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What Yoursquoll LearnChapter 10
Mendel and Meiosis
Chapter 11DNA and Genes
Chapter 12Patterns of Heredity and Human Genetics
Chapter 13Genetic Technology
Unit 4 ReviewBioDigest amp Standardized Test Practice
Why Itrsquos ImportantPhysical traits such as the stripes of these tigers areencoded in small segments of a chromosome called geneswhich are passed from one generation to the next Bystudying the inheritance pattern of a trait through severalgenerations the probability that future offspring willexpress that trait can be predicted
1863Lincoln writes the EmancipationProclamation
1865Mendel discovers the rulesof inheritance
Understanding the PhotoWhite tigers differ from orange tigers by having ice-blueeyes a pink nose and creamy white fur with brown orblack stripes They are not albinos The only time a whitetiger is born is when its parents each carry the white-coloring gene White tigers are very rare and today theyare only seen in zoos
250
Genetics
(t)Science Photo LibraryPhoto Researchers (crossover)Tom BrakefieldCORBIS
The following objectives are covered in Unit 4102 103 104 105 201 202 204 301 302 303 304 305 403 404 503
North Carolina Objectives
ncbdolglencoecomwebquest
0250-0251 UO4 BDOL-829900 8404 616 PM Page 250
1950The Korean War beginswhen North Koreainvades South Korea
1961The genetic codeis cracked
2000Most of thehuman DNAsequence iscompleted
1990The Human GenomeProject begins to mapand sequence the entirehuman genome
1952Alfred Hershey andMartha Chase showconclusively that DNAis the genetic material
1910Scientists determine that genesreside on chromosomes
1944Scientists suggest geneticmaterial is DNA not proteinThe results are not accepted
1964The Beatles maketheir first appearanceon American TV
1953Watson CrickWilkins andFranklin determinethe structure of DNA
(tl)OmikronScience SourcePhoto Researchers (tr)CNRIPhototake NYC
251
0250-0251 UO4 BDOL-829900 8404 308 PM Page 251
252
What Yoursquoll Learn You will identify the basic
concepts of genetics You will examine the process
of meiosis
Why Itrsquos ImportantGenetics explains why you haveinherited certain traits fromyour parents If you understandhow meiosis occurs you can seehow these traits were passed onto you
Mendel and MeiosisMendel and Meiosis
Dr GG DimijianPhoto Researchers
Visit tobull study the entire chapter
onlinebull access Web Links for more
information and activities ongenetics and meiosis
bull review content with theInteractive Tutor and self-check quizzes
Zebras usually travel in largegroups and each zebrarsquos stripesblend in with the stripes of thezebras around it This confusespredators Rather than seeingindividual zebras predators seea large striped mass Zebrastripe patterns are like humanfingerprintsmdashthey are geneti-cally determined and everyzebrarsquos stripe pattern is unique
Understandingthe Photo
ncbdolglencoecom
0252 C10Open BDOL-829900 8304 947 PM Page 252
101SECTION PREVIEWObjectivesRelate Mendelrsquos two lawsto the results he obtainedin his experiments withgarden peasPredict the possible off-spring of a genetic cross byusing a Punnett square
Review Vocabularyexperiment a procedure
that tests a hypothesisby the process of col-lecting data under con-trolled conditions (p 13)
New Vocabularyhereditytraitgeneticsgametefertilizationzygotepollinationhybridalleledominantrecessivelaw of segregationphenotypegenotypehomozygousheterozygouslaw of independentassortment
heredity from theLatin word hered-meaning ldquoheirrdquoHeredity describesthe way the gene-tic qualities youreceive from yourancestors arepassed on
Heredity Make the following Foldable to help you organize information about Mendelrsquos laws of heredity
Fold one piece of paper lengthwise into thirds
Fold the paper widthwise into fifths
STEP 1 STEP 2
MendelDescribe inYour Words
Give anExample
Rule ofunit factors
Rule of dominance
Law ofsegregation
Law of independentassortment
Make a Table As you read Chapter 10 complete the table describingMendelrsquos rules and laws of heredity
Unfold lay the paper length-wise and draw lines along the folds
Label your table as shown
STEP 3 STEP 4
101 MENDELrsquoS LAWS OF HEREDITY 253
Mendelrsquos Laws of Heredity
Why Mendel SucceededPeople have noticed for thousands of years that family resemblances are
inherited from generation to generation However it was not until themid-nineteenth century that Gregor Mendel an Austrian monk carriedout important studies of hereditymdashthe passing on of characteristics fromparents to offspring Characteristics that are inherited are called traitsMendel was the first person to succeed in predicting how traits are trans-ferred from one generation to the next A complete explanation requiresthe careful study of geneticsmdashthe branch of biology that studies heredity
Mendel chose his subject carefullyMendel chose to use the garden pea in his experiments for several rea-
sons Garden pea plants reproduce sexually which means that they producemale and female sex cells called gametes The male gamete forms in thepollen grain which is produced in the male reproductive organ Thefemale gamete forms in the female reproductive organ In a process calledfertilization the male gamete unites with the female gamete The resultingfertilized cell called a zygote (ZI goht) then develops into a seed
Objectives 303 Interpret and predict patterns of inheritanceDominant recessive and intermediate traits Multiple alleles Polygenic inheritance Sex-linked traits Independent assortment Punnett squares
North Carolina Objectives
0253-0262 C10S1 BDOL-829900 8304 845 PM Page 253
null
8941908
Figure 101 In his experimentsMendel often had totransfer pollen fromone plant to anotherplant with differenttraits This is called making a crossDescribe How didMendel make a cross
The transfer of pollen grains from amale reproductive organ to a femalereproductive organ in a plant is calledpollination In peas both organs arelocated in the same flower and aretightly enclosed by petals This pre-vents pollen from other flowers fromentering the pea flower As a result peasnormally reproduce by self-pollinationthat is the male and female gametescome from the same plant In many ofMendelrsquos experiments this is exactlywhat he wanted When he wanted tobreed or cross one plant with anotherMendel opened the petals of a flowerand removed the male organs as shownin Figure 101A He then dusted thefemale organ with pollen from the planthe wished to cross it with as shown inFigure 101B This process is calledcross-pollination By using this tech-nique Mendel could be sure of the par-ents in his cross
Mendel was a careful researcherMendel carefully controlled his
experiments and the peas he used Hestudied only one trait at a time to con-trol variables and he analyzed his datamathematically The tall pea plants heworked with were from populations ofplants that had been tall for manygenerations and had always producedtall offspring Such plants are said tobe true breeding for tallness Like-wise the short plants he worked withwere true breeding for shortness
Removemale parts
Transferpollen
Female part Male parts
Cross-pollination
Pollengrains
AA BB
254 MENDEL AND MEIOSIS
Observe and InferLooking at Pollen Pollen grainsare formed within the maleanthers of flowers What istheir role Pollen contains themale gametes or sperm cellsneeded for fertilization Thismeans that pollen grains carrythe hereditary units from maleparent plants to female parentplants The pollen grains thatMendel transferred from theanther of one pea plant to thefemale pistil of another plant carried the hereditary traits that he so carefully observed in the next generation
Procedure Examine a flower Using the diagram as a guide locate the
stamens of your flower There are usually several stamensin each flower
Remove one stamen and locate the enlarged endmdashtheanther
Add a drop of water to a microscope glass slide Place theanther in the water Add a coverslip Using the eraser endof a pencil tap the coverslip several times to squash theanther
$ Observe under low power Look for numerous small roundstructures These are pollen grains
Analysis1 Estimate Provide an estimate of the number of pollen
grains present in an anther2 Describe What does a single pollen grain look like3 Explain What is the role of pollen grains in plant
