The allele for round seeds ( is dominant over the allele for wrinkled seeds ( Each parent can pass either of its alleles , or , to its offspring . The boxes in the square represent the possible combinations of alleles that the offspring can inherit . gum What is a square ?

Copy the female parent I Copy the male alleles into the boxes to I alleles into the boxes their right . beneath them . Using a Square You can use a square to calculate the probability that offspring with a certain nation of alleles will result . In a genetic cross , the allele that each parent will pass on to its offspring is based on probability . The completed square in Figure four possible combinations of alleles . The probability that an offspring will be is in , or 25 percent . The probability that an offspring will be is also in , or 25 percent . Notice , however , that the allele combination appears in two boxes in the square . This is because there are two possible ways in which this combination can occur . So the probability that an offspring will be is in , or 50 percent . When Mendel crossed hybrid plants with round seeds , he discovered that about three fourths of the plants ( 75 percent ) had round seeds . The remaining one fourth of the plants ( 25 percent ) produced wrinkled seeds . Plants with the allele combination would produce round seeds . 50 too would those plants with the allele combination . Remember that the allele masks the recessive allele . Only those plants with the allele combination would have wrinkled seeds . Predicting Probabilities You can use a square to predict probabilities . For example . Figure shows a cross between a purebred black guinea pig and a purebred white guinea pig . The allele for black fur is dominant over the allele for white fur . Notice that only one allele tion is possible in the . All of the offspring will inherit the dominant allele for black fur . Because of this , all of the offspring will have black fur . There is a 100 percent probability that the offspring will have black fur . I am completed square shows all the possible allele combinations in the offspring . i re . FIGURE Ii Guinea Pig Square This square shows a cross between a black guinea pig ( and a white guinea pig ( Calculating What is the probability that an offspring will have white fur ?

Reviewing Math Algebra and Functions Analyzing Data What Are the ?

Mendel allowed several pea plants with yellow Of Mendel 005595 seeds to . The graph shows the approximate numbers of the offspring with yellow seeds and with green seeds . Reading Graphs many offspring had yellow seeds ?

How many had green seeds ?

Calculating Use the information in the graph to calculate the total number of offspring that resulted from this cross . Then calculate the percentage of the offspring with yellow peas . and the percentage with green peas . Yellow Green . Inferring Use the answers to Question to infer of the probable of the parent plants . Hint Construct squares with the possible of the parents . Number of Plants and Two terms that geneticists use are phenotype ( FEE noh ) and genotype ( JEN uh ) An organism phenotype is its physical appearance , or visible traits . An genotype is its genetic makeup , or allele combinations . To understand the difference between phenotype and type , look at Figure . The allele for smooth pea pods ( is dominant over the allele for pinched pea pods is ) All of the plants with at least one dominant allele have the same all produce smooth pods . However , the plants can have two different or . If you were to look at the plants with smooth pods , you would not be able to tell the difference between those with the genotype and those with the genotype . The plants with pinched pods , on the other hand , would all have the same well as the same . Geneticists use two additional terms to describe . An organism that has two identical alleles for a trait is said to FIGURE be homozygous ( hoh moh gus ) for that trait . A The 55 pod plant that has the alleles and a plant with a . the alleles are both homozygous . An organism that has two genotype IS genetic makeup . an I . How many i eren es or a trait is et ur gus ) are there for the A plant with the alleles is heterozygous . Phenotype ?

del used the term hybrid to describe heterozygous pea plants . 166 For all of the traits that Mendel studied , one allele was dominant while the other was recessive . This is not always the case . For sortie alleles , an inheritance pattern called exists . the alleles are neither dominant nor recessive . As a result , both alleles are expressed in the offspring . Look at Figure 10 . Mendel principle of dominant and recessive alleles does not explain why the chickens have both black and white feathers . The alleles for feather color are nor recessive . As you can see , neither allele is masked in the heterozygous chickens . Notice also that the codominant alleles are written as capital letters with for black feathers and for white feathers . As the square shows , heterozygous chickens have the allele combination . gating ) How are the symbols for codominant alleles written ?

Section Vocabulary Skill Use the meanings of the and to contrast the meanings of and . Reviewing Key Concepts a . Reviewing What is probability ?

Explaining If you know the parents alleles for a trait , how can you use a square to predict the probable of the offspring ?

