GETTING READY TO LEARN Preview Key Concepts 6.1 Chromosomes and Meiosis Gametes have half the number of chromosomes that body cells have.

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1 CHAPTER 6 Meiosis and Mendel GETTING READY TO LEARN Preview Key Concepts 6.1 Chromosomes and Meiosis Gametes have hal the number o chromosomes that body cells have. 6.2 Process o Meiosis During meiosis, diploid cells undergo two cell divisions that result in haploid cells. 6.3 Mendel and Heredity Mendel s research showed that traits are inherited as discrete units. 6.4 Traits, Genes, and Alleles Genes encode proteins that produce a diverse range o traits. 6.5 Traits and Probability The inheritance o traits ollows the rules o probability. 6.6 Meiosis and Genetic Variation Independent assortment and crossing over during meiosis result in genetic diversity. Review Academic Vocabulary Write the correct word or each deinition. mitosis data chromosome experiment 1. : recorded observations 2. : results in identical daughter cells 3. : long, continuous piece o DNA 4. : allows scientists to test hypothesis Preview Biology Vocabulary To see how many key terms you already know rom this chapter, choose the word that makes sense in each sentence. trait gene egg Punnett square 1. The shape o your ears is a(n). 2. A(n) is a grid system that helps predict inormation about the ospring o two parent organisms. 3. A(n) is a sex cell in a emale organism. 4. A segment o DNA that codes or a protein is a(n). 89

2 SECTION 6.1 Chromosomes and Meiosis KEY CONCEPT Gametes have hal the number o chromosomes that body cells have. Student text pages B.6.4 You have body cells and gametes. All o the dierent cells in your body can be divided into two groups: somatic cells and germ cells. Germ cells are the cells in your reproductive organs the ovaries or testes that develop into eggs or sperm. Somatic cells (soh-mat-ihk), or body cells, are all the other cells in your body. Somatic cells make up most o your tissues and organs. The DNA in your somatic cells will not be passed on to your children. Only the DNA in the egg or sperm cells gets passed on to ospring. Egg cells and sperm cells are called gametes. Each species has a characteristic number o chromosomes per cell. For example: Humans have 23 pairs o chromosomes. In other words, there are chromosomes in all body cells. Fruit lies have 4 pairs o chromosomes, or 8 chromosomes per cell. Yeast have 16 pairs o chromosomes, or 32 chromosomes per cell. The organism currently known to have the most chromosomes is a ern. It has more than 1200 chromosomes. Chromosome number is not related to the size or complexity o an organism. Do gametes come rom germ cells or somatic cells? egg cell sperm cell Egg cells and sperm cells are called gametes. Your cells have autosomes and sex chromosomes. Suppose you had 23 pairs o gloves. You would have a total o gloves. You could divide them into two sets: 23 right-hand and 23 lethand gloves. Similarly, your body cells have 23 pairs o chromosomes, or a total o 46. These can be divided into two sets: 23 rom your mother and 23 rom your ather. Just as you use both gloves i it is cold outside, your cells use both sets o chromosomes to unction properly. Each pair o chromosomes is called a homologous pair. Here, homologous means having the same structure. Homologous chromosomes are two chromosomes one rom the mother and one rom the ather that are the same size and have copies o the same genes. 90 McDougal Littell Biology

