sign in sign up
<<
Home , Dihybrid cross, Meiosis

0250-0251 UO4 BDOL-829900 8/4/04 6:16 PM Page 250

1863 Lincoln writes the Emancipation Proclamation.

1865 Mendel discovers the rules of inheritance.

What You’ll Learn Chapter 10 Mendel and Meiosis Chapter 11 DNA and Chapter 12 Patterns of and Human Genetics Chapter 13 Genetic Technology Unit 4 Review BioDigest & Standardized Test Practice

Why It’s Important Physical traits, such as the stripes of these tigers, are encoded in small segments of a called genes, which are passed from one generation to the next. By studying the inheritance pattern of a trait through several generations, the probability that future offspring will express that trait can be predicted. Indiana Standards The following standards are covered in Unit 4: B.1.8, B.1.21, B.1.22, B.1.23, B.1.24, B.1.25, B.1.26, B.1.27, B.1.28, B.1.29, B.1.30, B.2.4

Understanding the Photo White tigers differ from orange tigers by having ice-blue eyes, a pink nose, and creamy white fur with brown or black stripes. They are not albinos. The only time a white tiger is born is when its parents each carry the white- coloring . White tigers are very rare, and today, they are only seen in zoos.

250 in.bdol.glencoe.com/webquest

(t)Science Photo Library/Photo Researchers, (crossover)Tom Brakefield/CORBIS 0250-0251 UO4 BDOL-829900 8/4/04 3:08 PM Page 251

1950 1964 The Korean War begins The Beatles make when North Korea their first appearance invades South Korea. on American TV.

1910 1952 2000 Scientists determine that genes Alfred Hershey and 1953 Most of the reside on . Martha Chase show Watson, Crick, human DNA conclusively that DNA Wilkins, and sequence is is the genetic material. Franklin determine completed. 1944 the structure 1990 Scientists suggest genetic of DNA. The Human material is DNA, not . 1961 Project begins to map The results are not accepted. The genetic code and sequence the entire is cracked. .

(tl)Omikron/Science Source/Photo Researchers, (tr)CNRI/Phototake, NYC

251 0252 C10Open BDOL-829900 8/3/04 9:47 PM Page 252

Mendel and Meiosis

What You’ll Learn ■ You will identify the basic concepts of genetics. ■ You will examine the process of meiosis.

Why It’s Important Genetics explains why you have inherited certain traits from your parents. If you understand how meiosis occurs, you can see how these traits were passed on to you.

Understanding the Photo Zebras usually travel in large groups, and each zebra’s stripes blend in with the stripes of the zebras around it. This confuses predators. Rather than seeing individual zebras, predators see a large, striped mass. Zebra stripe patterns are like human fingerprints—they are geneti- cally determined, and every zebra’s stripe pattern is unique.

Visit in.bdol.glencoe.com to • study the entire chapter online • access Web Links for more information and activities on genetics and meiosis • review content with the Interactive Tutor and self- check quizzes

252 Dr. G.G. Dimijian/Photo Researchers 0253-0262 C10S1 BDOL-829900 8/3/04 8:45 PM Page 253

Mendel’s Laws of Heredity 10.1 Indiana Standards Standard B.1.22 Understand and explain the genetic basis for Mendel’s laws of segregation and independent assortment.

SECTION PREVIEW Objectives Heredity Make the following Foldable to help you Relate Mendel’s two laws organize information about Mendel’s laws of heredity. to the results he obtained in his experiments with garden . STEP 1 Fold one piece of paper STEP 2 Fold the paper Predict the possible off- lengthwise into thirds. widthwise into fifths. spring of a genetic cross by using a . Review Vocabulary experiment: a procedure that tests a hypothesis by the process of col- lecting data under con- STEP 3 STEP 4 trolled conditions (p. 13) Unfold, lay the paper length- Label your table wise, and draw lines along the folds. as shown. New Vocabulary heredity Describe in Give an Mendel Your Words trait Example Rule of genetics unit factors Rule of fertilization Law of segregation Law of independent assortment dominant recessive Make a Table As you read Chapter 10, complete the table describing law of segregation Mendel’s rules and laws of heredity. phenotype homozygous heterozygous Why Mendel Succeeded law of independent assortment People have noticed for thousands of years that resemblances are inherited from generation to generation. However, it was not until the mid-nineteenth century that , an Austrian monk, carried out important studies of heredity—the passing on of characteristics from parents to offspring. Characteristics that are inherited are called traits. Mendel was the first person to succeed in predicting how traits are trans- ferred from one generation to the next. A complete explanation requires the careful study of genetics—the branch of that studies heredity.

heredity from the Mendel chose his subject carefully Latin word hered-, Mendel chose to use the garden in his experiments for several rea- meaning “heir”; Heredity describes sons. Garden pea reproduce sexually, which means that they produce the way the gene- male and cells, called . The male gamete forms in the tic qualities you grain, which is produced in the male reproductive organ. The receive from your ancestors are female gamete forms in the female reproductive organ. In a process called passed on. fertilization, the male gamete unites with the female gamete. The resulting fertilized , called a zygote (ZI goht), then develops into a seed.

10.1 MENDEL’S LAWS OF HEREDITY 253 0253-0262 C10S1 BDOL-829900 8/3/04 8:46 PM Page 254

The transfer of pollen grains from a male reproductive organ to a female reproductive organ in a is called Observe and Infer pollination. In peas, both organs are Looking at Pollen Pollen grains located in the same flower and are are formed within the male tightly enclosed by petals. This pre- anthers of flowers. What is vents pollen from other flowers from their role? Pollen contains the male gametes, or cells, entering the pea flower. As a result, peas needed for fertilization. This Anther normally reproduce by self-pollination; means that pollen grains carry that is, the male and female gametes the hereditary units from male parent plants to female parent Pistil come from the same plant. In many of plants. The pollen grains that Mendel’s experiments, this is exactly Mendel transferred from the Stamens what he wanted. When he wanted to anther of one pea plant to the breed, or cross, one plant with another, female pistil of another plant carried the hereditary traits that he so carefully observed in the next generation. Mendel opened the petals of a flower and removed the male organs, as shown Procedure in Figure 10.1A. He then dusted the ! Examine a flower. Using the diagram as a guide, locate the female organ with pollen from the plant stamens of your flower. There are usually several stamens he wished to cross it with, as shown in in each flower. Figure 10.1B. This process is called @ Remove one stamen and locate the enlarged end—the cross-pollination. By using this tech- anther. nique, Mendel could be sure of the par- # Add a drop of water to a microscope glass slide. Place the ents in his cross. anther in the water. Add a coverslip. Using the eraser end of a pencil, tap the coverslip several times to squash the Mendel was a careful researcher anther. $ Observe under low power. Look for numerous small round Mendel carefully controlled his structures. These are pollen grains. experiments and the peas he used. He studied only one trait at a time to con- Analysis trol variables, and he analyzed his data 1. Estimate Provide an estimate of the number of pollen mathematically. The tall pea plants he grains present in an anther. worked with were from populations of 2. Describe What does a single pollen grain look like? plants that had been tall for many 3. Explain What is the role of pollen grains in plant generations and had always produced ? tall offspring. Such plants are said to be true breeding for tallness. Like- wise, the short plants he worked with were true breeding for shortness.

Figure 10.1 A B Pollen In his experiments, grains Mendel often had to transfer pollen from one plant to another plant with different traits. This is called Transfer making a cross. Remove pollen Describe How did male parts Female part Male parts Mendel make a cross?