Predicting A pea plant with round seeds has the genotype . You cross this plant with a plant , genotype . What is the probability that the offspring will have wrinkled seeds ?

Use a square . a . Defining genotype and . Relating Cause and Effect Explain how two organisms can have the same phenotype but different . Give an example . Applying Concepts A pea plant has a tall stem . What are its possible ?

Assessment . Ratios A scientist crossed a tall pea plant FIGURE 10 The offspring of the cross in this square will have both black and white feathers . Classifying Will the offspring be heterozygous or homozygous ?

Explain your answer . Binding a . Explaining What is ?

Give an example of codominant alleles and explain why they are codominant . Applying Concepts What is the of a chicken with the genotype ?

I II ' I II with a short pea plant . Of the offspring , 13 were tall and 12 were short . Write the ratio of each phenotype to the total number of offspring . Express the ratios as fractions . Percentage Use the fractions to calculate the percentage of the offspring that were tall and the percentage that were short . Chapters 161

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

Skills Focus making models , interpreting data Materials small paper bags marking pen blue marbles white marbles Procedure . Label one bag Bag , Female Label the other bag Bag , Male Then read over Part , Part , and Part of this lab . Write a prediction about the kinds of spring you expect from each cross . Crossing Two Homozygous Parents . Copy the data table and label it Data Table I . Then place two blue marbles in Bag . This pair of marbles represents the female ent alleles . Use the letter to represent the dominant allele for blue color . 13 . Place two white marbles in Bag . Use the letter to represent the recessive allele for white color . For Trial , remove one marble from Bag 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 . Record the result in your data table . Return the marble to the bag . In the column labeled Offspring Alleles , write if you removed two blue marbles , if you removed two white marbles , or if you removed one blue marble and one white marble . Repeat Steps and nine more times . Crossing Homozygous and Heterozygous Parents . Place two blue marbles in Bag . Place one white marble and one blue marble in Bag . Copy the data table again , and label it Data Table . Repeat Steps and ten times . Da Table Number Allele From Allele From Bag Offs i ( Male Parent ) Allele ( male Parent ) la . 1580

Crossing Two Heterozygous Parents . Place one blue marble and one white marble in Bag . Place one blue marble and one white marble in Bag . Copy the data table again and label it Data Table . 10 . Repeat Steps and ten times . Analyze and conclude . Making Models Make a square for each of the crosses you modeled in Part , Part , and Part . Interpreting Data According to your results in Part , how many different kinds of spring are possible when the homozygous parents ( 88 and ) are crossed ?

Do the results you obtained using the marble model agree with the results shown by a square ?

Predicting According to your results in Part , what percentage of offspring are likely to be homozygous when a parent ( and a parent ( are crossed ?

What percentage of spring are likely to be heterozygous ?

Does the model agree with the results shown by a square ?

Making Models According to your results in Part , what different kinds of offspring are possible when two heterozygous parents ( are crossed ?

What percentage of each type of offspring are likely to be Does the model agree with the results of a square ?

inferring For Part , if you did 100 trials instead of 10 trials , would your results be closer to the results shown in a square ?

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

How are they different ?

More to Explore In peas , the allele for yellow seeds ( is nant over the allele for green seeds ( What possible crosses do you think could produce a heterozygous plant with yellow seeds ( Use the marble model and squares to test your predictions . 0159 Students know sexual reproduction produces offspring that inherit half their genes from each parent . What role do chromosomes play in inheritance ?

What events occur during ( meiosis ?

What is the relationship between chromosomes and genes ?

Key Term sexual reproduction diploid meiosis FIGURE 11 Sex Cells The large egg is a female sex cell , and the smaller sperm is a male sex cell . Egg 170 The Cell and Inheritance Standards Focus Which Chromosome Which ?