3 Although each chromosome in a homologous pair has copies o the same genes, the two copies may dier. For example, each chromosome in a pair might have a gene that inluences eye color. But the gene on one chromosome o the pair may lead to brown eyes and the gene on the other chromosome may lead to green eyes. One o your 23 pairs o chromosomes is your pair o sex chromosomes. These chromosomes control the sex o an organism. Humans, and all mammals, have two dierent sex chromosomes called X and Y. Females have two X chromosomes. Males have one X chromosome and one Y chromosome. The rest o your chromosomes the other 22 pairs are called autosomes. These chromosomes contain genes or all o the rest o an organism s lie unctions. I a person s pair o sex chromosomes is XY, is the person male or emale? Body cells are diploid; gametes are haploid. Sexual reproduction involves two gametes an egg and a sperm joining together. Fertilization happens when the egg and sperm actually combine. The nucleus o the egg combines with the nucleus o the sperm to orm one nucleus. This new nucleus must have the correct number o chromosomes 46 or humans. Thereore, the egg and sperm each must each have hal that number o chromosomes 23 or humans. Diploid and Haploid Cells Gametes eggs and sperm are haploid (HAP-LOYD) cells. Haploid cells have one copy o each chromosome again, 23 or humans. A sperm and egg join together to orm a diploid (DIHP-LOYD) cell or a total o 46 chromosomes or humans. Body cells are all diploid. Only gametes are haploid. VISUAL VOCAB Meiosis The germ cells in your reproductive organs orm gametes through a process called meiosis. Meiosis (my-oh-sihs) is a process that divides a diploid cell into a haploid cell. In Chapter 5 you learned about mitosis, another process that divides a cell. The igure on the next page shows some o the dierences between mitosis and meiosis. Diploid cells have two copies o each chromosome: one copy rom the mother and one rom the ather. Body cells are diploid (2n). Gametes (sex cells) are haploid (n). Haploid cells have only one copy o each chromosome. Interactive Reader 91

4 COMPARING MITOSIS AND MEIOSIS MITOSIS MEIOSIS Produces genetically identical cells Results in diploid cells Produces genetically unique cells Results in haploid cells Takes place throughout an organism s lietime Takes place only at certain times in an organism s lie cycle Involved in asexual reproduction Involved in sexual reproduction Remember that mitosis results in two identical diploid cells. Mitosis is used or development, growth, and repair. In contrast, meiosis results in our haploid cells that are unique. Meiosis happens only in germ cells to make gametes. Meiosis will be presented in detail in the next section. What is the What dierence is the dierence between between the cells that the cells result that rom result mitosis and rom the mitosis cells that and result the cells rom that meiosis? result rom meiosis? 6.1 Vocabulary Check somatic cell sexual reproduction gamete ertilization homologous chromosome haploid sex chromosome diploid autosome meiosis Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. when the nucleus o an egg joins the nucleus o a sperm 2. a body cell 3. an egg or sperm cell 4. any chromosome except a sex chromosome 6.1 The Big Picture 5. I a diploid cell with 8 chromosomes goes through meiosis, how many chromosomes will the resulting haploid cells have? 6. Circle the sex o a person with the sex chromosomes XX: male / emale 92 McDougal Littell Biology

5 SECTION 6.2 Process o Meiosis KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells. Student text pages B.6.4 Cells go through two rounds o division in meiosis. Meiosis begins with a diploid cell that already has duplicated chromosomes. There are two rounds o cell division meiosis I and meiosis II. The phases o meiosis are similar to the phases o mitosis. To keep the two processes separate in your mind, ocus on the big picture. Mitosis results in identical diploid cells, and meiosis results in unique haploid cells. Homologous Chromosomes and Sister Chromatids Recall that homologous chromosomes are two separate chromosomes: one rom your mother and one rom your ather. Homologous chromosomes carry the same genes in the same order. However, the copies o the genes may dier. Homologous chromosomes are not copies o each other. In contrast, recall that a duplicated chromosome is made o two sister chromatids, attached at the centromere. Sister chromatids are identical copies o each other. homologous chromosomes sister chromatids sister chromatids Homologous chromosomes (shown duplicated) are two separate chromosomes one inherited rom the mother, and one rom the ather. The Process o Meiosis Beore meiosis begins, DNA has already been copied. Homologous chromosomes are separated in the irst hal o meiosis meiosis I. This results in two haploid cells with duplicated chromosomes. These cells are haploid because they each have only one o every pair o homologous chromosomes. Sister chromatids are separated in the second hal o meiosis meiosis II. This results in our haploid cells with undoubled chromosomes. Like mitosis, scientists describe this process in phases. Follow the process o meiosis illustrated on the next page. The igure is simpliied, showing only our chromosomes. Interactive Reader 93