reproduction
Pistil
Anther
Stamens
0253-0262 C10S1 BDOL-829900 8304 846 PM Page 254
Mendelrsquos MonohybridCrosses
What did Mendel do with the talland short pea plants he selected Hecrossed them to produce new plantsMendel referred to the offspring ofthis cross as hybrids A hybrid is theoffspring of parents that have differ-ent forms of a trait such as tall andshort height Mendelrsquos first experi-ments are called monohybrid crossesbecause mono means ldquoonerdquo and thetwo parent plants differed from eachother by a single traitmdashheight
The first generationMendel selected a six-foot-tall pea
plant that came from a population ofpea plants all of which were over sixfeet tall He cross-pollinated this tallpea plant with pollen from a short peaplant that was less than two feet talland which came from a population ofpea plants that were all short Whenhe planted the seeds from this crosshe found that all of the offspring grewto be as tall as the taller parent In thisfirst generation it was as if theshorter parent had never existed
The second generationNext Mendel allowed the tall
plants in this first generation to self-pollinate After the seeds formed heplanted them and counted more than1000 plants in this second generationMendel found that three-fourths ofthe plants were as tall as the tall plantsin the parent and first generationsHe also found that one-fourth of theoffspring were as short as the shortplants in the parent generation Inother words in the second genera-tion tall and short plants occurred ina ratio of about three tall plants to oneshort plant as shown in Figure 102The short trait had reappeared as iffrom nowhere
The original parents the true-breeding plants are known as the P1generation The P stands for ldquoparentrdquoThe offspring of the parent plants are known as the F1 generation The F stands for ldquofilialrdquomdashson ordaughter When you cross two F1plants with each other their offspringare the F2 generationmdashthe second fil-ial generation You might find it eas-ier to understand these terms if you
101 MENDELrsquoS LAWS OF HEREDITY 255
Short pea plant
All tall pea plants
3 tall 1 short
Tall pea plant
F2
F1
P1
Figure 102 When Mendel crossed true-breeding tall pea plantswith true-breeding short pea plants all the off-spring were tall When he allowed first-generationtall plants to self-pollinate three-fourths of the off-spring were tall and one-fourth were short
0253-0262 C10S1 BDOL-829900 8304 847 PM Page 255
look at your own family Your parentsare the P1 generation You are the F1generation and any children youmight have in the future would be theF2 generation
Mendel did similar monohybridcrosses with a total of seven pairs oftraits studying one pair of traits at atime These pairs of traits are shownin Figure 103 In every case hefound that one trait of a pair seemedto disappear in the F1 generationonly to reappear unchanged in one-fourth of the F2 plants
The rule of unit factorsMendel concluded that each
organism has two factors that controleach of its traits We now know that these factors are genes and thatthey are located on chromosomesGenes exist in alternative forms We call these different gene forms alleles (uh LEELZ) For example each
of Mendelrsquos pea plants had two allelesof the gene that determined itsheight A plant could have two allelesfor tallness two alleles for shortnessor one allele for tallness and one forshortness An organismrsquos two allelesare located on different copies of achromosomemdashone inherited fromthe female parent and one from themale parent
The rule of dominance Remember what happened when
Mendel crossed a tall P1 plant with a short P1 plant The F1 offspringwere all tall In other words only one trait was observed In such crossesMendel called the observed trait dominant and the trait that disap-peared recessive Mendel concludedthat the allele for tall plants is domi-nant to the allele for short plantsThus plants that had one allele fortallness and one for shortness were tall
256 MENDEL AND MEIOSIS
allele from theGreek wordallelon meaningldquoof each otherrdquoGenes exist inalternative formscalled alleles
round yellow purpleaxial(side) green inflated tall
wrinkled green whiteterminal
(tips) yellow constricted short
Seedshape
Dominanttrait
Recessivetrait
Seedcolor
Flowercolor
Flowerposition
Podcolor
Podshape
Plantheight
Figure 103 Mendel chose seventraits of peas for hisexperiments Each traithad two clearly differ-ent forms no interme-diate forms wereobserved CompareWhat genetic varia-tions are observed in plants
0253-0262 C10S1 BDOL-829900 8304 847 PM Page 256
Expressed another way the allele forshort plants is recessive to the allele fortall plants Pea plants with two allelesfor tallness were tall and those withtwo alleles for shortness were shortYou can see in Figure 104 how therule of dominance explained theresulting F1 generation
When recording the results ofcrosses it is customary to use the sameletter for different alleles of the samegene An uppercase letter is used forthe dominant allele and a lowercaseletter for the recessive allele Thedominant allele is always written firstThus the allele for tallness is writtenas T and the allele for shortness as t asit is in Figure 104
Describe Mendelrsquostwo rules of heredity
The law of segregationNow recall the results of Mendelrsquos
cross between F1 tall plants whenthe trait of shortness reappeared Toexplain this result Mendel formu-lated the first of his two laws ofheredity He concluded that each tallplant in the F1 generation carriedone dominant allele for tallness andone unexpressed recessive allele forshortness Each plant received theallele for tallness from its tall parentand the allele for shortness from itsshort parent in the P1 generationBecause each F1 plant has two differ-ent alleles it can produce two typesof gametesmdashldquotallrdquo gametes andldquoshortrdquo gametes This conclusionillustrated in Figure 105 on the nextpage is called the law of segregationThe law of segregation states thatevery individual has two alleles ofeach gene and when gametes areproduced each gamete receives oneof these alleles During fertilizationthese gametes randomly pair to pro-duce four combinations of alleles
Dwight R Kuhn
Tall plant
a
All tall plantsT t
Short plantT T t
tT
t
F1
Figure 104 The rule of dominance explains theresults of Mendelrsquos cross between P1 talland short plants (A) Tall pea plants areabout six feet tall whereas short plantsare less than two feet tall (B)
AA
BB
257
0253-0262 C10S1 BDOL-829900 8304 848 PM Page 257
Phenotypes andGenotypes
Mendel showed that tall plants arenot all the same Some tall plantswhen crossed with each other yieldedonly tall offspring These wereMendelrsquos original P1 true-breedingtall plants Other tall plants whencrossed with each other yielded bothtall and short offspring These werethe F1 tall plants in Figure 105 thatcame from a cross between a tall plantand a short plant
Two organisms therefore can lookalike but have different underlyingallele combinations The way anorganism looks and behaves is calledits phenotype (FEE noh tipe) Thephenotype of a tall plant is tallwhether it is TT or Tt The allele
combination an organism contains isknown as its genotype ( JEE noh tipe)The genotype of a tall plant that hastwo alleles for tallness is TT Thegenotype of a tall plant that has oneallele for tallness and one allele forshortness is Tt You can see that anorganismrsquos genotype canrsquot always beknown by its phenotype
An organism is homozygous (hohmoh ZI gus) for a trait if its two allelesfor the trait are the same The true-breeding tall plant that had two allelesfor tallness (TT) would be homozy-gous for the trait of height Becausetallness is dominant a TT individual is homozygous dominant for that trait A short plant would always have twoalleles for shortness (tt) It wouldtherefore always be homozygousrecessive for the trait of height
258 MENDEL AND MEIOSIS
phenotype fromthe Greek wordsphainein meaningldquoto showrdquo andtypos meaningldquomodelrdquo The visi-ble characteristicsof an organismmake up its phenotypegenotype fromthe Greek wordsgen or genomeaning ldquoracerdquoand typos mean-ing ldquomodelrdquo Theallele combinationof an organismmakes up its genotype