Sperm Mendel did not know about chromosomes or their role in genetics . Today we know that genes are located on chromosomes . Label two craft sticks with the letter A . The craft sticks represent a pair of chromosomes in the female parent . Turn the sticks face down on a piece of paper . Label two more craft sticks with the letter a . These represent a pair oi chromosomes in the male parent . Turn the sticks face down on another piece of paper . Turn over one craft stick chromosome from each piece of paper . Move both sticks to a third piece of paper . These represent a pair of chromosomes in the offspring . Note the allele combination that the offspring received . Think It Over Making Models Use this model to explain how chromosomes are involved in the inheritance of alleles . work showed that genes exist . But scientists in the early twentieth century did not know what structures in cells contained genes . The search for the answer to this puzzle is something like a mystery story . The story could be called The Clue in the Grasshoppers Cells . In 1903 , Walter Sutton , an American geneticist , was studying the cells of grasshoppers . He wanted to understand how sex cells ( sperm and egg ) form . ton focused on the movement of the formation of sex cells . Sex cells form during sexual duction . In sexual reproduction , genetic material from two parents combines to produce a new organism , which differs from both parents . Sutton hypothesized that chromosomes were the key to understanding how offspring have traits similar to those of their parents .

Grasshopper chromosomes Chromosomes and Inheritance i Sutton needed evidence to support his hypothesis that somes were important in the inheritance of traits . He found that evidence in grasshoppers cells . The body cells of a grasshopper have 24 chromosomes . To his surprise . Sutton found that the grasshoppers sex cells have only 12 chromosomes . In other words , a grasshoppers sex cells have exactly half the number of chromosomes found in its body cells . Chromosome Pairs Sutton observed what happened when a sperm cell and an egg cell joined during fertilization . The egg that was diploid . A diploid cell contains two sets of chromosomes , one set from each parent . The fertilized grasshopper egg had 24 chromosomes , or 12 pairs . One in each pair came from each parent . As result , the grasshopper offspring had exactly the same number of somes in its cells as did each of its parents . Genes on Chromosomes Recall that alleles are different forms of a gene . From the results of Mendel work , Sutton knew that alleles exist in pairs in an organism . One allele in a pair comes from the organism female parent and the other allele comes from the male parent . Sutton realized that paired alleles were carried on paired chromosomes . Sutton idea came to be known as the chromosome theory of inheritance . According to the chromosome theory of inheritance , genes are carried from parents to their offspring on chromosomes . What is the relationship between alleles and chromosomes ?

I I house 12 Grasshopper Chromosomes Grasshoppers body cells have twice the number of chromosomes as their sex cells . Applying Concepts What is the function of chromosomes ?

Chapters 111 FIGURE 13 Meiosis During meiosis , a cell produces sex cells with half the number of chromosomes . Interpreting Diagrams What happens before meiosis ?

Before Meiosis Before meiosis begins , Mel , A The chromosome the pairs separate Two cells form , each ' me pare I pairs line up in the and move to with half the number ce . center of the cell . opposite ends oi Each hold the two of the IL chromosome St . has together . two . MEIOSIS How do sex cells end up with half the number of chromosomes as body cells ?

To answer this question , you need to understand the events that occur during meiosis . Meiosis ( my OH sis ) is the process by which the number of chromosomes is reduced by half to form sex and eggs . or What Happens During Meiosis You can trace the events of meiosis in Figure 13 . In this example , each parent cell has four chromosomes arranged in two pairs . 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 . The sex cells in Figure 13 end up with only two chromosomes fa the number found in the parent cell . Each sex cell has one 11 chromosome from each original pair . When sex cells combine to form an organism , each sex cell ' i ?

contributes half the normal number of chromosomes . Thus . 335 ?

the offspring gets the normal number of from each parent . in , ' End of Four sex cells have been produced . Each cell has only half the number of chromosomes that the ?

A The chromosomes I The split , and only on with their two the separate . ad move to Single chromosomes move the center of the cell . to opposite ends of the cell . Meiosis and squares A 14 square is actually a way to show the events that ' occur at meiosis . When the chromosome pairs separate and go into two sex cells , so do the alleles carried on each chromosome . One allele from each pair goes to each sex cell . In Figure 14 , you can see how the square accounts for the separation of alleles during meiosis . As shown across the top of the square , half of the sperm cells from the male parent will receive the chromosome with the ' The other half of the sperm cells will receive the with the . In this example , the same is true for the egg cells from the female parent , as shown down the left side of the square . Depending on which sperm cell combines with which egg cell , one of the allele combinations shown in the boxes will result . Suppose both parents are heterozygous for the trait of stem height . The square shows the possible allele combinations after fertilization . Male parent Possible sperm cells Possible egg cells Chapters 113

