6 CHAPTER Meiosis And Mendel - Weebly

SECTION6.3Mendel and HeredityStudenttext pages@E;@8E8 JK8E;8I;J177–179B.5.2KEY CONCEPT Mendel’s research showed that traits are inherited asdiscrete units.Mendel laid the groundwork for genetics.Traits are characteristics* that are inherited, such as eye color, leaf shape,or tail length. Scientists recognized that traits are hereditary, or passedfrom one generation to the next, long before they understood how traitsare passed on. Genetics is the study of biological inheritance patternsand variation in organisms.The study of genetics started in the 1800s with an Austrian monknamed Gregor Mendel. He recognized that there are separate units ofinheritance—what we now call genes—that come from each parent.Mendel studied inheritance in pea plants.Highlight the sentence above that tells who Gregor Mendelwas.Mendel’s data revealed patternsof inheritance.Three things about Mendel’s experiments helped him develophis laws of inheritance.1 He controlled the breeding of the pea plants he studied.Pea flowers have both male and female parts. Theyusually self-pollinate. In other words, a plant mates withitself. As shown in the figure to the right, Mendel controlled the matings of his pea plants. He chose whichplants to cross. In genetics, the mating of two organismsis called a cross.2 He used “either-or” characteristics. Mendel studiedseven different pea traits, including flower color and peashape. All of the characteristics he studied had only twoforms, so all plants either had one form or the other. Forexample, all of the flowers were purple or white. All ofthe peas were wrinkled or round.MENDEL’S PROCESSMendel controlled the fertilizationof his pea plants by removing themale parts, or stamens.He then fertilized the female part,or pistil, with pollen from a different pea plant.* ACADEMIC VOCABULARYcharacteristic something that is recognizable, or that distinguishes someoneor something96McDougal Littell Biology

3He used purebred plants. If a line of plants self-pollinates for longenough, the plants become genetically uniform, or purebred. Theoffspring of a purebred parent inherits all of the parent organism’scharacteristics—they are all the same as the parent. BecauseMendel started with purebred plants, he knew that any variationin the offspring was a result of his crosses.ResultsMendel found that when he crossed purebred plants, one of the forms ofa trait was hidden in the offspring. But the form would reappear in thenext generation.MENDEL’S EXPERIMENTAL CROSSTraits that were hidden when parental purebred flowers were crossedreappeared when the F1 generation was allowed to self-pollinate.P F2F1(parentalgeneration)(first generationof offspring)Purebred whiteand purple plantswere crossed tomake F1. (second generationof offspring)Offspring wereallowed to selfpollinate tomake F2.White flowers reappearedon some offspring.Mendel studied many plants and made many crosses. He foundsimilar patterns in all of his results. In the figure above, you can see thatthe white flowers disappeared in the first generation of offspring. In thesecond generation, however, he found that about one-fourth of theplants had the form of the trait that had disappeared in the first generation. The other three-fourths of the plants had purple flowers. In otherwords there was a 3:1 ratio of purple-flowerd:white flowered plants inthe second generation.Interactive Reader97

ConclusionsThese observations helped Mendel form his first law, called thelaw of segregation.g gThere are two main parts of this law. Organisms inherit two copies of each gene, one from each parent. Only one copy of a gene goes into an organism’s gametes. The twocopies of a gene separate—or segregate—during gamete formation.the twoparts of Mendel’slaw of segregationHighlight Highlightthe two partsof Mendel’slaw of segregationlistedJK8E;8I;J:? :Babove. listed above.6.3traitgeneticscrossVocabulary Checkpurebredlaw of segregationMark It UpGo back and highlighteach sentence thathas a vocabularyword in bold.Choose the correct term from the list for each description.1. the study of biological inheritance2. the mating of two organisms3. a characteristic that is inherited6.3The Big Picture4. The law of segregation says that gametes receive only one chromosome from each homologous pair of chromosomes. Turn back to theimage on page 94 that shows the process of meiosis. In which stage ofmeiosis do homologous chromosomes separate?5. Give two examples of human traits that are not mentioned in the section above.98McDougal Littell Biology