Tall plant
3 1
TallT T
TallT t
TallT t
Shortt t
Tall plant
F1
F2
Law of segregation Tt timestimes Tt cross
T t T t
Figure 105 Mendelrsquos law of segregationexplains the results of hiscross between F1 tall plantsHe concluded that the twoalleles for each trait mustseparate when gametes areformed A parent thereforepasses on at random only oneallele for each trait to eachoffspring
0253-0262 C10S1 BDOL-829900 8304 848 PM Page 258
An organism is heterozygous (hehtuh roh ZI gus) for a trait if its twoalleles for the trait differ from eachother Therefore the tall plant thathad one allele for tallness and oneallele for shortness (Tt) is heterozy-gous for the trait of height
Now look at Figure 105 againCan you identify the phenotype andgenotype of each plant Is each planthomozygous or heterozygous Youcan practice determining genotypesand phenotypes in the BioLab at theend of this chapter
Mendelrsquos DihybridCrosses
Mendel performed another set ofcrosses in which he used peas that dif-fered from each other in two traitsrather than only one Such a crossinvolving two different traits is calleda dihybrid cross because di meansldquotwordquo In a dihybrid cross will thetwo traits stay together in the nextgeneration or will they be inheritedindependently of each other
The first generationMendel took true-breeding pea
plants that had round yellow seeds(RRYY) and crossed them with true-breeding pea plants that had wrinkledgreen seeds (rryy) He already knewthat when he crossed plants that pro-duced round seeds with plants thatproduced wrinkled seeds all theplants in the F1 generation producedseeds that were round In otherwords just as tall plants were domi-nant to short plants the round-seeded trait was dominant to thewrinkled-seeded trait Similarly whenhe crossed plants that produced yellow seeds with plants that producedgreen seeds all the plants in the F1 generation produced yellow seedsmdashyellow was dominant Therefore
Mendel was not surprised when hefound that the F1 plants of his dihy-brid cross all had the two dominanttraits of round and yellow seeds asFigure 106 shows
The second generationMendel then let the F1 plants polli-
nate themselves As you might expecthe found some plants that producedround yellow seeds and others thatproduced wrinkled green seeds Butthatrsquos not all He also found someplants with round green seeds andothers with wrinkled yellow seedsWhen Mendel sorted and countedthe plants of the F2 generation hefound they appeared in a definiteratio of phenotypesmdash9 round yellow3 round green 3 wrinkled yellow 1 wrinkled green To explain theresults of this dihybrid cross Mendelformulated his second law
101 MENDELrsquoS LAWS OF HEREDITY 259
heterozygousfrom the Greekwords heterosmeaning ldquootherrdquoand zygotosmeaning ldquojoinedtogetherrdquo A traitis heterozygouswhen an individualhas two differentalleles for thattrait
P1
F1
F2
Round yellow Wrinkled green
All roundyellow
9Roundyellow
3Roundgreen
3Wrinkledyellow
1Wrinkled
green
Dihybrid cross round yellow timestimes wrinkled green
Figure 106 When Mendel crossed true-breeding plants that produced round yellowseeds with true-breeding plants that produced wrinkled green seeds theseeds of all the offspring were round and yellow When the F1 plantswere allowed to self-pollinate they produced four different kinds ofplants in the F2 generation
0253-0262 C10S1 BDOL-829900 8304 849 PM Page 259
The law of independent assortment
Mendelrsquos second law states thatgenes for different traitsmdashfor exam-ple seed shape and seed colormdashareinherited independently of eachother This conclusion is known asthe law of independent assortmentWhen a pea plant with the genotypeRrYy produces gametes the alleles Rand r will separate from each other(the law of segregation) as well asfrom the alleles Y and y (the law of
independent assortment) and viceversa These alleles can then recom-bine in four different ways If the alle-les for seed shape and color wereinherited together only two kinds ofpea seeds would have been producedround yellow and wrinkled green
Punnett SquaresIn 1905 Reginald Punnett an
English biologist devised a shorthandway of finding the expected propor-tions of possible genotypes in the off-spring of a cross This method iscalled a Punnett square It takesaccount of the fact that fertilizationoccurs at random as Mendelrsquos law ofsegregation states If you know thegenotypes of the parents you can usea Punnett square to predict the possi-ble genotypes of their offspring
Monohybrid crossesConsider the cross between two F1
tall pea plants each of which has thegenotype Tt Half the gametes of eachparent would contain the T allele andthe other half would contain the t allele A Punnett square for thiscross is two boxes tall and two boxeswide because each parent can producetwo kinds of gametes for this traitThe two kinds of gametes from oneparent are listed on top of the squareand the two kinds of gametes fromthe other parent are listed on the leftside as Figure 107A shows It doesnrsquotmatter which set of gametes is on topand which is on the side that iswhich parent contributes the T alleleand which contributes the t alleleRefer to the Punnett square in Figure 107B to determine the possi-ble genotypes of the offspring Eachbox is filled in with the gametes aboveand to the left side of that box Youcan see that each box then containstwo allelesmdashone possible genotype
260 MENDEL AND MEIOSIS
Heterozygoustall parent
Heterozygoustall parent
T
T
T t
tt
T t
Figure 107 This Punnett square predicts the results of amonohybrid cross between two heterozygouspea plants
T
TT Tt
Tt
T
t
t tt
You can see that thereare three differentpossible genotypesmdashTT Tt and ttmdashand thatTt can result from twodifferent combinationsInterpret ScientificIllustrations Howmany possiblephenotypes resultfrom this cross
B
The gametes thateach parent formsare listed on thetop and left side of the Punnettsquare
A
0253-0262 C10S1 BDOL-829900 8304 849 PM Page 260
After the genotypes have beendetermined you can determine thephenotypes Looking again at thePunnett square in Figure 107B youcan see that three-fourths of the off-spring are expected to be tall becausethey have at least one dominant alleleOne-fourth are expected to be shortbecause they lack a dominant alleleOf the tall offspring one-third will behomozygous dominant (TT) and two-thirds will be heterozygous (Tt)Note that whereas the genotype ratiois 1TT 2Tt 1tt the phenotype ratio is3 tall 1 short You can practice doingcalculations such as Mendel did in theConnection to Math at the end of thischapter
Dihybrid crossesWhat happens in a Punnett square
when two traits are consideredThink again about Mendelrsquos crossbetween pea plants producing roundyellow seeds and plants producingwrinkled green seeds All the F1plants produced seeds that wereround and yellow and were heterozy-gous for each trait (RrYy) What kindof gametes will these F1 plants form
Mendel explained that the traits forseed shape and seed color would beinherited independently of eachother This means that each F1 plantwill produce gametes containing thefollowing combinations of genes withequal frequency round yellow (RY)round green (Ry) wrinkled yellow(rY) and wrinkled green (ry) APunnett square for a dihybrid crosswill then need to be four boxes oneach side for a total of 16 boxes asFigure 108 shows
ProbabilityPunnett squares are good for show-
ing all the possible combinations ofgametes and the likelihood that each
will occur In reality however youdonrsquot get the exact ratio of resultsshown in the square Thatrsquos becausein some ways genetics is like flippinga coinmdashit follows the rules of chance
When you toss a coin it landseither heads up or tails up The prob-ability or chance that an event willoccur can be determined by dividingthe number of desired outcomes bythe total number of possible out-comes Therefore the probability ofgetting heads when you toss a coinwould be one in two chances writtenas 12 or 1frasl2 A Punnett square can be used to determine the probabilityof getting a pea plant that producesround seeds when two plants that are heterozygous (Rr) are crossed
101 MENDELrsquoS LAWS OF HEREDITY 261
Gametes from RrYy parentRY
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
Ry rY ry
RY