A Lineup of Genes Each human body cell contains 23 chromosome pairs , or 46 chromosomes . Chromosomes are made up of many genes joined together like beads on a string . Plant and mal chromosomes contain many thousands of genes . Although you have only 23 pairs of chromosomes , your body cells each contain about genes . Each gene controls a trait . In Figure 15 , one chromosome in the pair came from the female parent . The other chromosome came from the male ent . There are usually two copies of every gene . The genes are lined up in the same order on both chromosomes . However , the alleles for some of the genes might be different For example , the organism has the A allele on one chromosome and the allele on the other . As you can see , this organism is heterozygous for some traits and homozygous for others . 15 Genes on Chromosomes The chromosomes in a pair may have different alleles for some genes and the same alleles for others . Classifying For which genes is this organism Chromosome pair homozygous ?

For which genes is it heterozygous ?

section Assessment Target Reading Skill Take Notes Use your a . Describing How are genes arranged on a notes to help answer the questions below . chromosome ?

Reviewing Key concepts . Comparing and Contrasting How does , the order of genes in one member of a a . Comparing and According to chromosome pail . Compare to the order of Sutton observations . how does the number gems on the other chromosome ?

of chromosomes in a grasshoppers body cells compare to the number in its sex cells ?

Describing Describe what happens to the number of chromosomes when two grasshopper sex cells join in fertilization . Explaining How do Sutton observations about chromosome number support the chromosome theory of inheritance ?

Defining What is meiosis ?

interpreting Diagrams describe meiosis I and meiosis II . Refer to Figure Sequencing Use the events of meiosis to explain why a sex cell normally does not receive both chromosomes from a pair . 174 000 Section Standards Focus Students know cells tion similarly in all living organisms . What forms the genetic code ?

How does a cell produce proteins ?

How can mutations affect an organism ?

Key Terms messenger RNA transfer RNA mutation Genes , DNA , and Proteins ?

Can You Crack the Code ?

Use the Morse code in the chart to decode the question in the message below . The letters are separated by slash ' Write your answer to the question in . Exchange your coded answer with a partner . Then decode your partner answer . Think It Over Forming Operational Based on your results from this activity , write a definition of the word code . Then compare your definition to one in a dictionary . The white kangaroo in the photograph below was born at the San Francisco zoo . The young kangaroo beside her is her offspring . Notice that the offspring coat is much natural coat color for eastern grey kangaroos . White kangaroos are extremely rare . Why was the mother born with such an uncommon phenotype ?

To answer this question , you need to know how the genes on a chromosome determine an ism inherited traits . A white kangaroo and her offspring 175 16 The DNA Code Chromosomes are made of DNA . Each chromosome contains thousands of genes . The sequence of bases in a gene forms a code that tells the cell what protein to produce . Interpreting Diagrams Where in the cell are chromosomes located ?

The Genetic Code The main function of genes is to control the production of proteins in an organism cells . Many proteins serve as enzymes that control chemical reactions in the cell . Proteins also help to determine the size , shape , color , and many other traits of an organism . Genes and DNA In Figure 16 , you can see the relationship between chromosomes and DNA . Notice that a DNA molecule is made up of four different nitrogen ( A ) thymine ( guanine ( and cytosine ( These bases form the rungs of the DNA A gene is a section of a DNA molecule that contains the information to code for one protein . A gene is made up of a series of bases in a row . The bases in a gene are arranged in a example , A single gene on a chromosome may contain anywhere from several hundred to a million or more of these bases . Each gene is located at a place on a chromosome . Order of the Bases A gene contains the code that mines the structure of a protein . The order of the nitrogen base along a gene forms a genetic code that specifies what type of protein will be produced . Remember that proteins are molecules made of individual amino acids . In the genetic code , a group of three DNA bases codes for one amino acid . For example , the base sequence ( always codes for the amino acid alanine . The order of the code units determines the order in which amino acids are put together to form a protein .

A Drawing Conclusions The following is a sequence of nitrogen bases on a strand of nucleic acid . How Cells Make Proteins The production of proteins in cells is called protein synthesis . During protein synthesis , the cell uses information from it gene on a chromosome to produce a protein . synthesis takes place on the ribosomes in the cytoplasm of a cell . As you know , the cytoplasm is outside the nucleus . The chromosomes , however , are found inside the nucleus . How , then , does the information needed to produce proteins get out of the nucleus and into the cytoplasm ?