6 MEIOSIS Meiosis I separates homologous chromosomes. rom mother rom ather 1 Prophase I The nuclear membrane breaks down. The duplicated chromosomes condense and homologous chromosomes begin to pair up. Notice that there are two pairs o duplicated homologous chromosomes. 2 Metaphase I The 3 Anaphase I The 4 chromosomes line up along the middle o the cell. paired homologous chromosomes separate. Sister chromatids remain attached. Telophase I Cytokinesis separates the cells. Each cell contains only one o each pair o chromosomes not both. In other words, the cells are now haploid. The chromosomes are still duplicated. Meiosis II separates sister chromatids. The overall process produces haploid cells. 5 Prophase II The 6 Metaphase II The 7 Anaphase II The 8 nuclear membrane breaks down and the cells prepare to divide. chromosomes line up along the middle o the cell. sister chromatids are separated and move to opposite sides o the cell. Telophase II The nuclear membranes orm again. The result o meiosis is our haploid cells with a combination o chromosomes rom both the mother and ather. Now that you ve seen how meiosis works, let s review two key dierences between the processes o meiosis and mitosis. Meiosis has two cell divisions. Mitosis has only one cell division. Meiosis results in haploid cells. Mitosis results in diploid cells. On the diagram On the above, diagram circle above, the circle part in the the part process in the o process meiosis when the o cells meiosis irst become when the haploid. cells irst become haploid. 94 McDougal Littell Biology

7 Haploid cells develop into mature gametes. Gametogenesis (guh-mee-tuh-jehn-ih-sihs) is the production o gametes eggs or sperm. Gametogenesis includes both meiosis and other changes that the haploid cells must go through. The sperm cell, the male gamete, is much smaller than the egg, the emale gamete. Ater meiosis, a cell that develops into a sperm will orm a compact shape with a long tail, or lagellum, that the cell uses to move. For egg production, only one o the cells rom meiosis becomes an egg. It receives most o the cytoplasm and organelles. The other cells produced by meiosis become polar bodies, smaller cells that contain little more than DNA, and are eventually broken down. How do mature gametes dier rom the immature haploid cells? GAMETOGENESIS 6.2 Vocabulary Check gametogenesis sperm egg polar body Choose the correct term rom the list above to complete the sentences below. Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. Sperm and eggs are ormed through the process o. 2. For egg ormation, one o the cells resulting rom meiosis becomes an egg and the others become. 6.2 The Big Picture 3. What is the end result o meiosis? 4. What are two dierences between meiosis and mitosis? Interactive Reader 95

8 SECTION 6.3 Mendel and Heredity KEY CONCEPT Mendel s research showed that traits are inherited as discrete units. Student text pages B.5.2 Mendel laid the groundwork or genetics. Traits are characteristics* that are inherited, such as eye color, lea shape, or tail length. Scientists recognized that traits are hereditary, or passed rom one generation to the next, long beore they understood how traits are passed on. Genetics is the study o biological inheritance patterns and variation in organisms. The study o genetics started in the 1800s with an Austrian monk named Gregor Mendel. He recognized that there are separate units o inheritance what we now call genes that come rom each parent. Mendel studied inheritance in pea plants. Highlight the sentence above that tells who Gregor Mendel was. Mendel s data revealed patterns o inheritance. Three things about Mendel s experiments helped him develop his laws o inheritance. 1 He controlled the breeding o the pea plants he studied. Pea lowers have both male and emale parts. They usually sel-pollinate. In other words, a plant mates with itsel. As shown in the igure to the right, Mendel controlled the matings o his pea plants. He chose which plants to cross. In genetics, the mating o two organisms is called a cross. 2 He used either-or characteristics. Mendel studied seven dierent pea traits, including lower color and pea shape. All o the characteristics he studied had only two orms, so all plants either had one orm or the other. For example, all o the lowers were purple or white. All o the peas were wrinkled or round. MENDEL S PROCESS Mendel controlled the ertilization o his pea plants by removing the male parts, or stamens. He then ertilized the emale part, or pistil, with pollen rom a dierent pea plant. * ACADEMIC VOCABULARY characteristic something that is recognizable, or that distinguishes someone or something 96 McDougal Littell Biology