Ry
rY
ry
round yellow
round green
wrinkled yellow
wrinkled green
F1 cross RrYy times RrYy
Punnett Square of Dihybrid Cross
Gam
etes
from
RrY
y pa
rent
Figure 108 A Punnett square for a dihybrid cross betweenheterozygous pea plants producing round yel-low seeds shows clearly that the offspring ful-fill Mendelrsquos observed ratio of 9 round yellow3 round green 3 wrinkled yellow 1 wrinkledgreen
0253-0262 C10S1 BDOL-829900 8304 850 PM Page 261
Understanding Main Ideas1 What structural features of pea plant flowers
made them suitable for Mendelrsquos genetic studies
2 What are the genotypes of a homozygous and aheterozygous tall pea plant
3 One parent is homozygous tall and the other isheterozygous Make a Punnett square to showhow many offspring will be heterozygous
4 How many different gametes can an RRYy parentform What are they
Thinking Critically5 In garden peas the allele for yellow peas is domi-
nant to the allele for green peas Suppose you have
a plant that produces yellow peas but you donrsquotknow whether it is homozygous dominant or het-erozygous What experiment could you do to findout Draw Punnett squares to help you
6 Observe and Infer The offspring of a crossbetween a plant with purple flowers and a plantwith white flowers are 23 plants with purple flow-ers and 26 plants with white flowers Use the letter P for purple and p for white What are thegenotypes of the parent plants Explain your rea-soning For more help refer to Observe and Inferin the Skill Handbook
SKILL REVIEWSKILL REVIEW
262 MENDEL AND MEIOSIS
R
R
RR
Rr
Rr
rr
r
r
Figure 109The probability that the offspring from amating of two heterozygotes will show adominant phenotype is 3 out of 4 or 34
The Punnett square in Figure 109shows three plants with round seedsout of four total plants so the proba-bility is 3frasl4 Yet if you calculate theprobability of round-seeded plantsfrom Mendelrsquos actual data in theProblem-Solving Lab on this page you will see that slightly less thanthree-fourths of the plants wereround-seeded It is important toremember that the results predictedby probability are more likely to beseen when there is a large number ofoffspring
Analyze InformationData Analysis In addition to crossing tall and short pea plants Mendel crossed plants that formed round seeds with plants that formed wrinkled seeds He found a 31 ratio of round-seeded plants to wrinkled-seeded plants in the F2 generation
Solve the ProblemMendelrsquos F2 results are shown to the right1 Calculate the actual
ratio of round-seeded plants to wrinkled-seeded plants by dividing the number of round-seeded plants by the number of wrinkled-seededplants Your answer tells you how many more times round-seeded plants resulted than wrinkled-seeded plants
2 To express your answer as a ratio write the number fromstep 1 followed by a colon and the numeral 1
Thinking Critically1 Compare How does Mendelrsquos observed ratio compare
with the expected 31 ratio 2 Analyze Why did the actual and expected ratios differ
RR
rr
Mendelrsquos Results
Kind of NumberPlant of Plants
Round-seeded 5474
Wrinkled-seeded 1850
ncbdolglencoecomself_check_quiz
0253-0262 C10S1 BDOL-829900 8304 850 PM Page 262
102SECTION PREVIEWObjectivesAnalyze how meiosismaintains a constant num-ber of chromosomeswithin a speciesInfer how meiosis leads tovariation in a speciesRelate Mendelrsquos laws ofheredity to the events ofmeiosis
Review Vocabularymitosis the orderly
process of nuclear divi-sion in which two newdaughter cells eachreceive a complete setof chromosomes (p 204)
New Vocabularydiploidhaploidhomologous chromosomemeiosisspermeggsexual reproductioncrossing overgenetic recombinationnondisjunction
102 MEIOSIS 263
Meiosis
Biophoto AssociatesPhoto Researchers
Solving the PuzzleUsing an Analogy Mendelrsquos studyof inheritance was based oncareful observations of peaplants but pieces of thehereditary puzzle were stillmissing Modern tech-nologies such as high-power microscopes allowus a glimpse of things thatMendel could only imag-ine Chromosomes such asthose shown here were themissing pieces of the puzzlebecause they carry the traitsthat Mendel described The keyto solving the puzzle was discoveringthe process by which these traits aretransmitted to the next generationOrganize Information As you read this section make a list of the ways in which meiosis explains Mendelrsquos results
Metaphase chromosomes
Color-enhanced SEM Magnification 650
Genes Chromosomes and NumbersOrganisms 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 onchromosomes Typically a chromosome can contain a thousand or moregenes along its length
Diploid and haploid cellsIf you examined the nucleus in a cell of one of Mendelrsquos pea plants
you would find it had 14 chromosomesmdashseven pairs In the body cells ofanimals and most plants chromosomes occur in pairs One chromosomein each pair came from the male parent and the other came from thefemale parent A cell with two of each kind of chromosome is called adiploid cell and is said to contain a diploid or 2n number of chromo-somes This pairing supports Mendelrsquos conclusion that organisms havetwo factorsmdashallelesmdashfor each trait One allele is located on each of thepaired 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 haploidcell and is said to contain a haploid or n number of chromosomes
Objective 302 Compare and contrast the characteristicsof asexual and sexual reproduction
North Carolina Objectives
0263-0279 C10S2 BDOL-829900 8304 738 PM Page 263
null
8563096
This fact supports Mendelrsquos conclusionthat parent organisms give one factoror allele for each trait to each of theiroffspring
Each species of organism contains acharacteristic number of chromo-somes Table 101 shows the diploidand haploid numbers of chromosomes
of some species Note the large rangeof chromosome numbers Note alsothat the chromosome number of aspecies is not related to the complexityof the organism
Homologous chromosomesThe two chromosomes of each pair
in a diploid cell are called homologous(hoh MAH luh gus) chromosomesEach of a pair of homologous chro-mosomes has genes for the sametraits such as plant height On homo-logous chromosomes these genes arearranged in the same order butbecause there are different possiblealleles for the same gene the twochromosomes in a homologous pairare not always identical to each otherIdentify the homologous chromo-somes in the Problem-Solving Lab
Letrsquos look at the seven pairs ofhomologous chromosomes in the cellsof Mendelrsquos peas These chromosomepairs are numbered 1 through 7 Eachpair contains certain genes located atspecific places on the chromosomeChromosome 4 contains the genes forthree of the traits that Mendel studiedMany other genes can be found onthis chromosome as well
Every pea plant has two copies ofchromosome 4 It received one fromeach of its parents and will give one atrandom to each of its offspringRemember however that the twocopies of chromosome 4 in a pea plantmay not necessarily have identicalalleles Each chromosome can haveone of the different alleles possible foreach gene The homologous chromo-somes diagrammed in Figure 1010show both alleles for each of threetraits Thus the plant represented bythese chromosomes is heterozygousfor each of the traits
Explain whathomologous chromosomes are
Interpret Scientific IllustrationsCan you identify homologous chromosomes Homologouschromosomes are paired chromosomes having genes for thesame trait located at the same place on the chromosome Thegene itself however may have different alleles producing dif-ferent forms of the trait
Solve the ProblemThe diagram below shows chromosome 1 with four differentgenes present These genes are represented by the letters F gh and J Possible homologous chromosomes of chromosome 1are labeled 2ndash5 Examine the five chromosomes and the genesthey contain to determine which of chromosomes 2ndash5 arehomologous with chromosome 1
Thinking Critically1 Classify Could chromosome 2 be homologous with
chromosome 1 Explain2 Classify Could chromosome 3 be homologous with
chromosome 1 Explain3 Classify Could chromosome 4 be homologous with
chromosome 1 Explain4 Classify Could chromosome 5 be homologous with
chromosome 1 Explain
1
K
m
n
O
4
F
g
J
h
5
F
g
J
h
2
F
G
j
h
3
F
G
K
h
264 MENDEL AND MEIOSIS