Cross-pollination 254 MENDEL AND MEIOSIS 0253-0262 C10S1 BDOL-829900 8/3/04 8:47 PM Page 255

Mendel’s Monohybrid The original parents, the true- breeding plants, are known as the P1 Crosses generation. The P stands for “parent.” What did Mendel do with the tall The offspring of the parent plants and short pea plants he selected? He are known as the F1 generation. crossed them to produce new plants. The F stands for “filial”—son or Mendel referred to the offspring of daughter. When you cross two F1 this cross as hybrids. A hybrid is the plants with each other, their offspring offspring of parents that have differ- are the F2 generation—the second fil- ent forms of a trait, such as tall and ial generation. You might find it eas- short height. Mendel’s first experi- ier to understand these terms if you ments are called monohybrid crosses because mono means “one” and the two parent plants differed from each other by a single trait—height. Figure 10.2 When Mendel crossed true-breeding tall pea plants The first generation with true-breeding short pea plants, all the off- Mendel selected a six-foot-tall pea spring were tall. When he allowed first-generation tall plants to self-pollinate, three-fourths of the off- plant that came from a population of spring were tall and one-fourth were short. pea plants, all of which were over six feet tall. He cross-pollinated this tall pea plant with pollen from a short pea P1 plant that was less than two feet tall and which came from a population of pea plants that were all short. When he planted the seeds from this cross, he found that all of the offspring grew Short pea plant Tall pea plant to be as tall as the taller parent. In this first generation, it was as if the shorter parent had never existed.

The second generation Next, Mendel allowed the tall F1 plants in this first generation to self- pollinate. After the seeds formed, he planted them and counted more than 1000 plants in this second generation. Mendel found that three-fourths of All tall pea plants the plants were as tall as the tall plants in the parent and first generations. He also found that one-fourth of the offspring were as short as the short plants in the parent generation. In other words, in the second genera- F2 tion, tall and short plants occurred in a ratio of about three tall plants to one short plant, as shown in Figure 10.2. The short trait had reappeared as if from nowhere. 3 tall : 1 short

10.1 MENDEL’S LAWS OF HEREDITY 255 0253-0262 C10S1 BDOL-829900 8/3/04 8:47 PM Page 256

Seed Seed Flower Flower Pod Pod Plant shape color color position color shape height

Dominant trait

axial round yellow purple (side) green inflated tall

Recessive trait

terminal wrinkled green white (tips) yellow constricted short

Figure 10.3 Mendel chose seven traits of peas for his experiments. Each trait look at your own family. Your parents of Mendel’s pea plants had two had two clearly differ- are the P generation. You are the F of the gene that determined its ent forms; no interme- 1 1 generation, and any children you height. A plant could have two alleles diate forms were observed. Compare might have in the future would be the for tallness, two alleles for shortness, What genetic varia- F2 generation. or one allele for tallness and one for tions are observed Mendel did similar monohybrid shortness. An ’s two alleles in plants? crosses with a total of seven pairs of are located on different copies of a traits, studying one pair of traits at a chromosome—one inherited from time. These pairs of traits are shown the female parent and one from the in Figure 10.3. In every case, he male parent. found that one trait of a pair seemed The rule of dominance to disappear in the F1 generation, only to reappear unchanged in one- Remember what happened when fourth of the F2 plants. Mendel crossed a tall P1 plant with a short P1 plant? The F1 offspring The rule of unit factors were all tall. In other words, only Mendel concluded that each one trait was observed. In such crosses, organism has two factors that control Mendel called the observed trait allele from the Greek word each of its traits. We now know dominant and the trait that disap- allelon, meaning that these factors are genes and that peared recessive. Mendel concluded “of each other”; they are located on chromosomes. that the allele for tall plants is domi- Genes exist in alternative forms Genes exist in alternative forms. nant to the allele for short plants. called alleles. We call these different gene forms Thus, plants that had one allele for alleles (uh LEELZ). For example, each tallness and one for shortness were tall.

256 MENDEL AND MEIOSIS 0253-0262 C10S1 BDOL-829900 8/3/04 8:48 PM Page 257

Expressed another way, the allele for Aa Figure 10.4 short plants is recessive to the allele for The rule of dominance explains the results of Mendel’s cross between P tall tall plants. Pea plants with two alleles 1 and short plants (A). Tall pea plants are for tallness were tall, and those with about six feet tall, whereas short plants two alleles for shortness were short. are less than two feet tall (B). You can see in Figure 10.4 how the rule of dominance explained the resulting F1 generation. When recording the results of crosses, it is customary to use the same letter for different alleles of the same Tall plant Short plant gene. An uppercase letter is used for TT t t the dominant allele and a lowercase letter for the recessive allele. The dominant allele is always written first. tT Thus, the allele for tallness is written as T and the allele for shortness as t, as it is in Figure 10.4. F1 Describe Mendel’s two rules of heredity.

The law of segregation All tall plants Now recall the results of Mendel’s Tt B cross between F1 tall plants, when B the trait of shortness reappeared. To explain this result, Mendel formu- lated the first of his two laws of heredity. He concluded that each tall plant in the F1 generation carried one dominant allele for tallness and one unexpressed recessive allele for shortness. Each plant received the allele for tallness from its tall parent and the allele for shortness from its short parent in the P1 generation. Because each F1 plant has two differ- ent alleles, it can produce two types of gametes—“tall” gametes and “short” gametes. This conclusion, illustrated in Figure 10.5 on the next page, is called the law of segregation. The law of segregation states that every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles. During fertilization, these gametes randomly pair to pro- duce four combinations of alleles.

257 Dwight R. Kuhn 0253-0262 C10S1 BDOL-829900 8/3/04 8:48 PM Page 258

Figure 10.5 Law of segregation Tt × Tt cross Mendel’s law of segregation explains the results of his

cross between F1 tall plants. He concluded that the two alleles for each trait must separate when gametes are formed. A parent, therefore, F1 passes on at random only one allele for each trait to each offspring.

Tall plant Tall plant Tt Tt

F2

Tall Tall Tall Short TT Tt Tt tt 31

Phenotypes and combination an organism contains is phenotype from known as its genotype ( JEE noh tipe). the Greek words The genotype of a tall plant that has phainein, meaning “to show,” and Mendel showed that tall plants are two alleles for tallness is TT. The typos, meaning not all the same. Some tall plants, genotype of a tall plant that has one “model”; The visi- when crossed with each other, yielded allele for tallness and one allele for ble characteristics of an organism only tall offspring. These were shortness is Tt. You can see that an make up its Mendel’s original P1 true-breeding organism’s genotype can’t always be phenotype. tall plants. Other tall plants, when known by its phenotype. genotype from crossed with each other, yielded both An organism is homozygous (hoh the Greek words gen or geno, tall and short offspring. These were moh ZI gus) for a trait if its two alleles meaning “race,” the F1 tall plants in Figure 10.5 that for the trait are the same. The true- and typos, mean- came from a cross between a tall plant breeding tall plant that had two alleles ing “model”; The allele combination and a short plant. for tallness (TT) would be homozy- of an organism Two , therefore, can look gous for the trait of height. Because makes up its alike but have different underlying tallness is dominant, a TT individual genotype. allele combinations. The way an is homozygous dominant for that trait. organism looks and behaves is called A short plant would always have two its phenotype (FEE noh tipe). The alleles for shortness (tt). It would, phenotype of a tall plant is tall, therefore, always be homozygous whether it is TT or Tt. The allele recessive for the trait of height.