9 3 He used purebred plants. I a line o plants sel-pollinates or long enough, the plants become genetically uniorm, or purebred. The ospring o a purebred parent inherits all o the parent organism s characteristics they are all the same as the parent. Because Mendel started with purebred plants, he knew that any variation in the ospring was a result o his crosses. Results Mendel ound that when he crossed purebred plants, one o the orms o a trait was hidden in the ospring. But the orm would reappear in the next generation. MENDEL S EXPERIMENTAL CROSS Traits that were hidden when parental purebred lowers were crossed reappeared when the F 1 generation was allowed to sel-pollinate. P (parental generation) F 1 (irst generation o ospring) F 2 (second generation o ospring) Purebred white and purple plants were crossed to make F 1. Ospring were allowed to selpollinate to make F 2. White lowers reappeared on some ospring. Mendel studied many plants and made many crosses. He ound similar patterns in all o his results. In the igure above, you can see that the white lowers disappeared in the irst generation o ospring. In the second generation, however, he ound that about one-ourth o the plants had the orm o the trait that had disappeared in the irst generation. The other three-ourths o the plants had purple lowers. In other words there was a 3:1 ratio o purple-lowerd:white lowered plants in the second generation. Interactive Reader 97

10 Conclusions These observations helped Mendel orm his irst law, called the law o segregation. g There are two main parts o this law. Organisms inherit two copies o each gene, one rom each parent. Only one copy o a gene goes into an organism s gametes. The two copies o a gene separate or segregate during gamete ormation. Highlight Highlight the two parts the two o Mendel s parts o Mendel s law o segregation law o segregation listed above. listed above. 6.3 Vocabulary Check trait genetics cross purebred law o segregation Choose the correct term rom the list or each description. 1. the study o biological inheritance 2. the mating o two organisms 3. a characteristic that is inherited 6.3 The Big Picture Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 4. The law o segregation says that gametes receive only one chromosome rom each homologous pair o chromosomes. Turn back to the image on page 94 that shows the process o meiosis. In which stage o meiosis do homologous chromosomes separate? 5. Give two examples o human traits that are not mentioned in the section above. 98 McDougal Littell Biology

11 SECTION 6.4 Traits, Genes, and Alleles KEY CONCEPT Genes encode proteins that produce a diverse range o traits. Student text pages B.5.2 The same gene can have many versions. As you learned, the units o inheritance that Mendel studied are now called genes. You can think o a gene as a piece o DNA that stores instructions to make a certain protein. Each gene is located at a particular place on a chromosome called a locus. Just like a house has an address on a street, a gene has a locus on a chromosome. Many things come in dierent orms. For example, bread can be wheat, white, or rye. Most genes have many orms, too. An allele (uh-leel) is any o the dierent orms o a gene. The gene or pea shape, or example, has two alleles one or round peas and another or wrinkled peas. Your cells, like the pea plant s cells, have two alleles or each gene one on each chromosome o a homologous pair. The term homozygous (HOH-moh-ZY-guhs) means the two alleles o a gene are the same or example, both alleles are or round peas. The term heterozygous (HEHT-uhr-uh-ZYguhs) means the two alleles are dierent or example, one allele is or wrinkled peas and one is or round peas. Draw a circle Draw around a circle each around o the each alleles o the shown alleles in the Visual shown Vocab in to the the Visual right. Vocab to the right. Genes inluence the development o traits. For Mendel s peas, i a plant was heterozygous or pea shape, the pea shape would be round. This is because the allele or round peas is dominant, or expressed when two dierent alleles are present. A recessive allele is expressed only when there are two copies o the recessive allele. A dominant allele is not better or stronger or more common; it is simply the allele that is expressed when there are two dierent alleles. Mendel studied traits that had just two alleles, one that was dominant and one that was recessive. Most traits involve much more complicated patterns o inheritance. Alleles are represented with letters capital letters or dominant alleles and lowercase letters or recessive alleles. For example, the dominant allele or round pea shape can be VISUAL VOCAB Homozygous alleles are identical to each other. homozygous alleles heterozygous alleles wrinkled wrinkled wrinkled round Heterozygous alleles are dierent rom each other. The drawing on p. 93 shows a homologous pair o duplicated chromosomes. Notice that here the chromosomes are drawn unduplicated. These are two homologous pairs o unduplicated chromosomes. VISUAL VOCAB A dominant allele is expressed when two dierent alleles are present. genotype phenotype wrinkled recessive wrinkled recessive genotype round dominant wrinkled recessive phenotype A recessive allele is expressed only when two copies are present. R r Interactive Reader 99