0263-0279 C10S2 BDOL-829900 8304 741 PM Page 264
Why meiosisWhen cells divide by mitosis the
new cells have exactly the same num-ber and kind of chromosomes as theoriginal cells Imagine if mitosis werethe only means of cell division Eachpea plant parent which has 14 chro-mosomes would produce gametesthat contained a complete set of 14 chromosomes That means thateach offspring formed by fertilizationof gametes would have twice the num-ber of chromosomes as each of its par-ents The F1 pea plants would havecell nuclei with 28 chromosomes andthe F2 plants would have cell nucleiwith 56 chromosomes
Clearly there must be anotherform of cell division that allows off-spring to have the same number ofchromosomes as their parents Thiskind of cell division which producesgametes containing half the numberof chromosomes as a parentrsquos bodycell is called meiosis (mi OH sus)Meiosis occurs in the specialized bodycells of each parent that producegametes
Meiosis consists of two separate divi-sions known as meiosis I and meiosisII Meiosis I begins with one diploid(2n) cell By the end of meiosis II there are four haploid (n) cells These
haploid cells are called sex cellsmdashgametes Male gametes are calledsperm Female gametes are calledeggs When a sperm fertilizes an eggthe resulting zygote once again hasthe diploid number of chromosomes
102 MEIOSIS 265(tl)Rob amp Ann SimpsonVisuals Unlimited (tr)Aaron Haupt (b)Robert J EllisonPhoto Researchers
Chromosome 4
a
Terminal
Tall
Inflated
Axial
Short
Constricted
T
D
t
d
A
Homologous Chromosome 4
Figure 1010Each chromosome 4 in garden peas contains genes for flower positionpod shape and height among others Flower position can be either axial(flowers located along the stems) or terminal (flowers clustered at the topof the plant) Pod shape can be either inflated or constricted Plant heightcan be either tall or short
Table 101 Chromosome Numbers of Common Organisms
Organism Body Cell (2n) Gamete (n)
Fruit fly 8 4
Garden pea 14 7
Corn 20 10
Tomato 24 12
Leopard frog 26 13
Apple 34 17
Human 46 23
Chimpanzee 48 24
Dog 78 39
Adderrsquos tongue fern 1260 630
Adderrsquostongue fern
Corn
Leopardfrog
0263-0279 C10S2 BDOL-829900 8304 742 PM Page 265
The zygote then develops by mitosisinto a multicellular organism This pat-tern of reproduction involving theproduction and subsequent fusion ofhaploid sex cells is called sexualreproduction It is illustrated inFigure 1011
Explain why meiosisis necessary in organisms
The Phases of MeiosisDuring meiosis a spindle forms
and the cytoplasm divides in the sameways they do during mitosisHowever what happens to the chro-mosomes in meiosis is very differentFigure 1012 illustrates interphaseand the phases of meiosis Examinethe diagram and photo of each phaseas you read about it
InterphaseRecall from Chapter 8 that dur-
ing interphase the cell replicates itschromosomes The chromosomes are
replicated during interphase that pre-cedes meiosis I also After replicationeach chromosome consists of two iden-tical sister chromatids held together bya centromere
Prophase I A cell entering prophase I behaves
in a similar way to one enteringprophase of mitosis The DNA of thechromosomes coils up and a spindleforms As the DNA coils homologouschromosomes line up with each othergene by gene along their length toform a four-part structure called atetrad A tetrad consists of two homol-ogous chromosomes each made up oftwo sister chromatids The chro-matids in a tetrad pair tightly In factthey pair so tightly that non-sisterchromatids from homologous chro-mosomes can actually break andexchange genetic material in a processknown as crossing over Crossingover can occur at any location on achromosome and it can occur at sev-eral locations at the same time
266 MENDEL AND MEIOSIS
Figure 1011 In sexual reproduction the dou-bling of the chromosome num-ber that results from fertilizationis balanced by the halving of thechromosome number that resultsfrom meiosis
Fertilization
Meiosis
Mitosis andDevelopment
Diploid zygote(2n = 46)
Haploid gametes(n = 23)
Sperm cell
Multicellular diploid adults
(2n = 46)
Meiosis
Egg cell
meiosis from theGreek word meiounmeaning ldquoto dimin-ishrdquo Meiosis is celldivision that resultsin a gamete contain-ing half the numberof chromosomes ofits parent
0263-0279 C10S2 BDOL-829900 8304 745 PM Page 266
102 MEIOSIS 267(clockwise from top) (1ndash4 7 9)John D CunninghamVisuals Unlimited (5 6 8)Carolina Biological SupplyPhototake NYC
Figure 1012Compare these diagrams of meiosis with those of mitosis in Chapter 8 After telophase IImeiosis is finished and gametes form Compare and Contrast In what other ways aremitosis and meiosis different
Meiosis I
Metaphase I
Metaphase II
Prophase I
Prophase II
Interphase
Telophase II
Telophase IAnaphase II
Anaphase I
Meiosis II
LM Magnification 255
LM Magnification 255
LM Magnification 255
LM Magnification 255
Stained LM Magnification 580Stained LM Magnification 580
LM Magnification 255
Stained LM Magnification 580
LM Magnification 255
0263-0279 C10S2 BDOL-829900 8304 746 PM Page 267
It is estimated that during prophase Iof meiosis in humans there is an aver-age of two to three crossovers for eachpair of homologous chromosomes
This exchange of genetic material is diagrammed in Figure 1013BCrossing over results in new combina-tions of alleles on a chromosome asyou can see in Figure 1013C Youcan practice modeling crossing over inthe MiniLab at the left
Metaphase IDuring metaphase I the centromere
of each chromosome becomes attachedto a spindle fiber The spindle fiberspull the tetrads into the middle orequator of the spindle This is animportant step unique to meiosisNote that homologous chromosomesare lined up side by side as tetrads Inmitosis on the other hand they lineup on the spindlersquos equator independ-ently of each other
Anaphase I Anaphase I begins as homologous
chromosomes each with its two chro-matids separate and move to oppositeends of the cell This separation occursbecause the centromeres holding thesister chromatids together do not splitas they do during anaphase in mitosisThis critical step ensures that each newcell will receive only one chromosomefrom each homologous pair
Telophase IEvents occur in the reverse order
from the events of prophase I Thespindle is broken down the chromo-somes uncoil and the cytoplasmdivides to yield two new cells Each cellhas half the genetic information of theoriginal cell because it has only onechromosome from each homologouspair However another cell division isneeded because each chromosome isstill doubled
268 MENDEL AND MEIOSIS
Formulate ModelsModeling Crossing OverCrossing over occurs duringmeiosis and involves onlythe nonsister chromatidsthat are present duringtetrad formation Theprocess is responsible forthe appearance of newcombinations of alleles ingamete cells
Procedure Copy the data table Roll out four long strands of clay at least 10 cm long to
represent two chromosomes each with two chromatids Use the figure above as a guide to joining and labeling
these model chromatids Although there are four chro-matids assume that they started out as a single pair ofhomologous chromosomes prior to replication The figureshows tetrad formation during prophase I of meiosis
$ First assume that no crossing over takes place Model theappearance of the chromosomes in the four gamete cellsthat will result at the end of meiosis Record your modelrsquosappearance by drawing the gametesrsquo chromosomes andtheir genes in your data table
Next repeat steps 2ndash4 This time however assume thatcrossing over occurs between genes B and C
Analysis1 Predict What will be the appearance of the chromosomes
prior to replication2 Compare Are there any differences in the combinations
of alleles on chromosomes in gamete cells when crossingover occurs and when it does not occur
3 Analogy Crossing over has been compared to ldquoshufflingthe deckrdquo in cards Explain what this means
4 Think Critically What would be accomplished if crossingover occurred between sister chromatids Explain
5 Evaluate Does your model adequately represent crossingover in a cell
Data Table
No Crossing Over Crossing Over
Appearance of chromosomes Appearance of chromosomes
2 chromosomes with chromatids
Nonsister chromatids
Twist tie
Mark geneswith a pencilpoint