258 MENDEL AND MEIOSIS 0253-0262 C10S1 BDOL-829900 8/3/04 8:49 PM Page 259

An organism is heterozygous (heh Mendel was not surprised when he tuh roh ZI gus) for a trait if its two found that the F1 plants of his dihy- alleles for the trait differ from each brid cross all had the two dominant other. Therefore, the tall plant that traits of round and yellow seeds, as had one allele for tallness and one Figure 10.6 shows. heterozygous allele for shortness (Tt) is heterozy- from the Greek The second generation words heteros, gous for the trait of height. meaning “other,” Now look at Figure 10.5 again. Mendel then let the F1 plants polli- and zygotos, Can you identify the phenotype and nate themselves. As you might expect, meaning “joined together”; A trait genotype of each plant? Is each plant he found some plants that produced is heterozygous homozygous or heterozygous? You round yellow seeds and others that when an individual can practice determining genotypes produced wrinkled green seeds. But has two different alleles for that and phenotypes in the BioLab at the that’s not all. He also found some trait. end of this chapter. plants with round green seeds and others with wrinkled yellow seeds. Mendel’s Dihybrid When Mendel sorted and counted the plants of the F2 generation, he Crosses found they appeared in a definite Mendel performed another set of ratio of phenotypes—9 round yellow: crosses in which he used peas that dif- 3 round green: 3 wrinkled yellow: fered from each other in two traits 1 wrinkled green. To explain the rather than only one. Such a cross results of this dihybrid cross, Mendel involving two different traits is called formulated his second law. a dihybrid cross because di means “two.” In a dihybrid cross, will the two traits stay together in the next generation or will they be inherited independently of each other? Dihybrid cross round yellow × wrinkled green The first generation Mendel took true-breeding pea P1 plants that had round yellow seeds (RRYY) and crossed them with true- Round yellow Wrinkled green breeding pea plants that had wrinkled green seeds (rryy). He already knew All round F1 that when he crossed plants that pro- yellow duced round seeds with plants that produced wrinkled seeds, all the plants in the F1 generation produced seeds that were round. In other F2 words, just as tall plants were domi- 9 3 3 1 nant to short plants, the round- Round Round Wrinkled Wrinkled seeded trait was dominant to the yellow green yellow green wrinkled-seeded trait. Similarly, when he crossed plants that produced Figure 10.6 When Mendel crossed true-breeding plants that produced round yellow yellow seeds with plants that produced seeds with true-breeding plants that produced wrinkled green seeds, the green seeds, all the plants in the seeds of all the offspring were round and yellow. When the F1 plants F1 generation produced yellow seeds— were allowed to self-pollinate, they produced four different kinds of yellow was dominant. Therefore, plants in the F2 generation.

10.1 MENDEL’S LAWS OF HEREDITY 259 0253-0262 C10S1 BDOL-829900 8/3/04 8:49 PM Page 260

The law of independent independent assortment), and vice assortment versa. These alleles can then recom- Mendel’s second law states that bine in four different ways. If the alle- genes for different traits—for exam- les for seed shape and color were ple, seed shape and seed color—are inherited together, only two kinds of inherited independently of each pea seeds would have been produced: other. This conclusion is known as round yellow and wrinkled green. the law of independent assortment. When a pea plant with the genotype Punnett Squares RrYy produces gametes, the alleles R and r will separate from each other In 1905, Reginald Punnett, an (the law of segregation) as well as English biologist, devised a shorthand from the alleles Y and y (the law of way of finding the expected propor- tions of possible genotypes in the off- spring of a cross. This method is called a Punnett square. It takes account of the fact that fertilization Figure 10.7 occurs at random, as Mendel’s law of This Punnett square predicts the results of a segregation states. If you know the between two heterozygous genotypes of the parents, you can use pea plants. Heterozygous a Punnett square to predict the possi- tall parent ble genotypes of their offspring. A The gametes that Tt each parent forms Monohybrid crosses are listed on the Tt Consider the cross between two F1 top and left side tall pea plants, each of which has the of the Punnett genotype Tt. Half the gametes of each square. T parent would contain the T allele, and the other half would contain the t allele. A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait. t The two kinds of gametes from one T t parent are listed on top of the square, Heterozygous and the two kinds of gametes from tall parent the other parent are listed on the left T t side, as Figure 10.7A shows. It doesn’t B You can see that there matter which set of gametes is on top are three different possible genotypes— and which is on the side, that is, TT, Tt, and tt—and that T TT Tt which parent contributes the T allele Tt can result from two and which contributes the t allele. different combinations. Refer to the Punnett square in Interpret Scientific Figure 10.7B to determine the possi- Illustrations How ble genotypes of the offspring. Each many possible phenotypes result tttTt box is filled in with the gametes above from this cross? and to the left side of that box. You can see that each box then contains two alleles—one possible genotype.

260 MENDEL AND MEIOSIS 0253-0262 C10S1 BDOL-829900 8/3/04 8:50 PM Page 261

After the genotypes have been Punnett Square of Dihybrid Cross determined, you can determine the Gametes from RrYy parent phenotypes. Looking again at the RY Ry rY ry Punnett square in Figure 10.7B, you RRYY RRYy RrYY RrYy can see that three-fourths of the off- RY spring are expected to be tall because they have at least one dominant allele. One-fourth are expected to be short RRYy RRyy RrYy Rryy because they lack a dominant allele. Of the tall offspring, one-third will be Ry homozygous dominant (TT) and two- thirds will be heterozygous (Tt). Note that whereas the genotype ratio RrYY RrYy rrYY rrYy is 1TT: 2Tt: 1tt, the phenotype ratio is 3 tall: 1 short. You can practice doing rY

calculations such as Mendel did in the Gametes from RrYy parent Connection to Math at the end of this chapter. RrYy Rryy rrYy rryy

Dihybrid crosses ry What happens in a Punnett square when two traits are considered? Think again about Mendel’s cross Figure 10.8 × F1 cross: RrYy RrYy between pea plants producing round A Punnett square for a dihybrid cross between yellow seeds and plants producing heterozygous pea plants producing round yel- round yellow low seeds shows clearly that the offspring ful- wrinkled green seeds. All the F 1 fill Mendel’s observed ratio of 9 round yellow: round green plants produced seeds that were 3 round green: 3 wrinkled yellow: 1 wrinkled round and yellow and were heterozy- green. wrinkled yellow gous for each trait (RrYy). What kind wrinkled green of gametes will these F1 plants form? Mendel explained that the traits for will occur. In reality, however, you seed shape and seed color would be don’t get the exact ratio of results inherited independently of each shown in the square. That’s because, other. This means that each F1 plant in some ways, genetics is like flipping will produce gametes containing the a coin—it follows the rules of chance. following combinations of genes with When you toss a coin, it lands equal frequency: round yellow (RY), either heads up or tails up. The prob- round green (Ry), wrinkled yellow ability or chance that an event will (rY), and wrinkled green (ry). A occur can be determined by dividing Punnett square for a dihybrid cross the number of desired outcomes by will then need to be four boxes on the total number of possible out- each side for a total of 16 boxes, as comes. Therefore, the probability of Figure 10.8 shows. getting heads when you toss a coin would be one in two chances, written 1 as 1:2 or ⁄ 2. A Punnett square can Probability be used to determine the probability Punnett squares are good for show- of getting a pea plant that produces ing all the possible combinations of round seeds when two plants that gametes and the likelihood that each are heterozygous (Rr) are crossed.