12 written as R, or round. The recessive allele, or wrinkled pea shape, can be represented with the same letter, but lowercase r. A genotype is the set o alleles an organism has or a trait. For example, a genotype could be homozygous dominant (RR), heterozygous (Rr), or homozygous recessive (rr). A phenotype is what the resulting trait looks like or example, round or wrinkled. A genome is all o an organism s genetic material all o the genes on all o the chromosomes. What is the dierence between a genotype and a phenotype? 6.4 Vocabulary Check Mark It Up gene allele homozygous heterozygous dominant recessive genotype phenotype genome Go back and highlight each sentence that has a vocabulary word in bold. 1. What is the dierence between a gene and an allele? 2. What is the dierence between a dominant allele and a recessive allele? 6.4 The Big Picture 3. Fill in the blanks in the chart below regarding pea shape. GENOTYPE PHENOTYPE HOMOZYGOUS OR HETEROZYGOUS RR homozygous dominant Rr round peas rr homozygous recessive 4. Which o the alleles in the chart above is dominant? 100 McDougal Littell Biology

13 SECTION 6.5 Traits and Probability KEY CONCEPT The inheritance o traits ollows the rules o probability. Student text pages B.7.1 B.7.3 Punnett squares illustrate genetic crosses. A Punnett square is a grid* system or predicting all possible genotypes resulting rom a cross. The outside edges, or axes*, o the grid represent the possible genotypes o gametes rom each parent. The grid boxes show the possible genotypes o ospring rom those two parents. Let s briely review what you ve learned about meiosis and segregation to examine how the Punnett square works. Both parents have two alleles or each gene. These alleles are represented on the axes o the Punnett square. During meiosis, the chromosomes and thereore the alleles are separated. Each gamete can receive only one o the alleles, but not both. When ertilization happens, gametes rom each parent join together and orm a diploid cell with two copies o each chromosome. The new cell has two alleles or each gene. This is why each box shows two alleles. One is rom each parent. What do the letters on the axes o the Punnett square represent? A monohybrid cross involves one trait. Thus ar, we have studied crosses o one trait. Monohybrid crosses are crosses that examine the inheritance o only one speciic trait or example, lower color. I we know the genotypes o the parents, we can use a Punnett square to predict the genotypes o the ospring. VISUAL VOCAB The Punnett squares on the next page show the results o three dierent crosses: Homozygous dominant crossed with homozygous recessive (FF ) Heterozygous crossed with heterozygous (F F ) Heterozygous crossed with homozygous recessive (F ) The Punnett square is a grid system or predicting possible genotypes o ospring. Parent 2 alleles Parent 1 alleles A a A AA Aa a Aa aa possible genotypes o ospring * ACADEMIC VOCABULARY grid a layout o squares, like on graph paper axes lines that act as points o reerence Interactive Reader 101