0263-0279 C10S2 BDOL-829900 8304 747 PM Page 268
The phases of meiosis IIThe newly formed cells in some
organisms undergo a short restingstage In other organisms howeverthe cells go from late anaphase ofmeiosis I directly to metaphase ofmeiosis II
The second division in meiosis issimply a mitotic division of the prod-ucts of meiosis I Meiosis II consists ofprophase II metaphase II anaphase IIand telophase II During prophase II aspindle forms in each of the two newcells and the spindle fibers attach to thechromosomes The chromosomes stillmade up of sister chromatids arepulled to the center of the cell and lineup randomly at the equator duringmetaphase II Anaphase II begins as thecentromere of each chromosome splitsallowing the sister chromatids to sepa-rate and move to opposite polesFinally nuclei re-form the spindlesbreak down and the cytoplasm dividesduring telophase II The events ofmeiosis II are identical to those youstudied for mitosis except that thechromosomes do not replicate beforethey divide at the centromeres
At the end of meiosis II four hap-loid cells have been formed from onediploid cell Each haploid cell con-tains one chromosome from eachhomologous pair These haploid cellswill become gametes transmittingthe genes they contain to offspring
Meiosis Provides forGenetic Variation
Cells that are formed by mitosis areidentical to each other and to the par-ent cell Crossing over during meiosishowever provides a way to rearrangeallele combinations Rather than thealleles from each parent stayingtogether new combinations of allelescan form Thus variability is increased
Genetic recombinationHow many different kinds of sperm
can a pea plant produce Each cellundergoing meiosis has seven pairs ofchromosomes Because each of theseven pairs of chromosomes can lineup at the cellrsquos equator in two differ-ent ways 128 different kinds of spermare possible (2n 27 128)
102 MEIOSIS 269
pro- from theGreek word promeaning ldquobeforerdquometa- from theGreek word metameaning ldquoafterrdquoana- from theGreek word anameaning ldquoupback againrdquotelo- from theGreek telosmeaning ldquoendrdquoThe four phases ofcell division areprophasemetaphaseanaphase andtelophase
Homologous chromosomesCrossing over in tetrad
Gametes
Tetrad
A A
Sister chromatids Nonsister chromatids
B B
a a
b b b
a a
B Bb b
b
AA
BB
a a
A AFigure 1013 Late in prophase I the homologouschromosomes come together to formtetrads (A) Arms of nonsister chro-matids wind around each other (B) andgenetic material may be exchanged (C)
AA BB
CC
0263-0279 C10S2 BDOL-829900 8304 747 PM Page 269
270 MENDEL AND MEIOSIS
Figure 1014 If a cell has two pairs ofchromosomesmdashA and aB and b (n 2)mdashfourkinds of gametes (22)are possible dependingon how the homologouschromosomes line up atthe equator duringmeiosis I (A) This eventis a matter of chanceWhen zygotes areformed by the union ofthese gametes 22 22
or 16 possible combina-tions may occur (B)
ab
aB
Ab
AB
AB
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
AaBB AaBb aaBB aaBb
AaBb Aabb aaBb aabb
Possible combinations ofchromosomes in zygotes (in boxes)
Poss
ible
com
bina
tion
ofch
rom
osom
es in
egg
s
Possible combination ofchromosomes in sperm
Ab aB ab
Possible gametesPossible gametes
MEIOSIS I
MEIOSIS II
Chromosome A Chromosome aChromosome B Chromosome b
In the same way any pea plant can form 128 different eggs Becauseany egg can be fertilized by anysperm the number of different possi-ble offspring is 16 384 (128 128) A simple example of how geneticrecombination occurs is shown inFigure 1014A You can see that thegene combinations in the gametesvary depending on how each pair of homologous chromosomes lines up during metaphase I a randomprocess
These numbers increase greatly asthe number of chromosomes in thespecies increases In humans n 23so the number of different kinds ofeggs or sperm a person can produce ismore than 8 million (223) When fertil-ization occurs 223 223 or 70 trilliondifferent zygotes are possible Itrsquos nowonder that each individual is unique
In addition crossing over can occuralmost anywhere at random on a chro-mosome This means that an almostendless number of different possiblechromosomes can be produced bycrossing over providing additionalvariation to the variation already pro-duced by the random assortment ofchromosomes This reassortment of
chromosomes and the genetic infor-mation they carry either by crossingover or by independent segregation ofhomologous chromosomes is calledgenetic recombination It is a majorsource of variation among organismsVariation is important to a speciesbecause it is the raw material thatforms the basis for evolution
Explain how cross-ing over increases genetic variability
Meiosis explains Mendelrsquos resultsThe behavior of the chromosomes
in meiosis provides the physical ba-sis for explaining Mendelrsquos resultsThe segregation of chromosomes inanaphase I of meiosis explains Men-delrsquos observation that each parentgives one allele for each trait at ran-dom to each offspring regardless ofwhether the allele is expressed Thesegregation of chromosomes at ran-dom during anaphase I also explainshow factors or genes for differenttraits are inherited independently ofeach other Today Mendelrsquos laws andthe events of meiosis together formthe foundation of the chromosometheory of heredity
AA BB
0263-0279 C10S2 BDOL-829900 8304 748 PM Page 270
271
NondisjunctionAlthough the events of meiosis usu-
ally proceed accurately sometimeschromosomes fail to separate correctlyThe failure of homologous chromo-somes to separate properly duringmeiosis is called nondisjunctionRecall that during meiosis I onechromosome from each homologouspair moves to each pole of the cell Innondisjunction both chromosomesof a homologous pair move to thesame pole of the cell
In one form of nondisjunction twokinds of gametes result One has anextra chromosome and the other ismissing a chromosome The effects ofnondisjunction are often seen aftergametes fuse For example when agamete with an extra chromosome isfertilized by a normal gamete thezygote will have an extra chromo-some This condition is called trisomy(TRI soh mee) In humans if a gametewith an extra chromosome number 21is fertilized by a normal gamete theresulting zygote has 47 chromosomesinstead of 46 This zygote will developinto a baby with Down syndrome
Although organisms with extrachromosomes often survive organ-isms lacking one or more chromo-somes usually do not When a gametewith a missing chromosome fuseswith a normal gamete during fertil-ization the resulting zygote lacks achromosome This condition is calledmonosomy In humans most zygoteswith monosomy do not survive If azygote with monosomy does survivethe resulting organism usually doesnot An example of monosomy that isnot lethal is Turner syndrome inwhich human females have only a single X chromosome instead of two
Another form of nondisjunctioninvolves a total lack of separation ofhomologous chromosomes When thishappens a gamete inherits a completediploid set of chromosomes like thoseshown in Figure 1015 When agamete with an extra set of chromo-somes is fertilized by a normal haploidgamete the offspring has three sets ofchromosomes and is triploid Thefusion of two gametes each with anextra set of chromosomes producesoffspring with four sets of chromo-somesmdasha tetraploid
Male parent (2n)
Meiosis
Abnormalgamete (2n)
Nondisjunction
Zygote(4n)
Female parent (2n)
Meiosis
Abnormalgamete (2n)
Nondisjunction
Figure 1015 Follow the steps to seehow a tetraploid plantsuch as this chrysanthe-mum is produced
RB Satterthwaite
0263-0279 C10S2 BDOL-829900 8304 749 PM Page 271
Chromosome MappingFigure 1016Crossing over the exchange of genetic material by nonsisterchromatids provides information that can be used to make chro-mosome maps Crossing over occurs more frequently betweengenes that are far apart on a chromosome than between genesthat are closer together Critical Thinking Why is the fre-quency of crossing over related to the distance betweengenes on a chromosome
272 MENDEL AND MEIOSISB JohnCabiscoVisuals Unlimited
AA BB50
AA DD BB CC10 5
DD AA CC BB510
or or
CC DD
DD CC35
35
or
AA BBCCDD
50
10 35 5
Crossing over In prophase I ofmeiosis nonsister chromatids crossover as shown in the photo aboveEach X-shaped region is a crossover
AA