10.1 MENDEL’S LAWS OF HEREDITY 261 0253-0262 C10S1 BDOL-829900 8/3/04 8:50 PM Page 262

The Punnett square in Figure 10.9 shows three plants with round seeds out of four total plants, so the proba- 3 bility is ⁄4. Yet, if you calculate the probability of round-seeded plants Analyze Information RR from Mendel’s actual data in the Data Analysis In addition to crossing rr Problem-Solving Lab on this page, tall and short pea plants, Mendel crossed plants that formed round seeds you will see that slightly less than with plants that formed wrinkled seeds. three-fourths of the plants were He found a 3:1 ratio of round-seeded plants round-seeded. It is important to to wrinkled-seeded plants in the F2 generation. remember that the results predicted by probability are more likely to be seen when there is a large number of Solve the Problem offspring. Mendel’s F2 results are Mendel’s Results shown to the right. Kind of Number 1. Calculate the actual Plant of Plants Figure 10.9 ratio of round-seeded The probability that the offspring from a Round-seeded 5474 plants to wrinkled- of two heterozygotes will show a seeded plants by Wrinkled-seeded 1850 dominant phenotype is 3 out of 4, or 3/4. dividing the number R r of round-seeded plants by the number of wrinkled-seeded plants. Your answer tells you how many more times round- RR Rr seeded plants resulted than wrinkled-seeded plants. 2. To express your answer as a ratio, write the number from R step 1 followed by a colon and the numeral 1.

Thinking Critically Rr rr 1. Compare How does Mendel’s observed ratio compare with the expected 3:1 ratio? r 2. Analyze Why did the actual and expected ratios differ?

Understanding Main Ideas a plant that produces yellow peas, but you don’t 1. What structural features of pea plant flowers know whether it is homozygous dominant or het- made them suitable for Mendel’s genetic studies? erozygous. What experiment could you do to find 2. What are the genotypes of a homozygous and a out? Draw Punnett squares to help you. heterozygous tall pea plant? 3. One parent is homozygous tall and the other is SSKILLKILL RREVIEWEVIEW heterozygous. Make a Punnett square to show how many offspring will be heterozygous. 6. Observe and Infer The offspring of a cross between a plant with purple flowers and a plant 4. How many different gametes can an RRYy parent with white flowers are 23 plants with purple flow- form? What are they? ers and 26 plants with white flowers. Use the letter P for purple and p for white. What are the Thinking Critically genotypes of the parent plants? Explain your rea- 5. In garden peas, the allele for yellow peas is domi- soning. For more help, refer to Observe and Infer nant to the allele for green peas. Suppose you have in the Skill Handbook.

262 MENDEL AND MEIOSIS in.bdol.glencoe.com/self_check_quiz 0263-0279 C10S2 BDOL-829900 8/3/04 7:38 PM Page 263

Meiosis Indiana Standards Standard B.1.28 Illustrate that the sorting and recombination of genes in 10.2 results in a great variety of possible gene combinations from the offspring of any two parents. Recognize that can occur from such processes as crossing over, jumping genes, and and duplication of genes. SECTION PREVIEW Solving the Puzzle Color-enhanced SEM Magnification: 650 Objectives Using an Analogy Mendel’s study Analyze how meiosis of inheritance was based on maintains a constant num- careful observations of pea ber of chromosomes within a species. plants, but pieces of the Infer how meiosis leads to hereditary puzzle were still variation in a species. missing. Modern tech- Relate Mendel’s laws of nologies such as high- heredity to the events of meiosis. power microscopes allow us a glimpse of things that Review Vocabulary : the orderly Mendel could only imag- process of nuclear divi- ine. Chromosomes, such as sion in which two new those shown here, were the daughter cells each receive a complete set missing pieces of the puzzle of chromosomes (p. 204) because they carry the traits New Vocabulary that Mendel described. The key diploid to solving the puzzle was discovering haploid the process by which these traits are chromosomes meiosis transmitted to the next generation. sperm Organize Information As you read this section, make a list of the ways in which meiosis explains Mendel’s results. sexual reproduction crossing over Genes, Chromosomes, and Numbers Organisms have tens of thousands of genes that determine individual traits. Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes. Typically, a chromosome can contain a thousand or more genes along its length.

Diploid and haploid cells If you examined the nucleus in a cell of one of Mendel’s pea plants, you would find it had 14 chromosomes—seven pairs. In the body cells of and most plants, chromosomes occur in pairs. One chromosome in each pair came from the male parent, and the other came from the female parent. A cell with two of each kind of chromosome is called a diploid cell and is said to contain a diploid, or 2n, number of chromo- somes. This pairing supports Mendel’s conclusion that organisms have two factors—alleles—for each trait. One allele is located on each of the paired chromosomes. Organisms produce gametes that contain one of each kind of chromo- some. A cell containing one of each kind of chromosome is called a haploid cell and is said to contain a haploid, or n, number of chromosomes.

10.2 MEIOSIS 263 Biophoto Associates/Photo Researchers 0263-0279 C10S2 BDOL-829900 8/3/04 7:41 PM Page 264

This fact supports Mendel’s conclusion of some species. Note the large range that parent organisms give one factor, of chromosome numbers. Note also or allele, for each trait to each of their that the chromosome number of a offspring. species is not related to the complexity Each species of organism contains a of the organism. characteristic number of chromo- somes. Table 10.1 shows the diploid Homologous chromosomes and haploid numbers of chromosomes The two chromosomes of each pair in a diploid cell are called homologous (hoh MAH luh gus) chromosomes. Each of a pair of homologous chro- mosomes has genes for the same traits, such as plant height. On homo- logous chromosomes, these genes are arranged in the same order, but Interpret Scientific Illustrations because there are different possible Can you identify homologous chromosomes? Homologous alleles for the same gene, the two chromosomes are paired chromosomes having genes for the chromosomes in a homologous pair same trait located at the same place on the chromosome. The are not always identical to each other. gene itself, however, may have different alleles, producing dif- Identify the homologous chromo- ferent forms of the trait. somes in the Problem-Solving Lab. Let’s look at the seven pairs of Solve the Problem homologous chromosomes in the cells The diagram below shows with four different of Mendel’s peas. These chromosome genes present. These genes are represented by the letters F, g, pairs are numbered 1 through 7. Each h, and J. Possible homologous chromosomes of chromosome 1 pair contains certain genes located at are labeled 2–5. Examine the five chromosomes and the genes they contain to determine which of chromosomes 2–5 are specific places on the chromosome. homologous with chromosome 1. contains the genes for three of the traits that Mendel studied. 1 2 3 4 5 Many other genes can be found on K F F F F this chromosome as well. m Every pea plant has two copies of g G G g chromosome 4. It received one from h h h h each of its parents and will give one at J j K J random to each of its offspring. n Remember, however, that the two

O copies of chromosome 4 in a pea plant may not necessarily have identical Thinking Critically alleles. Each chromosome can have 1. Classify Could be homologous with one of the different alleles possible for chromosome 1? Explain. each gene. The homologous chromo- 2. Classify Could be homologous with somes diagrammed in Figure 10.10 chromosome 1? Explain. show both alleles for each of three 3. Classify Could chromosome 4 be homologous with traits. Thus, the plant represented by chromosome 1? Explain. these chromosomes is heterozygous 4. Classify Could be homologous with chromosome 1? Explain. for each of the traits. Explain what homologous chromosomes are.