Skills Activity The Role of RNA Before protein synthesis can take place . a messenger must carry the genetic code from the DNA inside the nucleus into the cytoplasm . This genetic messenger is called ribonucleic acid , or RNA . a Although RNA 15 similar to DNA , the two molecules differ Explain your in some important ways . Unlike DNA , which has two strands , RNA has only one strand . RNA also contains a sugar molecule from the sugar found in DNA . Another difference between DNA and RNA is in their nitrogen bases . Like DNA , RNA contains adenine , guanine , and cytosine . However , instead of thymine , RNA contains uracil ( uh sil ) of 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 the message to the in the cytoplasm . Another type of RNA , called transfer RNA . carries amino acids to the and adds them to the growing protein . How is RNA different from DNA ?

Nitrogen bases Chapter 117 Translating the Code The process of protein synthesis is shown in Figure 17 . Look at the illustration as you read the following steps . The first step is for a DNA molecule to unzip between its base pairs . Then one of the strands of DNA directs the tion of a strand of messenger RNA . To form the RNA strand , RNA bases pair up with the DNA bases . The process is similar to the process in which DNA replicates . Cytosine always pairs with guanine . However , pairs with adenine . 17 Protein Synthesis To make proteins . messenger RNA copies information from DNA in The messenger RNA then leaves the nucleus and enters the the nucleus . Messenger RNA and cytoplasm . In the cytoplasm , messenger RNA attaches to a RNA the this some . On the , the messenger RNA provides the code information to produce proteins . In which for the protein molecule that Will form . During protein of the ( Sis . the moves along the messenger RNA strand . manufactured ?

Messenger RNA to a When the messenger RNA enters the cytoplasm . it attaches to a , where production of the protein chain begins The moves Messenger RNA along the messenger RNA strand . Amino acid Transfer RNA Messenger RNA Production In the nucleus , a DNA serves as a pattern for making messenger RNA . The DNA molecule unzips between base pairs . RNA bases match up along one of the DNA strands . The genetic information in the DNA is transferred to the messenger RNA strand . Messenger RNA 113

Molecules of transfer RNA attach to the messenger RNA . The bases on the transfer RNA read the message by pairing up codes to bases on the messenger RNA . For example , you can see that a molecule of transfer RNA with the bases pairs with the bases on the messenger RNA . The molecules of transfer RNA carry amino acids . The amino acids link in a chain . The order of the amino acids in the chain is determined by the order of the codes on the messenger RNA . The protein molecule grows longer as each transfer RNA molecule puts the amino acid it is carrying along the growing protein chain . Once an amino acid is added to the protein chain , the transfer RNA is released into the cytoplasm and can pick up another amino acid . Each transfer RNA molecule always picks up the same kind of amino acid . what is the function of transfer RNA ?

Chiral ! For Protein Synthesis activity Visit Web Code Transfer RNA Attaches to Messenger RNA Transfer RNA molecules carry specific amino acids to the . There they read the message in messenger RNA by matching up with codes of bases . The protein chain grows as each amino acid is attached . Go Growing protein Start of protein Amino acid Transfer RNA Protein Production Continues The protein chain continues to grow until the reaches a code that acts as a stop sign . The then releases the completed protein . Chapter 119

Original DNA sequence 18 Mutations in Genes The illustration shows three types of mutations that can occur in genes . Comparing and Contrasting How are these mutations different from the mutations that occur when chromosomes do not separate during meiosis ?

Hill Ill ! Genetics The Science of Heredity 1800 one base pair is added . Mutations Suppose that a mistake occurred in one gene of a chromosome . Instead of the base A , for example , the DNA molecule might have the base Such a mistake is one type of mutation that can occur in a cell hereditary material . A mutation is any change in a gene or chromosome . Mutations can cause a cell to produce an incorrect protein during protein synthesis . As a result , the organism trait , or phenotype , may be ent from what it normally would have been . In fact , the term mutation comes from a Latin word that means If a mutation occurs in a body cell , such as a skin cell , the mutation will not be passed on to the organism offspring . If , however , a mutation occurs in a sex cell , the mutation can be passed on to an offspring and affect the offspring phenotype . of Mutations Some mutations are the result of small changes in an organism hereditary material . For example , a single base may be substituted for another , or one or more bases may be removed from a section of DNA . This type of mutation can occur during the DNA replication process . Other mutations may occur when chromosomes don separate during meiosis . When this type of mutation occurs , a cell can end up with too many or too few chromosomes . The cell could also end up with extra segments of chromosomes .