SECTION6.4Studenttext pages@E;@8E8 JK8E;8I;J180–182Traits, Genes, and AllelesB.5.2KEY CONCEPT Genes encode proteins that produce a diverse range of traits.The same gene can have many versions.As you learned, the units of inheritance that Mendel studiedare now called genes. You can think of a gene as a piece ofDNA that stores instructions to make a certain protein. Eachgene is located at a particular place on a chromosome called alocus. Just like a house has an address on a street, a gene has alocus on a chromosome.Many things come in different forms. For example, breadcan be wheat, white, or rye. Most genes have many forms,too. An allele (uh-LEEL) is any of the different forms of agene. The gene for pea shape, for example, has two alleles—one for round peas and another for wrinkled peas.Your cells, like the pea plant’s cells, have two alleles foreach gene—one on each chromosome of a homologous pair.The term homozygous (HOH-moh-ZY-guhs) means the twoalleles of a gene are the same—for example, both alleles arefor round peas. The term heterozygous (HEHT-uhr-uh-ZYguhs) means the two alleles are different—for example, oneallele is for wrinkled peas and one is for round peas.Drawarounda circleeacharoundeachof theallelescircleof theallelesshowninthe VisualshownVocabintothetheVisualright.Vocab to the right.Draw aJK8E;8I;J:? :BGenes influence the developmentof traits.For Mendel’s peas, if a plant was heterozygous for pea shape,the pea shape would be round. This is because the allele forround peas is dominant, or expressed when two differentalleles are present. A recessive allele is expressed only whenthere are two copies of the recessive allele. A dominant alleleis not better or stronger or more common; it is simply theallele that is expressed when there are two different alleles.Mendel studied traits that had just two alleles, one that wasdominant and one that was recessive. Most traits involvemuch more complicated patterns of inheritance.Alleles are represented with letters—capital letters fordominant alleles and lowercase letters for recessive alleles. Forexample, the dominant allele for round pea shape can beVISUAL VOCABHomozygous alleles are identical toeach wrinkledwrinkledroundHeterozygous alleles are different fromeach other.The drawing on p. 93 shows a homologous pair of duplicated chromosomes.Notice that here the chromosomes aredrawn unduplicated. These are twohomologous pairs of unduplicatedchromosomes.VISUAL VOCABA dominant allele is expressed whentwo different alleles are cessiverA recessive allele is expressed onlywhen two copies are present.Interactive Reader99

written as R, for round. The recessive allele, for wrinkled pea shape, canbe represented with the same letter, but lowercase—r.A genotype is the set of alleles an organism has for a trait. For example, a genotype could be homozygous dominant (RR), heterozygous(Rr), or homozygous recessive (rr). A phenotype is what the resultingtrait looks like—for example, round or wrinkled. A genome is all of anorganism’s genetic material—all of the genes on all of the chromosomes.What is the difference between a genotype and a phenotype?6.4Vocabulary sivegenotypephenotypegenomeMark It UpGo back and highlighteach sentence thathas a vocabularyword in bold.1. What is the difference between a gene and an allele?2. What is the difference between a dominant allele and a recessiveallele?6.4The Big Picture3. Fill in the blanks in the chart below regarding pea shape.GENOTYPEPHENOTYPERRRrHOMOZYGOUS OR HETEROZYGOUShomozygous dominantround peasrrhomozygous recessive4. Which of the alleles in the chart above is dominant?100McDougal Littell Biology

SECTION6.5Studenttext pages@E;@8E8 JK8E;8I;J183–187Traits and ProbabilityB.7.1 B.7.3KEY CONCEPT The inheritance of traits follows the rules of probability.Punnett squares illustrate genetic crosses.VISUAL VOCABThe Punnett square is a grid systemfor predicting possible genotypes ofoffspring.Parent 1allelesParent 2allelesA Punnett square is a grid* system for predicting all possiblegenotypes resulting from a cross. The outside edges, or axes*,of the grid represent the possible genotypes of gametes fromeach parent. The grid boxes show the possible genotypes ofoffspring from those two parents.Let’s briefly review what you’ve learned about meiosis andsegregation to examine how the Punnett square works. Bothparents have two alleles for each gene. These alleles are represented on the axes of the Punnett square. During meiosis, thechromosomes—and therefore the alleles—are separated.Each gamete can receive only one of the alleles, but not both.When fertilization happens, gametes from each parent jointogether and form a diploid cell with two copies of eachchromosome. The new cell has two alleles for each gene. Thisis why each box shows two alleles. One is from each parent.AaAAAAaaAaaapossiblegenotypesof offspringWhat do the letters on the axes of the Punnett squarerepresent?A monohybrid cross involves one trait.Thus far, we have studied crosses of one trait. Monohybrid crosses arecrosses that examine the inheritance of only one specific trait—forexample, flower color. If we know the genotypes of the parents, we canuse a Punnett square to predict the genotypes of the offspring.The Punnett squares on the next page show the results of three differentcrosses: Homozygous dominant crossed with homozygous recessive (FF ff ) Heterozygous crossed with heterozygous (Ff Ff ) Heterozygous crossed with homozygous recessive (Ff ff )* ACADEMIC VOCABULARYgrid a layout of squares, like on graph paperaxes lines that act as points of referenceInteractive Reader101