264 MENDEL AND MEIOSIS 0263-0279 C10S2 BDOL-829900 8/3/04 7:42 PM Page 265

Table 10.1 Chromosome Numbers of Common Organisms Organism Body Cell (2n) Gamete (n) Fruit fly 8 4 Garden pea 14 7 Corn 20 10 24 12 Leopard frog 26 13 Apple 34 17 Corn Human 46 23 Adder’s Chimpanzee 48 24 tongue Dog 78 39 Adder’s tongue fern 1260 630 Leopard frog Why meiosis? haploid cells are called sex cells— When cells divide by mitosis, the gametes. Male gametes are called new cells have exactly the same num- sperm. Female gametes are called ber and kind of chromosomes as the . When a sperm fertilizes an egg, original cells. Imagine if mitosis were the resulting zygote once again has the only means of . Each the diploid number of chromosomes. pea plant parent, which has 14 chro- mosomes, would produce gametes Figure 10.10 that contained a complete set of Each chromosome 4 in garden peas contains genes for flower position, pod shape, and height, among others. Flower position can be either axial 14 chromosomes. That means that (flowers located along the stems) or terminal (flowers clustered at the top each offspring formed by fertilization of the plant). Pod shape can be either inflated or constricted. Plant height of gametes would have twice the num- can be either tall or short. ber of chromosomes as each of its par- Homologous Chromosome 4 ents. The F1 pea plants would have cell nuclei with 28 chromosomes, and the F plants would have cell nuclei 2 a A with 56 chromosomes. Clearly, there must be another form of cell division that allows off- spring to have the same number of Terminal Axial chromosomes as their parents. This kind of cell division, which produces gametes containing half the number of chromosomes as a parent’s body cell, is called meiosis (mi OH sus). Inflated Meiosis occurs in the specialized body cells of each parent that produce D d Constricted gametes. Meiosis consists of two separate divi- sions, known as meiosis I and meiosis T t II. Meiosis I begins with one diploid Short (2n) cell. By the end of meiosis II, Tall there are four haploid (n) cells. These Chromosome 4

10.2 MEIOSIS 265 (tl)Rob & Ann Simpson/Visuals Unlimited, (tr)Aaron Haupt, (b)Robert J. Ellison/Photo Researchers 0263-0279 C10S2 BDOL-829900 8/3/04 7:45 PM Page 266

The zygote then develops by mitosis replicated during that pre- into a . This pat- cedes meiosis I, also. After replication, tern of reproduction, involving the each chromosome consists of two iden- production and subsequent fusion of tical sister , held together by haploid sex cells, is called sexual a . reproduction. It is illustrated in Figure 10.11. I A cell entering prophase I behaves Explain why meiosis in a similar way to one entering is necessary in organisms. prophase of mitosis. The DNA of the chromosomes coils up and a spindle forms. As the DNA coils, homologous The Phases of Meiosis chromosomes line up with each other, During meiosis, a spindle forms gene by gene along their length, to and the cytoplasm divides in the same form a four-part structure called a meiosis from the ways they do during mitosis. tetrad. A tetrad consists of two homol- Greek word meioun, However, what happens to the chro- meaning “to dimin- ogous chromosomes, each made up of ish”; Meiosis is cell mosomes in meiosis is very different. two . The chro- division that results Figure 10.12 illustrates interphase matids in a tetrad pair tightly. In fact, in a gamete contain- and the phases of meiosis. Examine ing half the number they pair so tightly that non-sister of chromosomes of the diagram and photo of each phase chromatids from homologous chro- its parent. as you read about it. mosomes can actually break and exchange genetic material in a process Interphase known as crossing over. Crossing Recall from Chapter 8 that, dur- over can occur at any location on a ing interphase, the cell replicates its chromosome, and it can occur at sev- chromosomes. The chromosomes are eral locations at the same time.

Figure 10.11 Haploid gametes Meiosis In sexual reproduction, the dou- (n = 23) Sperm cell bling of the chromosome num- ber that results from fertilization is balanced by the halving of the chromosome number that results Meiosis from meiosis.

Egg cell Fertilization

Diploid zygote (2n = 46) Multicellular diploid adults Mitosis and (2n = 46) Development

266 MENDEL AND MEIOSIS 0263-0279 C10S2 BDOL-829900 8/3/04 7:46 PM Page 267

Figure 10.12 Compare these diagrams of meiosis with those of mitosis in Chapter 8. After II, meiosis is finished and gametes form. Compare and Contrast In what other ways are mitosis and meiosis different?

LM Magnification: 255

LM Magnification: 255 LM Magnification: 255 Prophase I

Interphase Metaphase I

Anaphase I

Meiosis I Telophase II Meiosis II

Stained LM Magnification: 580 LM Magnification: 255 II Telophase I

Metaphase II Prophase II

LM Magnification: 255 LM Magnification: 255

Stained LM Magnification: 580 Stained LM Magnification: 580

10.2 MEIOSIS 267 (clockwise from top) (1–4 7 9)John D. Cunningham/Visuals Unlimited, (5 6 8)Carolina Biological Supply/Phototake, NYC 0263-0279 C10S2 BDOL-829900 8/3/04 7:47 PM Page 268

It is estimated that during prophase I of meiosis in humans, there is an aver- age of two to three crossovers for each Formulate Models 2 chromosomes with chromatids pair of homologous chromosomes. Modeling Crossing Over This exchange of genetic material Crossing over occurs during is diagrammed in Figure 10.13B. meiosis and involves only Twist tie Crossing over results in new combina- the nonsister chromatids that are present during tions of alleles on a chromosome, as tetrad formation. The you can see in Figure 10.13C. You process is responsible for can practice modeling crossing over in Mark genes the appearance of new the MiniLab at the left. combinations of alleles in with a pencil gamete cells. point. Metaphase I Procedure Nonsister chromatids During metaphase I, the centromere ! Copy the data table. of each chromosome becomes attached @ Roll out four long strands of clay at least 10 cm long to to a spindle fiber. The spindle fibers represent two chromosomes, each with two chromatids. pull the tetrads into the middle, or # Use the figure above as a guide to joining and labeling equator, of the spindle. This is an these model chromatids. Although there are four chro- important step unique to meiosis. matids, assume that they started out as a single pair of homologous chromosomes prior to replication. The figure Note that homologous chromosomes shows tetrad formation during prophase I of meiosis. are lined up side by side as tetrads. In $ First, assume that no crossing over takes place. Model the mitosis, on the other hand, they line appearance of the chromosomes in the four gamete cells up on the spindle’s equator independ- that will result at the end of meiosis. Record your model’s ently of each other. appearance by drawing the gametes’ chromosomes and their genes in your data table. Anaphase I % Next, repeat steps 2–4. This time, however, assume that Anaphase I begins as homologous crossing over occurs between genes B and C. chromosomes, each with its two chro- Data Table matids, separate and move to opposite No Crossing Over Crossing Over ends of the cell. This separation occurs Appearance of chromosomes Appearance of chromosomes because the holding the sister chromatids together do not split as they do during anaphase in mitosis. This critical step ensures that each new cell will receive only one chromosome from each homologous pair. Analysis 1. Predict What will be the appearance of the chromosomes Telophase I prior to replication? Events occur in the reverse order 2. Compare Are there any differences in the combinations from the events of prophase I. The of alleles on chromosomes in gamete cells when crossing over occurs and when it does not occur? spindle is broken down, the chromo- 3. Analogy Crossing over has been compared to “shuffling somes uncoil, and the cytoplasm the deck” in cards. Explain what this means. divides to yield two new cells. Each cell 4. Think Critically What would be accomplished if crossing has half the genetic information of the over occurred between sister chromatids? Explain. original cell because it has only one 5. Evaluate Does your model adequately represent crossing chromosome from each homologous over in a cell? pair. However, another cell division is needed because each chromosome is still doubled.