of Mutations Because mutations can introduce changes in an organism , they can be a source of genetic variety . Some mutations are harmful to an organism . A few mutations , however , are helpful , and others are neither harmful nor ful . A mutation is harmful to an organism if it reduces the 19 organisms chance for survival and reproduction . Whether a mutation is harmful or not depends partly on , ii the organisms environment . The mutation that led to the each from paw , production of a white kangaroo would probably be harmful to an organism in the wild . The kangaroo white color would make it more visible , and thus easier for predators to . However , a white kangaroo in a zoo has the same chance for survival as a brown kangaroo . In a zoo , the mutation neither helps nor harms the kangaroo . Helpful mutations , on the other hand , improve an ism chances for survival and reproduction . Antibiotic in bacteria is an example . Antibiotics are chemicals that kill bacteria . Gene mutations have enabled some kinds of bacteria to become resistant to certain is , the antibiotics do not kill the bacteria that have the mutations . The mutations have improved the bacteria ability to survive and reproduce . Reading What are two types of mutations ?

Section Assessment Target Reading Skill Take Notes Use your Inferring Does transfer RNA perform its notes to help answer the questions below . function in the nucleus or cytoplasm ?

Explain . Reviewing Key concepts . Reviewing How does a mutation in a gene . affect the order of DNA bases ?

a . Explaining What 15 the relationship cause and Effect How can a between a gene , a DNA molecule , and a mutation in a gene cause a change in an protein ?

organism phenotype ?

Relating Cause and Effect How does a DNA molecule determine the structure of a protein ?

Inferring The DNA base sequence ' codes for the amino acid proline . Could this same base sequence code for a different amino acid ?

Why or why not ?

a . Listing List the sequence of events that happens during protein synthesis . Describing What is messenger RNA ?

Describe how it performs its function . Chapter 181 Chapter Study Guide 711 I a Organisms produced by sexual reproduction inherit half their DNA from each parent . The new combination of DNA determines an organism traits . Mendel work Key Concepts In all of Mendel crosses , only one form of the trait appeared in the generation . However , in the generation , the lost form of the trait reappeared in about one fourth of the plants . An organism traits are controlled by the alleles it inherits from its parents . Some alleles are dominant , while other alleles are recessive . Key Terms heredity alleles trait dominant allele genetics recessive allele fertilization hybrid purebred gene ( Probability and Heredity Key Concepts Probability is the likelihood that a particular event will occur . In a genetic cross , the allele that each parent will pass on to its offspring is based on probability . An organism phenotype is its physical appearance , or visible traits . An organisms genotype is its genetic makeup , or allele combinations . In , the alleles are neither dominant nor recessive . As a result , both alleles are expressed in the offspring . Key Terms probability square phenotype genotype homozygous heterozygous 1820 The Cell and Inheritance Key Concepts According to the chromosome theory of inheritance , genes are carried from parents to their offspring on chromosomes . During meiosis , the chromosome pairs separate and are distributed to two different cells . The resulting sex cells have half as many chromosomes as the other cells in the organism . Chromosomes are made up of many genes joined together like beads on a string . Key Term sexual reproduction diploid meiosis Genes , DNA , and Proteins Key Concepts The order of the nitrogen bases along a gene forms a genetic code that what type of protein will be produced . During protein synthesis , the cell uses information from a gene on a chromosome to produce a protein . Mutations can cause a cell to produce an incorrect protein during protein synthesis . As a result , the organism trait , or phenotype , may be different from what it normally would have been . Key Terms messenger RNA mutation transfer RNA

Review and Assessment . i For Visit Web Code I Target Reading Skill Take Notes To help review Section , take notes on the text that follows the what is heading Principles of an my ( pages ) The notes have ' been started for you as shown at the expressing the probability that I coin will land heads up ?