MONOHYBRID CROSSES]dbdon\djh Ydb cVci eVgZci (FF)All offspring receive a dominantallele, F, from one parent and arecessive allele, f, from the otherparent. So all offspring—100percent—have the heterozygousgenotype Ff. And 100 percent ofoffspring have purple flowers.]dbdon\djh gZXZhh kZ eVgZci (ff)FFFfFfFfFfff]ZiZgdon\djh eVgZci (Ff)From each parent, half of theoffspring receive a dominant al]ZiZgdon\djh]ZiZgdon\djheVgZci (Ff)eVgZci (Ff)lele, F, and half receive a recessiveFallele, f. Therefore, one-fourth ofthe offspring have an FF genotype,one-half are Ff, and one-fourth areff. In other words, the genotypicratio is 1:2:1 of FF : Ff : ff. Rememberfthat both FF and Ff genotypes havea purple phenotype. The phenotypicratio is 3 :1 of purple:white flowers.FfFFFfFfff]dbdon\djh gZXZhh kZ eVgZci (ff)All of the offspring receive arecessive allele, f, from the homozygous recessive parent. Halfreceive a dominant allele, F, fromthe heterozygous parent, and halfreceive the recessive allele, f. Theresulting genotypic ratio is 1:1 ofFf:ff. The phenotypic ratio is 1:1 ofpurple:white.ffFfFfffff]ZiZgdon\djh ]ZiZgdon\djh eVgZci (Ff)eVgZci (Ff)FfSuppose that we had a purple-flowered pea plant but did not know itsgenotype. It could be FF or Ff. We could figure out its genotype by crossing the purple-flowered plant with a white-flowered plant. We know thatthe white-flowered plant is ff, because it has the recessive phenotype. Ifthe purple-flowered plant is FF, the offspring will all be purple. If thepurple-flowered plant is Ff, half of the offspring will have purple flowers,and half will have white flowers. Crossing a homozygous recessive organism with an organism of unknown genotype is called a testcross.is the phenotypeof theoffspringtheJK8E;8I;JWhat are Whatthe genotypesof offspringfroman FF fromff cross?:? :Bcross FF x ff?102McDougal Littell Biology

A dihybrid cross involves two traits.VOCABULARYMono- means “one,” anddi- means “two.” A monohybrid cross looks at onetrait and a dihybrid crosslooks at two traits.So far, we have examined monohybrid crosses, or crosses that examineonly one trait. Mendel also performed dihybrid crosses, or crosses thatexamine the inheritance of two different traits.For example, Mendel crossed a purebredplant that had yellow round peas witha purebred plant that had greenDIHYBRID CROSSwrinkled peas. He wanted to see if theThis dihybrid cross isheterozygous-heterozygous.two traits—pea shape and color—were inherited together. The firstYyRrgeneration of offspring all looked the;& \ZcZgVi dcYryRYRsame, and they were all heterozygousfor both traits. The second generationYRof offspring is shown in the figure toYYRRYYRrYyRRthe right. In addition to green wrinkled and yellow round peas, thereYrwere also green round and yellowYYRrYYrrYyRrwrinkled peas. In other words,YyRryRMendel found that pea shape andyyRRYyRRYyRrcolor were independent of eachother—they were not inheritedyrtogether. Mendel’s second law ofyyRrYyRrYyrrgenetics is the law of independentassortment, which states that alleles;' \ZcZgVi dcof different genes separate independently of one another during gameteformation, or meiosis. Different traits are inherited separately.yrYyRrYyrryyRryyrrWhat is the difference between a monohybrid cross and adihybrid cross?Heredity patterns can be calculated withprobability.Probability is the likelihood, or chance, that a particular event willhappen. It predicts the average number of times something happens, notthe exact number of times.number of ways a specific event can occurProbability number of total possible outcomesSuppose you flip a coin. There is a 12 chance it will land on heads, and a 12chance that it will land on tails. Suppose you flip two coins. For eachone, the chance it will land on heads is 12 . But for both to land on heads,the chance is 12 12 14 .Interactive Reader103