268 MENDEL AND MEIOSIS 0263-0279 C10S2 BDOL-829900 8/3/04 7:47 PM Page 269

The phases of meiosis II At the end of meiosis II, four hap- The newly formed cells in some loid cells have been formed from one organisms undergo a short resting diploid cell. Each haploid cell con- stage. In other organisms, however, tains one chromosome from each the cells go from late anaphase of homologous pair. These haploid cells meiosis I directly to metaphase of will become gametes, transmitting meiosis II. the genes they contain to offspring. pro- from the The second division in meiosis is Greek word pro, meaning “before” simply a mitotic division of the prod- Meiosis Provides for meta- from the ucts of meiosis I. Meiosis II consists of Greek word meta, prophase II, metaphase II, anaphase II, Genetic Variation meaning “after” and telophase II. During prophase II, a Cells that are formed by mitosis are ana- from the spindle forms in each of the two new identical to each other and to the par- Greek word ana, meaning “up, cells and the spindle fibers attach to the ent cell. Crossing over during meiosis, back, again” chromosomes. The chromosomes, still however, provides a way to rearrange telo- from the made up of sister chromatids, are allele combinations. Rather than the Greek telos, pulled to the center of the cell and line alleles from each parent staying meaning “end” up randomly at the equator during The four phases of together, new combinations of alleles cell division are metaphase II. Anaphase II begins as the can form. Thus, variability is increased. prophase, centromere of each chromosome splits, metaphase, allowing the sister chromatids to sepa- Genetic recombination anaphase, and telophase. rate and move to opposite poles. How many different kinds of sperm Finally, nuclei re-form, the spindles can a pea plant produce? Each cell break down, and the cytoplasm divides undergoing meiosis has seven pairs of during telophase II. The events of chromosomes. Because each of the meiosis II are identical to those you seven pairs of chromosomes can line studied for mitosis except that the up at the cell’s equator in two differ- chromosomes do not replicate before ent ways, 128 different kinds of sperm they divide at the centromeres. are possible (2n 27 128).

Sister chromatids Nonsister chromatids

A B

AA aa A A a a

BB bb b BB b

Tetrad Crossing over in tetrad Homologous chromosomes

C Figure 10.13 Late in prophase I, the homologous AAaa chromosomes come together to form tetrads (A). Arms of nonsister chro- BBbb matids wind around each other (B), and Gametes genetic material may be exchanged (C).

10.2 MEIOSIS 269 0263-0279 C10S2 BDOL-829900 8/3/04 7:48 PM Page 270

In the same way, any pea plant chromosomes and the genetic infor- can form 128 different eggs. Because mation they carry, either by crossing any egg can be fertilized by any over or by independent segregation of sperm, the number of different possi- homologous chromosomes, is called ble offspring is 16 384 (128 128). genetic recombination. It is a major A simple example of how genetic source of variation among organisms. recombination occurs is shown in Variation is important to a species Figure 10.14A. You can see that the because it is the raw material that gene combinations in the gametes forms the basis for . vary depending on how each pair of homologous chromosomes lines Explain how cross- up during metaphase I, a random ing over increases . process. These numbers increase greatly as Meiosis explains Mendel’s results the number of chromosomes in the The behavior of the chromosomes species increases. In humans, n 23, in meiosis provides the physical ba- so the number of different kinds of sis for explaining Mendel’s results. eggs or sperm a person can produce is The segregation of chromosomes in Figure 10.14 more than 8 million (223). When fertil- anaphase I of meiosis explains Men- If a cell has two pairs of ization occurs, 223 223, or 70 trillion, del’s observation that each parent chromosomes—A and a, B and b (n 2)—four different are possible! It’s no gives one allele for each trait at ran- kinds of gametes (22) wonder that each individual is unique. dom to each offspring, regardless of are possible, depending In addition, crossing over can occur whether the allele is expressed. The on how the homologous almost anywhere at random on a chro- segregation of chromosomes at ran- chromosomes line up at mosome. This means that an almost dom during anaphase I also explains the equator during endless number of different possible how factors, or genes, for different meiosis I (A). This event is a matter of chance. chromosomes can be produced by traits are inherited independently of When zygotes are crossing over, providing additional each other. Today, Mendel’s laws and formed by the union of variation to the variation already pro- the events of meiosis together form these gametes, 22 22 duced by the random assortment of the foundation of the chromosome or 16 possible combina- chromosomes. This reassortment of theory of heredity. tions may occur (B).

A B Possible combination of chromosomes in sperm

AB Ab aB ab

AB AABB AABb AaBB AaBb MEIOSIS I

Ab AABb AAbb AaBb Aabb

aB AaBB AaBb aaBB aaBb MEIOSIS II chromosomes in eggs Possible combination of ab AaBb Aabb aaBb aabb

Possible gametes Possible gametes Possible combinations of Chromosome A BChromosome Chromosome aChromosome b chromosomes in zygotes (in boxes)

270 MENDEL AND MEIOSIS 0263-0279 C10S2 BDOL-829900 8/3/04 7:49 PM Page 271

Nondisjunction Although organisms with extra chromosomes often survive, organ- Although the events of meiosis usu- isms lacking one or more chromo- ally proceed accurately, sometimes somes usually do not. When a gamete chromosomes fail to separate correctly. with a missing chromosome fuses The failure of homologous chromo- with a normal gamete during fertil- somes to separate properly during ization, the resulting zygote lacks a meiosis is called nondisjunction. chromosome. This condition is called Recall that during meiosis I, one . In humans, most zygotes chromosome from each homologous with monosomy do not survive. If a pair moves to each pole of the cell. In zygote with monosomy does survive, nondisjunction, both chromosomes the resulting organism usually does of a homologous pair move to the not. An example of monosomy that is same pole of the cell. not lethal is , in In one form of nondisjunction, two which human have only a kinds of gametes result. One has an single instead of two. extra chromosome, and the other is Another form of nondisjunction missing a chromosome. The effects of involves a total lack of separation of nondisjunction are often seen after homologous chromosomes. When this gametes fuse. For example, when a happens, a gamete inherits a complete gamete with an extra chromosome is diploid set of chromosomes, like those fertilized by a normal gamete, the shown in Figure 10.15. When a zygote will have an extra chromo- gamete with an extra set of chromo- some. This condition is called somes is fertilized by a normal haploid (TRI soh mee). In humans, if a gamete gamete, the offspring has three sets of with an extra chromosome number 21 chromosomes and is triploid. The is fertilized by a normal gamete, the fusion of two gametes, each with an resulting zygote has 47 chromosomes extra set of chromosomes, produces instead of 46. This zygote will develop offspring with four sets of chromo- into a baby with . somes—a tetraploid.

Male parent (2n) Figure 10.15 Follow the steps to see Meiosis how a tetraploid plant, such as this chrysanthe- Nondisjunction mum, is produced.

Abnormal Zygote gamete (2n) (4n)

Female parent (2n)

Meiosis

Nondisjunction

Abnormal gamete (2n)

271 R.B. Satterthwaite 0263-0279 C10S2 BDOL-829900 8/3/04 7:49 PM Page 272

Chromosome Mapping Figure 10.16 Crossing over, the exchange of genetic material by nonsister chromatids, provides information that can be used to make chro- mosome maps. Crossing over occurs more frequently between genes that are far apart on a chromosome than between genes that are closer together. Critical Thinking Why is the fre- quency of crossing over related to the distance between genes on a chromosome? Stained TEM Magnification: 1905 A Crossing over In prophase I of meiosis, nonsister chromatids cross over, as shown in the photo above. B Mapping Crossing over produces new allele combinations. Each X-shaped region is a crossover. Geneticists use the frequency of crossing over to map the relative positions of genes on a chromosome. Genes that are farther apart on a chromosome are more likely to have crossing over occur between them than are genes that are closer together.

A 50 B

A 10 D B 5 C or or

D 10 A C 5 B

D 35 C or

C 35 D

C Frequencies and distance Suppose there are four genes—A, B, C, and D—on a chromosome. Geneticists determine that the frequencies of recombination among them are as follows: between A and B—50%; between A and D—10%; between B and C—5%; between C and D—35%. The recombination frequencies can be converted to map units: A–B 50; A–D 10; B–C 5; C–D 35. These map units are not actual distances on the chromosome, but they give relative distances between genes. Geneticists line up the genes as shown above.

D Making the map A 10 D 35C 5 B The genes can be arranged in the sequence that reflects 50 the recombination data. This sequence is a chromosome map.