Reviewing Key Terms Choose the letter of the best answer . Complete the following sentences so that the . The different forms of a gene are called Pan ' he first Part I . Round seed shape in peas is an example of a trait controlled by a dominant allele because . An organism with a genotype of is a . The likelihood that a particular event will hybrid because . occur . The results of a genetic cross can be shown in ' a square , which is . urine . example of a recessive . An organism with two identical alleles for a 10 ' The ofa base Pair of DNA during an is meiosis is an example of a mutation because heterozygous . homozygous . recessive . Writing in Science . The process by which the number of chromosomes is . red by half to form ma sperm eggs is called fol ankle ) mitosis . meiosis . harmful . purebred . During protein synthesis , which of the following provides a code from DNA in the Genetics The Science of Heredity nucleus ?

amino acid transfer RNA guanine messenger RNA Chapter 183 Review and Assessment Checking Concepts 11 . Describe what happened when Mendel crossed purebred tall pea plants with purebred short pea plants . 12 . You toss a coin times and it lands heads up each time . What is the probability that it will land heads up on the sixth toss ?

Explain . guinea pigs , the allele for black fur ( is dominant over the allele for white fur ( In a cross between a heterozygous black guinea pig ( and a homozygous white guinea pig ( what is the probability that an offspring will have white fur ?

Use a square to answer the question . 14 . Describe the role of transfer RNA in protein synthesis . 15 . How can mutations affect protein synthesis ?

Thinking Critically 16 . Applying Concepts in rabbits , the allele for a spotted coat is dominant over the allele for a coat . A spotted rabbit was crossed with a rabbit . The offspring all had spotted coats . What are the probable of the parents ?

17 . Interpreting Diagrams The diagram below shows a chromosome pair . For which genes is the organism heterozygous 18 . Predicting A new mutation in mice causes the coat to be twice as thick as normal . In what environments would this mutation be helpful ?

Why ?

19 . Applying Concepts If the body cells have 12 chromosomes . how many will the sex cells have ?

1800 20 . Relating Cause and Effect Why are mutations that occur in an organism body cells not passed on to its offspring ?

Math Practice 21 . Percentage A garden has 80 pea plants . Of the plants . 20 have short stems and 60 have tall stems . What percentage of the plants have short stems ?

What percentage have tall stems ?

Applying Skills Use the information in the table to answer Questions . In peas , the allele for green pods ( is dominant over the allele for yellow pods ( The table shows the of offspring produced from a cross of two plants with green pods . Phenotype Green pods Number of Offspring 22 . Calculating Percent ( what percent of the offspring produce green pods . Calculate what percent have yellow pods . 23 . Inferring What is the genotype of the offspring with yellow pods ?

What are the possible of the offspring with green pods ?

24 . Drawing Conclusions What are the of the parents ?

How do you know ?

Standards Investigation Performance Assessment Finalize your display of your pet family . Be prepared to discuss the inheritance patterns in your pet family . Examine your classmates exhibits . See which offspring look most like . and least like , their parents . Can you find any offspring that break the laws of inheritance ?

Standards Practice Choose the letter of the best . Which of the following is the first step in the formation of sex cells in an organism that has eight chromosomes ?

A The two of chromosome separate . Chromosome pairs line up next to each other in the center of the cell . The DNA in the eight chromosomes is copied . The move apart , producing cells with four chromosomes each . An organism contains two chromosomes with the genes shown . Which of the following most likely represents the of each parent ?

A Father Aa Mother Father Aa Mother Father AA Mother a Father AA ( Mother AA . Which of the following events begins the process of protein synthesis ?

A Messenger RNA enters the cytoplasm and attaches to a . The coded message in DNA is copied when a molecule of messenger RNA is formed . The protein chain grows until a stop code is reached . Transfer RNA molecules carrying amino acids attach to messenger RNA . Success , Tracker at Use the square below to answer Questions . The square below shows a cross between two pea plants , each with round seeds . The missing genotype in the empty square is correctly written as A . Which statement is true about the cross shown in the square ?

A Both parents are heterozygous for the trait . Both parents are homozygous for the trait . One parent is heterozygous and the other is homozygous for the trait . The trait is controlled by codominant alleles . What percentage of the offspring of this cross will produce round seeds ?

A 25 50 100 . A section of DNA has the base sequence . The corresponding messenger RNA made from this strand will have the base sequence A . Idea . Summarize the stages of meiosis . Chapter 185