Mapping Crossing over produces new allele combinationsGeneticists use the frequency of crossing over to map the relativepositions of genes on a chromosome Genes that are farther aparton a chromosome are more likely to have crossing over occurbetween them than are genes that are closer together
BB
Frequencies and distance Suppose there are four genesmdashA B C and Dmdashona chromosome Geneticists determine that the frequencies of recombination amongthem are as follows between A and Bmdash50 between A and Dmdash10 between Band Cmdash5 between C and Dmdash35 The recombination frequencies can beconverted to map units AndashB 50 AndashD 10 BndashC 5 CndashD 35 These map unitsare not actual distances on the chromosome but they give relative distancesbetween genes Geneticists line up the genes as shown above
CC
Making the mapThe genes can bearranged in thesequence that reflectsthe recombinationdata This sequence is a chromosome map
DD
Stained TEM Magnification 1905
0263-0279 C10S2 BDOL-829900 8304 749 PM Page 272
Understanding Main Ideas1 How are the cells at the end of meiosis different
from the cells at the beginning of meiosis Usethe terms chromosome number haploid anddiploid in your answer
2 What is the significance of meiosis to sexualreproduction
3 Why are there so many varied phenotypes withina species such as humans
4 If the diploid number of a plant is 10 how manychromosomes would you expect to find in itstriploid offspring
Thinking Critically5 How do the events that take place during
meiosis explain Mendelrsquos law of independentassortment
6 Get the Big Picture Compare Figures 1012 and813 of meiosis and mitosis Explain why crossingover between nonsister chromatids of homolo-gous chromosomes cannot occur during mitosisFor more help refer to Get the Big Picture in theSkill Handbook
SKILL REVIEWSKILL REVIEW
102 MEIOSIS 273Bildarchiv OkapiPhoto Researchers
PolyploidyOrganisms with more than the usual
number of chromosome sets are calledpolyploids Polyploidy is rare in animalsand almost always causes death of thezygote However polyploidy frequentlyoccurs in plants Often the flowers andfruits of these plants are larger thannormal and the plants are healthierMany polyploid plants such as the ster-ile banana plant shown in Figure 1017are of great commercial value
Meiosis is a complex process andthe results of an error occurring aresometimes unfortunate However theresulting changes can be beneficialsuch as those that have occurred inagriculture Hexaploid (6n) wheattriploid (3n) apples and polyploidchrysanthemums all are availablecommercially You can see that a thor-ough understanding of meiosis andgenetics would be very helpful toplant breeders In fact plant breedershave learned to produce polyploidplants artificially by using chemicalsthat cause nondisjunction
Gene Linkage and MapsGenes sometimes appear to be
inherited together instead of inde-pendently If genes are close togetheron the same chromosome they usually
are inherited together These genesare said to be linked In fact all thegenes on a chromosome usually arelinked and inherited together It is thechromosomes rather than the individ-ual genes that follow Mendelrsquos law ofindependent assortment
Linked genes may become sepa-rated on different homologous chro-mosomes as a result of crossing overWhen crossing over produces newgene combinations geneticists canuse the frequencies of these new genecombinations to make a chromosomemap showing the relative locations ofthe genes Figure 1016 illustratesthis process
Figure 1017The banana plant is anexample of a triploidplant Think CriticallyWhy do you think thebanana plant is sterile
ncbdolglencoecomself_check_quiz
0263-0279 C10S2 BDOL-829900 8304 750 PM Page 273
Before YouBegin
Itrsquos difficult to predict thetraits of plants if all thatyou see is their seeds Butif these seeds are plantedand allowed to grow cer-tain traits will appear Byobserving these traits youmight be able to deter-mine the possible pheno-types and genotypes ofthe parent plants that pro-duced these seeds In thislab you will determinethe genotypes of plantsthat grow from twogroups of tobacco seedsEach group of seeds camefrom different parentsPlants will be either greenor albino (white) in colorUse the following geno-types for this cross CC green Cc greenand cc albino
How can phenotypes and genotypes of plants be determined
ProblemCan the phenotypes and genotypes of the parent plants thatproduced two groups of seeds be determined from the phe-notypes of the plants grown from the seeds
HypothesesHave your group agree on a hypothesis to be tested that willanswer the problem question Record your hypothesis
ObjectivesIn this BioLab you will Analyze the results of growing two groups of seeds Draw conclusions about phenotypes and genotypes based
on those results Use the Internet to collect and compare data from other
students
Possible Materialspotting soil light sourcesmall flowerpots or seedling thermometer or
flats temperature probetwo groups of tobacco seeds plant-watering bottlehand lens
Safety PrecautionsCAUTION Always wash your hands after handling plantmaterials Always wear goggles in the lab
Skill HandbookIf you need help with this lab refer to the Skill Handbook
1 Examine the materials provided by your teacher As agroup make a list of the possible ways you might testyour hypothesis
2 Agree on one way that your group could investigate yourhypothesis
PLAN THE EXPERIMENTPLAN THE EXPERIMENT
PREPARATIONPREPARATION
Matt Meadows
274 MENDEL AND MEIOSIS
0263-0279 C10S2 BDOL-829900 8304 830 PM Page 274
102 MEIOSIS 275
ANALYZE AND CONCLUDEANALYZE AND CONCLUDE
1 Think Critically Why was it necessary to grow plants from the seeds inorder to determine the phenotypes of the plants that formed the seeds
2 Draw Conclusions Using the information in the introduction describehow the gene for green color (C) is inherited
3 Make Inferences For the group of seeds that yielded all green plants areyou able to determine exactly the genotypes of the parents that formed theseseeds Can you determine the genotype of each plant observed Explain
4 Make Inferences For the group of seedsthat yielded some green and some albinoplants are you able to determine exactlythe genotypes of the plants that formedthese seeds Can you determine the geno-type of each plant observed Explain
5 Use the data posted onto
compare your experimental design withthat of other students Were your resultssimilar What might account for the differences
ERROR ANALYSIS
Find this BioLab using the link below andpost your results in the table providedBriefly describe your experimental design
3 Design an experiment that will allow you to collect quanti-tative data For example how many plants do you thinkyou will need to examine
4 Prepare a numbered list of directions Include a listof materials and the quantities you will need
5 Make a data table for recording your observations
Check the Plan1 Carefully determine what data you are going
to collect How many seeds will you need Howlong 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 plantsin each group and complete your data table
5 Visit to post your data6 Make wise choices in the disposal of
materialsCLEANUP AND DISPOSAL
Ray PfortnerPeter Arnold Inc
ncbdolglencoecominternet_lab
ncbdolglencoecominternet_lab ncbdolglencoecominternet_lab
0263-0279 C10S2 BDOL-829900 8504 948 AM Page 275
276 MENDEL AND MEIOSIS
A Solution from Ratios
In 1866 Gregor Mendel an Austrian monkpublished the results of eight years of experi-
ments with garden peas His work was ignoreduntil 1900 when it was rediscovered
Mendel had three qualities that led to his dis-covery of the laws of heredity First he was curi-ous impelled to find out why things happenedSecond he was a keen observer Third he was askilled mathematician Mendel was the first biol-ogist who relied heavily on statistics for solu-tions to how traits are inherited