272 MENDEL AND MEIOSIS B. John/Cabisco/Visuals Unlimited 0263-0279 C10S2 BDOL-829900 8/3/04 7:50 PM Page 273

Polyploidy Organisms with more than the usual number of chromosome sets are called polyploids. is rare in animals and almost always causes death of the zygote. However, polyploidy frequently occurs in plants. Often, the flowers and fruits of these plants are larger than normal, and the plants are healthier. Many polyploid plants, such as the ster- ile banana plant shown in Figure 10.17, are of great commercial value. Meiosis is a complex process, and the results of an error occurring are sometimes unfortunate. However, the resulting changes can be beneficial, such as those that have occurred in Figure 10.17 agriculture. Hexaploid (6n) wheat, are inherited together. These genes The banana plant is an triploid (3n) apples, and polyploid are said to be linked. In fact, all the example of a triploid chrysanthemums all are available genes on a chromosome usually are plant. Think Critically linked and inherited together. It is the Why do you think the commercially. You can see that a thor- banana plant is sterile? ough understanding of meiosis and chromosomes, rather than the individ- genetics would be very helpful to ual genes, that follow Mendel’s law of plant breeders. In fact, plant breeders independent assortment. have learned to produce polyploid Linked genes may become sepa- plants artificially by using chemicals rated on different homologous chro- that cause nondisjunction. mosomes as a result of crossing over. When crossing over produces new gene combinations, geneticists can Gene Linkage and Maps use the frequencies of these new gene Genes sometimes appear to be combinations to make a chromosome inherited together instead of inde- map showing the relative locations of pendently. If genes are close together the genes. Figure 10.16 illustrates on the same chromosome, they usually this process.

Understanding Main Ideas Thinking Critically 1. How are the cells at the end of meiosis different 5. How do the events that take place during from the cells at the beginning of meiosis? Use meiosis explain Mendel’s law of independent the terms chromosome number, haploid, and assortment? diploid in your answer. 2. What is the significance of meiosis to sexual SSKILLKILL RREVIEWEVIEW reproduction? 6. Get the Big Picture Compare Figures 10.12 and 3. Why are there so many varied phenotypes within 8.13 of meiosis and mitosis. Explain why crossing a species such as humans? over between nonsister chromatids of homolo- 4. If the diploid number of a plant is 10, how many gous chromosomes cannot occur during mitosis. chromosomes would you expect to find in its For more help, refer to Get the Big Picture in the triploid offspring? Skill Handbook.

in.bdol.glencoe.com/self_check_quiz 10.2 MEIOSIS 273 Bildarchiv Okapi/Photo Researchers 0263-0279 C10S2 BDOL-829900 8/3/04 8:30 PM Page 274

How can phenotypes and genotypes of plants be determined? Before You Begin It’s difficult to predict the PPREPARATIONREPARATION traits of plants if all that you see is their seeds. But Problem if these seeds are planted Can the phenotypes and genotypes of the parent plants that and allowed to grow, cer- produced two groups of seeds be determined from the phe- tain traits will appear. By notypes of the plants grown from the seeds? observing these traits, you might be able to deter- Hypotheses mine the possible pheno- Have your group agree on a hypothesis to be tested that will types and genotypes of the parent plants that pro- answer the problem question. Record your hypothesis. duced these seeds. In this lab, you will determine Objectives the genotypes of plants In this BioLab, you will: that grow from two ■ Analyze the results of growing two groups of seeds. groups of tobacco seeds. ■ Draw conclusions about phenotypes and genotypes based Each group of seeds came on those results. from different parents. ■ Use the Internet to collect and compare data from other Plants will be either green or albino (white) in color. students. Use the following geno- types for this cross. Possible Materials CC green, Cc green, potting soil light source and cc albino small flowerpots or seedling thermometer or flats temperature probe two groups of tobacco seeds plant-watering bottle hand lens

Safety Precautions CAUTION: Always wash your hands after handling plant materials. Always wear goggles in the lab.

Skill Handbook If you need help with this lab, refer to the Skill Handbook.

PPLANLAN THETHE EEXPERIMENTXPERIMENT 1. Examine the materials provided by your teacher. As a group, make a list of the possible ways you might test your hypothesis. 2. Agree on one way that your group could investigate your hypothesis.

274 MENDEL AND MEIOSIS Matt Meadows 0263-0279 C10S2 BDOL-829900 8/5/04 9:48 AM Page 275

3. Design an experiment that will allow you to collect quanti- tative data. For example, how many plants do you think you will need to examine? 4. Prepare a numbered list of directions. Include a list of materials and the quantities you will need. 5. Make a data table for recording your observations.

Check the Plan 1. Carefully determine what data you are going to collect. How many seeds will you need? How long will you carry out the experiment? 2. What variables, if any, will have to be con- trolled? (Hint: Think about the growing condi- tions for the plants.) 3. Make sure your teacher has approved your experi- mental plan before you proceed further. 4. Carry out your experiment. Make any needed obser- vations, such as the numbers of green and albino plants in each group, and complete your data table. 5. Visit in.bdol.glencoe.com/internet_lab to post your data. 6. CLEANUP AND DISPOSAL Make wise choices in the disposal of materials.

AANALYZENALYZE ANDAND CCONCLUDEONCLUDE

1. Think Critically Why was it necessary to grow plants from the seeds in order to determine the phenotypes of the plants that formed the seeds? 2. Draw Conclusions Using the information in the introduction, describe how the gene for green color (C) is inherited. 3. Make Inferences For the group of seeds that yielded all green plants, are you able to determine exactly the genotypes of the parents that formed these seeds? Can you determine the genotype of each plant observed? Explain. 4. Make Inferences For the group of seeds that yielded some green and some albino plants, are you able to determine exactly the genotypes of the plants that formed these seeds? Can you determine the geno- Find this BioLab using the link below and post your results in the table provided. type of each plant observed? Explain. Briefly describe your experimental design. 5. ERROR ANALYSIS Use the data posted on in.bdol.glencoe.com/internet_lab to in.bdol.glencoe.com/internet_lab compare your experimental design with that of other students. Were your results similar? What might account for the differences?

10.2 MEIOSIS 275 Ray Pfortner/Peter Arnold, Inc. 0263-0279 C10S2 BDOL-829900 8/3/04 8:15 PM Page 276

A Solution from Ratios

n 1866, Gregor Mendel, an Austrian monk, Ipublished the results of eight years of experi- Draw Table B in your notebook or journal. ments with garden peas. His work was ignored Calculate the ratios for the data in Table A and until 1900, when it was rediscovered. complete Table B by following these steps: Mendel had three qualities that led to his dis- • Step 1 Divide the larger number by the covery of the laws of heredity. First, he was curi- smaller number. ous, impelled to find out why things happened. • Step 2 Round to the nearest hundredth. Second, he was a keen observer. Third, he was a • Step 3 To express your answer as a ratio, skilled mathematician. Mendel was the first biol- write the number from step 2 followed by ogist who relied heavily on statistics for solu- a colon and the number 1. tions to how traits are inherited. Table A Mendel’s Results Darwin missed his chance About the same Seed Flower Pod Pod time that Mendel was carrying out his experi- Color Position Color Shape ments with pea plants, Charles Darwin was gath- Yellow Axial Green Inflated ering data on snapdragon flowers. When Darwin 6022 651 428 882 crossed plants that had normal-shaped flowers with plants that had odd-shaped flowers, all the Green Terminal Yellow Constricted offspring had normal-shaped flowers. He thought 2001 207 152 299 the two traits had blended. When he allowed the Table B Calculating Ratios for F1 plants to self-pollinate, his results were 88 plants with normal-shaped flowers and 37 plants Mendel’s Results with odd-shaped flowers. Darwin was puzzled by Seed Flower Pod Pod the results and did not continue his studies with Color Position Color Shape these plants. Lacking Mendel’s statistical skills, Calculation 6022 3.00 Darwin failed to see the significance of the ratio 2001 of normal-shaped flowers to odd-shaped flowers in the F generation. What was this ratio? Was it Ratio 3:1 2 yellow: green similar to Mendel’s ratio of dominant to recessive traits in pea plants?