These probabilities can be applied to meiosis,too. Suppose a germ cell has heterozygous allelesfor a trait, for example, Ff.f A ggamete has a 12chance of getting an F and a 12 chance of gettingan ff. If two heterozygous plants are crossed, whatis the chance that the offspring will be FF? Thereis a 12 chance that the sperm will carry an F anda 12 chance that the egg will carry an F. Therefore,there is a 12 12 14 chance that the offspring willbe FF. Probability can be used to determine all ofthe possible genotypic outcomes of a cross.PROBABILITY AND HEREDITYThe coins areequally likelyto land headsup or tails up.&'HIld h YZh d[ Xd c & & ' T& ' HIld h YZh d[ Xd c ' &)HH&) HT&)HT&)TTIf you flip two coins, what is the probability that they will both land on tails?6.5Mark It UpVocabulary CheckPunnett squaremonohybrid crosstestcrossGo back and highlighteach sentence thathas a vocabularyword in bold.dihybrid crosslaw of independent assortmentprobabilityChoose the correct term from the list for each description.1. crossing an organism of unknown genotype with a homozygous recessive organism2. a cross to examine one trait only3. a cross to examine two different traits6.5The Big Picture4. Fill in the Punnett square and list thegenotype and phenotype ratios.FGenotype ratio:Phenotype ratio:ff104McDougal Littell BiologyFf&' Tf

SECTION6.6Studenttext pages@E;@8E8 JK8E;8I;J189–191Meiosis andGenetic VariationB.6.5KEY CONCEPT Independent assortment and crossing over during meiosisresult in genetic diversity.Sexual reproduction creates unique genecombinations.Sexual reproduction produces a lot of variety within a species. ThisVOCABULARYgenetic variety comes from the events of meiosis and from the fertilizaRecall Mendel’s law ofindependent assortmenttion of gametes, which is a random process. Recall that humans have 23described on p. 103.pairs of chromosomes, and that each pair assorts independently from theothers. As a result, there are about 8 million different combinations ofchromosomes that can be produced during meiosis of one human cell.Suppose a human sperm cell that has one of 8 million different possible combinations fertilizes a human egg cell that has one of 8 milliondifferent possible combinations. Since any sperm cell canCROSSING OVERfertilize any egg, more than 64 trillion possible combinationsCrossing over exchanges pieces ofcan result.DNA between homologousFor all sexually reproducing organisms, sexual reproductionchromosomes.results in unique combinations of the two parents’ genes.Therefore, their offspring have unique phenotypes. This varietyhelps some organisms of a species survive and reproduce inconditions where other organisms of the species cannot.are twoparts ofsexual reproductionWhat are Whattwo partsof sexualreproductionthatJK8E;8I;J:? :Bthat produce genetic variation?produce genetic variation?1Two homologous chromosomespair up with each other duringprophase I in meiosis.2In this position, some chromatidsare very close to each other andpieces cross.3Some of these pieces break off andreattach to the other homologouschromosome.Crossing over during meiosis increasesgenetic diversity.Crossing over is a process that occurs during meiosis and alsocontributes to genetic variation. Crossing over is the exchange*of chromosome pieces between homologous chromosomes.This happens during prophase I of meiosis I. The process isshown in the figure to the right. Crossing over can happenmany times—even within the same pair of homologouschromosomes.* ACADEMIC VOCABULARYexchange to give and receive, or to trade somethingInteractive Reader105

Recall that a single chromosome hasGENETIC LINKAGEmany genes, each with its own locus, orA and B are not linked to C and D because they are so farplace, on the chromosome. Two genes onapart. Crossing over is likely to occur in the space betweengenes B and C, thereby separating A and B from C and D.the same chromosome may be closetogether or far apart. For example, in the6 VcY 7 VgZ gZ[ZggZY id Vh a c ZY WZXVjhZ\ZcZ 6i]Z

Now that you've seen how meiosis works, let's review two key differ-ences between the processes of 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 above, circle the part in the process of meiosis