Darwin missed his chance About the sametime that Mendel was carrying out his experi-ments with pea plants Charles Darwin was gath-ering data on snapdragon flowers When Darwincrossed plants that had normal-shaped flowerswith plants that had odd-shaped flowers all theoffspring had normal-shaped flowers He thoughtthe two traits had blended When he allowed theF1 plants to self-pollinate his results were 88plants with normal-shaped flowers and 37 plantswith odd-shaped flowers Darwin was puzzled bythe results and did not continue his studies withthese plants Lacking Mendelrsquos statistical skillsDarwin failed to see the significance of the ratioof normal-shaped flowers to odd-shaped flowersin the F2 generation What was this ratio Was itsimilar to Mendelrsquos ratio of dominant to recessivetraits in pea plants
Finding the ratios for four other traitsFigure 103 on page 256 shows seven traits thatMendel studied in pea plants You have alreadylooked at Mendelrsquos data for plant height andseed shape Now use the data for seed colorflower position pod color and pod shape to findthe ratios of dominant to recessive for thesetraits in the F2 generation
Draw Table B in your notebook or journalCalculate the ratios for the data in Table A andcomplete Table B by following these steps
bull Step 1 Divide the larger number by the smaller number
bull Step 2 Round to the nearest hundredthbull Step 3 To express your answer as a ratio
write the number from step 2 followed by a colon and the number 1
RW VanNormanVisuals Unlimited
Think Critically Why are ratios so important inunderstanding how dominant and recessive traitsare inherited
To find out more about Mendelrsquos work visit ncbdolglencoecommath
Table A Mendelrsquos Results
Seed Flower Pod PodColor Position Color Shape
Yellow Axial Green Inflated6022 651 428 882
Green Terminal Yellow Constricted2001 207 152 299
Table B Calculating Ratios for Mendelrsquos Results
Seed Flower Pod PodColor Position Color Shape
Calculation 6022 300
2001
Ratio 31yellow green
0263-0279 C10S2 BDOL-829900 8304 815 PM Page 276
Section 101STUDY GUIDESTUDY GUIDE
CHAPTER 10 ASSESSMENT 277
Section 102
To help you reviewMendelrsquos work use the OrganizationalStudy Fold on page 253
Key Concepts Genes are located on chromosomes and
exist in alternative forms called alleles Adominant allele can mask the expression ofa recessive allele
When Mendel crossed pea plants differingin one trait one form of the trait disap-peared until the second generation of off-spring To explain his results Mendelformulated the law of segregation
Mendel formulated the law of independentassortment to explain that two traits areinherited independently
Events in genetics are governed by thelaws of probability
Vocabularyallele (p 256)dominant (p 256)fertilization (p 253)gamete (p 253)genetics (p 253)genotype (p 258)heredity (p 253)heterozygous (p 259)homozygous (p 258)hybrid (p 255)law of independent
assortment (p 260)law of segregation
(p 257)phenotype (p 258)pollination (p 254)recessive (p 256)trait (p 253)zygote (p 253)
Mendelrsquos Lawsof Heredity
Key Concepts In meiosis one diploid (2n) cell produces
four haploid (n) cells providing a way foroffspring to have the same number ofchromosomes as their parents
In prophase I of meiosis homologous chro-mosomes come together and pair tightlyExchange of genetic material called cross-ing over takes place
Mendelrsquos results can be explained by the dis-tribution of chromosomes during meiosis
Random assortment and crossing over dur-ing meiosis provide for genetic variationamong the members of a species
The outcome of meiosis may vary due tonondisjunction the failure of chromosomesto separate properly during cell division
All the genes on a chromosome are linkedand are inherited together It is the chro-mosomes rather than the individual genesthat are assorted independently
Vocabularycrossing over (p 266)diploid (p 263)egg (p 265)genetic recombination
(p 270)haploid (p 263)homologous chromo-
some (p 264)meiosis (p 265)nondisjunction (p 271)sexual reproduction
(p 266)sperm (p 265)
Meiosis
ncbdolglencoecomvocabulary_puzzlemaker
0263-0279 C10S2 BDOL-829900 8304 813 PM Page 277
Review the Chapter 10 vocabulary words listed inthe Study Guide on page 277 For each set ofvocabulary words choose the one that does notbelong Explain why it does not belong
1 eggmdashspermmdashzygote2 homozygousmdashhybridmdashheterozygous3 phenotypemdashgenotypemdashallele4 nondisjunctionmdashgenetic recombinationmdash
crossing over5 zygotemdashdiploidmdashgamete
6 At the end of meiosis how many haploidcells have been formed from the originalcellA one C threeB two D four
7 When Mendel transferred pollen from onepea plant to another he was ________ theplantsA self-pollinating C self-fertilizingB cross-pollinating D cross-fertilizing
8 Which of these does NOT show a recessivetrait in garden peasA B C D
9 During what phase of meiosis do sister chromatids separateA prophase I C anaphase IIB telophase I D telophase II
10 During what phase of meiosis do nonsisterchromatids cross overA prophase I C telophase IB anaphase I D telophase II
11 A dihybrid cross between two heterozygotesproduces a phenotypic ratio of ________A 31 C 9331B 121 D 169
12 Open Ended On the average each humanhas about six recessive alleles that would belethal if expressed Why do you think thathuman cultures have laws against marriagebetween close relatives
13 Open Ended How does separation of homol-ogous chromosomes during anaphase I ofmeiosis increase variation among offspring
14 Open Ended Relating to the methods ofscience why do you think it was importantfor Mendel to study only one trait at a timeduring his experiments
15 Open Ended Explain why sexual reproduc-tion is an advantage to a population thatlives in a rapidly changing environment
16 Observe and Infer Why is it possible tohave a family of six girls and no boys butextremely unlikely that there will be a publicschool with 500 girls and no boys
17 Recognize Cause and Effect Why is itsometimes impossible to determine the genotype of an organism that has a domi-nant phenotype
18 Observe and Infer While examining a cellin prophase I of meiosis you observe a pairof homologous chromosomes pairing tightlyWhat is the significance of the places atwhich the chromosomes are joined
19 Several humangenetic disorders result from nondisjunctionin meiosis including Down syndromeKleinfelterrsquos syndrome and Turner syn-drome Visit to inves-tigate these disorders What characteristic iscommon to each Choose one of these dis-orders or another human disorder causedby nondisjunction and prepare a visual dis-play that explains the disorder Explain thedisorder to your class
REAL WORLD BIOCHALLENGE
278 CHAPTER 10 ASSESSMENT
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Multiple ChoiceUse the diagram to answer questions 20ndash23
20 Which of the following is trueA Individual 1 is heterozygousB Individuals 2 and 3 are homozygousC Individual 4 is recessiveD All individuals will be male
21 Which of the following has the Tt genotypeA 1 C 3B 2 D 2 and 3
22 If T is the allele for purple flowers and t isthe allele for white flowers the resultswould be ________A 3 out of 4 are purpleB 3 out of 4 are whiteC equal numbers of white and purpleD all of the same color
23 Which of Mendelrsquos observations woulddescribe the results of the experimental cross in question 22A rule of dominanceB law of segregationC law of independent assortmentD rule of unit factors
24 Recessive traits appear only when an organism is ________A matureB different from its parentsC heterozygousD homozygous
25 The stage of meiosis shown here is ________A anaphase IB metaphase IIC telophase ID telophase II
Study the diagram and answer questions 26ndash28
26 What name is given to the process shownaboveA fertilization C meiosisB zygote D gametes
27 What name is given to the cells shown inthe diagram aboveA fertilization C meiosisB zygotes D gametes
28 If each of the cells shown in the diagram has16 chromosomes how many chromosomeswould you expect to find in a skin cell of theresulting organismA 16 C 32B 64 D 8
T
T 1 2
43t
t
CHAPTER 10 ASSESSMENT 279
Constructed ResponseGrid InRecord your answers on your answer document
29 Open Ended Explain the difference between trisomy and triploidy Describe a way that eachcondition could occur Use diagrams to clarify your answer
30 Open Ended Compare metaphase of mitosis with metaphase I of meiosis Explain the significance ofthe differences between the two stages in terms of sexual reproduction and genetic variation
The assessed North Carolina objective appears next to the question
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