Finding the ratios for four other traits Figure 10.3 on page 256 shows seven traits that Mendel studied in pea plants. You have already Think Critically Why are ratios so important in looked at Mendel’s data for plant height and understanding how dominant and recessive traits seed shape. Now use the data for seed color, are inherited? flower position, pod color, and pod shape to find To find out more about Mendel’s work, visit the ratios of dominant to recessive for these in.bdol.glencoe.com/math traits in the F2 generation.

276 MENDEL AND MEIOSIS R.W. VanNorman/Visuals Unlimited 0263-0279 C10S2 BDOL-829900 8/3/04 8:13 PM Page 277

STUDY GUIDE Section 10.1 Key Concepts Vocabulary Mendel’s Laws ■ Genes are located on chromosomes and allele (p. 256) exist in alternative forms called alleles. A dominant (p. 256) of Heredity fertilization (p. 253) dominant allele can mask the expression of gamete (p. 253) a recessive allele. genetics (p. 253) ■ When Mendel crossed pea plants differing genotype (p. 258) in one trait, one form of the trait disap- heredity (p. 253) heterozygous (p. 259) peared until the second generation of off- homozygous (p. 258) spring. To explain his results, Mendel hybrid (p. 255) formulated the law of segregation. law of independent ■ Mendel formulated the law of independent assortment (p. 260) law of segregation assortment to explain that two traits are (p. 257) inherited independently. phenotype (p. 258) ■ Events in genetics are governed by the pollination (p. 254) laws of probability. recessive (p. 256) trait (p. 253) zygote (p. 253)

Section 10.2 Key Concepts Vocabulary ■ In meiosis, one diploid (2n) cell produces crossing over (p. 266) Meiosis four haploid (n) cells, providing a way for diploid (p. 263) egg (p. 265) offspring to have the same number of genetic recombination chromosomes as their parents. (p. 270) ■ In prophase I of meiosis, homologous chro- haploid (p. 263) mosomes come together and pair tightly. homologous chromo- some (p. 264) Exchange of genetic material, called cross- meiosis (p. 265) ing over, takes place. nondisjunction (p. 271) ■ Mendel’s results can be explained by the dis- sexual reproduction tribution of chromosomes during meiosis. (p. 266) sperm (p. 265) ■ Random assortment and crossing over dur- ing meiosis provide for genetic variation among the members of a species. ■ The outcome of meiosis may vary due to nondisjunction, the failure of chromosomes to separate properly during cell division. ■ All the genes on a chromosome are linked and are inherited together. It is the chro- mosomes rather than the individual genes that are assorted independently.

To help you review Mendel’s work, use the Organizational Study Fold on page 253.

in.bdol.glencoe.com/vocabulary_puzzlemaker CHAPTER 10 ASSESSMENT 277 0263-0279 C10S2 BDOL-829900 8/3/04 8:13 PM Page 278

Review the Chapter 10 vocabulary words listed in 12. Open Ended On the average, each human the Study Guide on page 277. For each set of has about six recessive alleles that would be vocabulary words, choose the one that does not lethal if expressed. Why do you think that belong. Explain why it does not belong. human cultures have laws against marriage 1. egg—sperm—zygote between close relatives? 2. homozygous—hybrid—heterozygous 13. Open Ended How does separation of homol- 3. phenotype—genotype—allele ogous chromosomes during anaphase I of 4. nondisjunction—genetic recombination— meiosis increase variation among offspring? crossing over 14. Open Ended Relating to the methods of 5. zygote—diploid—gamete science, why do you think it was important for Mendel to study only one trait at a time during his experiments? 15. Open Ended Explain why sexual reproduc- 6. At the end of meiosis, how many haploid tion is an advantage to a population that cells have been formed from the original lives in a rapidly changing environment. cell? A. one C. three B. two D. four 7. When Mendel transferred pollen from one pea plant to another, he was ______the 16. Observe and Infer Why is it possible to plants. have a family of six girls and no boys, but A. self-pollinating C. self-fertilizing extremely unlikely that there will be a public B. cross-pollinating D. cross-fertilizing school with 500 girls and no boys? 8. Which of these does NOT show a recessive 17. Recognize Cause and Effect Why is it trait in garden peas? sometimes impossible to determine the A. B. C. D. genotype of an organism that has a domi- nant phenotype? 18. Observe and Infer While examining a cell in prophase I of meiosis, you observe a pair of homologous chromosomes pairing tightly. What is the significance of the places at 9. During what phase of meiosis do sister which the chromosomes are joined? chromatids separate? 19. REAL WORLD BIOCHALLENGE Several human A. prophase I C. anaphase II genetic disorders result from nondisjunction B. telophase I D. telophase II in meiosis, including Down syndrome, 10. During what phase of meiosis do nonsister Kleinfelter’s syndrome, and Turner syn- chromatids cross over? drome. Visit in.bdol.glencoe.com to inves- A. prophase I C. telophase I tigate these disorders. What characteristic is B. anaphase I D. telophase II common to each? Choose one of these dis- 11. A dihybrid cross between two heterozygotes orders, or another human disorder caused produces a phenotypic ratio of ______. by nondisjunction, and prepare a visual dis- A. 3:1 C. 9:3:3:1 play that explains the disorder. Explain the B. 1:2:1 D. 1:6:9 disorder to your class.

278 CHAPTER 10 ASSESSMENT in.bdol.glencoe.com/chapter_test 0263-0279 C10S2 BDOL-829900 8/3/04 8:10 PM Page 279

The assessed Indiana standard appears next to the question.

Multiple Choice 24. Recessive traits appear only when an B.1.29 Use the diagram to answer questions 20–23. organism is ______. A. mature T t B. different from its parents C. heterozygous T 12 D. homozygous 25. The stage of meiosis shown B.1.8here is ______. A. anaphase I t B. metaphase II 43 C. telophase I D. telophase II

20. Which of the following is true? B.1.28 A. Individual 1 is heterozygous. Study the diagram and answer questions 26–28. B. Individuals 2 and 3 are homozygous. C. Individual 4 is recessive. D. All individuals will be male. 21. Which of the following has the Tt genotype? B.1.29 A. 1 C. 3 B. 2 D. 2 and 3 26. What name is given to the process shown 22. If T is the allele for purple flowers and t is above? B.1.29 the allele for white flowers, the results A. fertilization C. meiosis would be ______. B. zygote D. gametes A. 3 out of 4 are purple 27. What name is given to the cells shown in B. 3 out of 4 are white the diagram above? C. equal numbers of white and purple A. fertilization C. meiosis D. all of the same color B. zygotes D. gametes 23. Which of Mendel’s observations would B.1.22 describe the results of the experimental 28. If each of the cells shown in the diagram has cross in question 22? B.1.816 chromosomes, how many chromosomes A. rule of dominance would you expect to find in a skin cell of the B. law of segregation resulting organism? C. law of independent assortment A. 16 C. 32 D. rule of unit factors B. 64 D. 8

Constructed Response/Grid In Record your answers on your answer document. 29. Open Ended Explain the difference between trisomy and triploidy. Describe a way that each B.1.29 condition could occur. Use diagrams to clarify your answer. 30. Open Ended Compare metaphase of mitosis with metaphase I of meiosis. Explain the significance of B.1.8 the differences between the two stages in terms of sexual reproduction and genetic variation.

in.bdol.glencoe.com/standardized_test CHAPTER 10 ASSESSMENT 279