14 MONOHYBRID CROSSES All ospring receive a dominant allele, F, rom one parent and a recessive allele,, rom the other parent. So all ospring 100 percent have the heterozygous genotype F. And 100 percent o ospring have purple lowers. () F F F (FF) F F F From each parent, hal o the ospring receive a dominant allele, F, and hal receive a recessive allele,. Thereore, one-ourth o the ospring have an FF genotype, one-hal are F, and one-ourth are. In other words, the genotypic ratio is 1:2:1 o FF : F :. Remember that both FF and F genotypes have a purple phenotype. The phenotypic ratio is 3 :1 o purple:white lowers. (F) F F FF F (F) F All o the ospring receive a recessive allele,, rom the homozygous recessive parent. Hal receive a dominant allele, F, rom the heterozygous parent, and hal receive the recessive allele,. The resulting genotypic ratio is 1:1 o F:. The phenotypic ratio is 1:1 o purple:white. (F) F F () F Suppose that we had a purple-lowered pea plant but did not know its genotype. It could be FF or F. We could igure out its genotype by crossing the purple-lowered plant with a white-lowered plant. We know that the white-lowered plant is, because it has the recessive phenotype. I the purple-lowered plant is FF, the ospring will all be purple. I the purple-lowered plant is F, hal o the ospring will have purple lowers, and hal will have white lowers. Crossing a homozygous recessive organism with an organism o unknown genotype is called a testcross. What are What the genotypes is the phenotype o ospring the rom ospring an FF rom cross? the cross FF x? 102 McDougal Littell Biology

15 A dihybrid cross involves two traits. So ar, we have examined monohybrid crosses, or crosses that examine only one trait. Mendel also perormed dihybrid crosses, or crosses that examine the inheritance o two dierent traits. For example, Mendel crossed a purebred plant that had yellow round peas with a purebred plant that had green DIHYBRID CROSS wrinkled peas. He wanted to see i the two traits pea shape and color were inherited together. The irst generation o ospring all looked the same, and they were all heterozygous or both traits. The second generation o ospring is shown in the igure to YR the right. In addition to green wrinkled and yellow round peas, there Yr were also green round and yellow wrinkled peas. In other words, YyRr Mendel ound that pea shape and yr color were independent o each other they were not inherited yr together. Mendel s second law o genetics is the law o independent assortment, which states that alleles o dierent genes separate independently o one another during gamete ormation, or meiosis. Dierent traits are inherited separately. This dihybrid cross is heterozygous-heterozygous. What is the dierence between a monohybrid cross and a dihybrid cross? YR YyRr Yr yr yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr YyRr Yyrr yyrr yyrr VOCABULARY Mono- means one, and di- means two. A monohybrid cross looks at one trait and a dihybrid cross looks at two traits. yyrr yyrr Heredity patterns can be calculated with probability. Probability is the likelihood, or chance, that a particular event will happen. It predicts the average number o times something happens, not the exact number o times. number o ways a speciic event can occur Probability = number o total possible outcomes Suppose you lip a coin. There is a _ 1 2 chance it will land on heads, and a _ 1 2 chance that it will land on tails. Suppose you lip two coins. For each one, the chance it will land on heads is _ 1 2. But or both to land on heads, the chance is _ 1 2 _ 1 2 _ 1 4. Interactive Reader 103

16 These probabilities can be applied to meiosis, too. Suppose a germ cell has heterozygous alleles or a trait, or example, F. A gamete has a _ 1 2 chance o getting an F and a _ 1g 2 chance o getting an. I two heterozygous plants are crossed, what is the chance that the ospring will be FF? There is a _ 1 2 chance that the sperm will carry an F and a _ 1 2 chance that the egg will carry an F. Thereore, there is a _ 1 2 _ 1 2 _ 1 4 chance that the ospring will be FF. Probability can be used to determine all o the possible genotypic outcomes o a cross. I you lip two coins, what is the probability that they will both land on tails? PROBABILITY AND HEREDITY The coins are equally likely to land heads up or tails up. H T H HH HT T HT TT 6.5 Vocabulary Check Punnett square dihybrid cross monohybrid cross law o independent assortment testcross probability Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. Choose the correct term rom the list or each description. 1. crossing an organism o unknown genotype with a homozygous recessive organism 2. a cross to examine one trait only 3. a cross to examine two dierent traits 6.5 The Big Picture 4. Fill in the Punnett square and list the genotype and phenotype ratios. Genotype ratio: Phenotype ratio: F F 104 McDougal Littell Biology

17 SECTION 6.6 Meiosis and Genetic Variation KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity. Student text pages B.6.5 Sexual reproduction creates unique gene combinations. Sexual reproduction produces a lot o variety within a species. This genetic variety comes rom the events o meiosis and rom the ertilization o gametes, which is a random process. Recall that humans have 23 pairs o chromosomes, and that each pair assorts independently rom the others. As a result, there are about 8 million dierent combinations o chromosomes that can be produced during meiosis o one human cell. Suppose a human sperm cell that has one o 8 million dierent possible combinations ertilizes a human egg cell that has one o 8 million dierent possible combinations. Since any sperm cell can ertilize any egg, more than 64 trillion possible combinations can result. For all sexually reproducing organisms, sexual reproduction results in unique combinations o the two parents genes. Thereore, their ospring have unique phenotypes. This variety helps some organisms o a species survive and reproduce in conditions where other organisms o the species cannot. What are What two parts are two o sexual parts o reproduction sexual reproduction that produce genetic that produce variation? genetic variation? CROSSING OVER VOCABULARY Recall Mendel s law o independent assortment described on p Crossing over exchanges pieces o DNA between homologous chromosomes. 1 Two homologous chromosomes pair up with each other during prophase I in meiosis. Crossing over during meiosis increases genetic diversity. Crossing over is a process that occurs during meiosis and also contributes to genetic variation. Crossing over is the exchange* o chromosome pieces between homologous chromosomes. This happens during prophase I o meiosis I. The process is shown in the igure to the right. Crossing over can happen many times even within the same pair o homologous chromosomes. * ACADEMIC VOCABULARY exchange to give and receive, or to trade something 2 3 In this position, some chromatids are very close to each other and pieces cross. Some o these pieces break o and reattach to the other homologous chromosome. Interactive Reader 105

18 Recall that a single chromosome has many genes, each with its own locus, or place, on the chromosome. Two genes on the same chromosome may be close together or ar apart. For example, in the igure to the right, genes A and B are close together, but they are both ar apart rom genes C and D. When crossing over occurs, it is likely that genes A and B will be separated rom genes C and D. But it is unlikely that genes A and B will be separated rom each other or that C and D will be separated because they are so close together. Genes located close together tend to be inherited together, which is called genetic linkage. GENETIC LINKAGE In the igure above, which genes are likely to be separated by crossing over? A and B are not linked to C and D because they are so ar apart. Crossing over is likely to occur in the space between genes B and C, thereby separating A and B rom C and D. 6.6 Vocabulary Check crossing over genetic linkage Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. Draw a picture that shows two chromosomes crossing over. 2. Draw a picture that shows genetic linkage o two genes on a chromosome. 6.6 The Big Picture 3. How is genetic diversity beneicial to a species? 4. How does crossing over contribute to genetic diversity? 106 McDougal Littell Biology

19 Chapter 6 Review 1. Which pair o sex chromosomes makes a person male: XX or XY? 2. Which cells in a multicellular organism undergo meiosis? B.6.4 B What is the inal product o meiosis? B.6.4 a. identical diploid cells c. identical haploid cells b. unique diploid cells d. unique haploid cells 4. In pea plants, the allele or tall stems, T, is dominant to the allele or short stems, t. Draw and ill in a Punnett square that shows the cross o a heterozygous plant, Tt, with a homozygous dominant plant, TT. B List all the possible genotypes o the ospring rom your Punnett square in question 4. Next to each genotype write the corresponding phenotype short stems or tall stems. B What part o meiosis is responsible or Mendel s law o segregation? a. DNA condensing into tightly packaged chromosomes b. homologous chromosomes crossing over c. alleles assorting independently into gametes d. homologous pairs o chromosomes separating into dierent gametes 7. Which human cells are haploid? 8. Explain in words or with drawings how the processes o ertilization and crossing over contribute to genetic diversity. B.6.5 B.6.4 B.6.4 Interactive Reader 107