chapter 10: mendel and meiosis - polson...
TRANSCRIPT
GeneticsGeneticsPhysical traits, such as the colors of thesesnapdragons, are encoded in small segmentsof a chromosome called genes, which arepassed from one generation to the next. Bystudying the inheritance pattern of a traitthrough several generations, the probabilitythat future offspring will express that traitcan be predicted.
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UNIT CONTENTSUNIT CONTENTS
Mendel and Meiosis
DNA and Genes
Patterns of Heredity andHuman Genetics
Genetic Technology
Unit 4Unit 4
GeneticsBIODIGESTBIODIGEST
UNIT PROJECTUNIT PROJECT
Use the Glencoe Science Web site for more project
activities that are connected to this unit.science.glencoe.com
BIOLOGY
258 MENDEL AND MEIOSIS
Mendel and Meiosis
What You’ll Learn■ You will identify the basic
concepts of genetics.■ You will examine the process
of meiosis.
Why It’s ImportantGenetics explains why you haveinherited certain traits fromyour parents. If you understandhow meiosis occurs, you can seehow these traits were passedon to you.
Look over the chapter, andmake a list of severalvocabulary words that arefamiliar. Review the list. Foreach word, think of whereand how you may haveheard the word usedbefore. As you read thetext, note if the words areused differently in a sciencecontext than elsewhere.
To find out more aboutgenetics, visit the GlencoeScience Web site.science.glencoe.com
READING BIOLOGYREADING BIOLOGY
10ChapterChapter
Organisms usuallyresemble their parentsbecause they inheritcertain characteristicsfrom them. Thesecharacteristics, alsocalled traits, are de-termined by geneticinformation on chromo-somes such as thoseshown in the inset photo.
BIOLOGY
Section
Why Mendel SucceededGregor Mendel carried out the
first important studies of heredity,the passing on of characteristics from parents to offspring. Althoughpeople had noticed for thousands ofyears that family resemblances wereinherited from generation to genera-tion, a complete explanation requiredthe careful study of genetics—the branch of biology that studiesheredity. Characteristics that areinherited are called traits. Mendelwas the first person to succeed inpredicting how traits would be trans-ferred from one generation to thenext. How was he able to solve thisproblem of heredity?
Mendel chose his subject carefully
Mendel studied many plantsbefore deciding to use the garden peain his experiments. Garden pea plantsreproduce sexually, which means theyhave two distinct sex cells—male andfemale. Sex cells are called gametes.
In peas, both male and femalegametes are in the same flower. Themale gamete is in the pollen grain,which is produced by the anther. Thefemale gamete is in the ovule, which islocated in the pistil. The transfer ofthe male pollen grains to the pistil of aflower is called pollination. The unit-ing of male and female gametes, in aprocess called fertilization, occurs
10.1 MENDEL’S LAWS OF HEREDITY 259
An Austrian monastery in the mid-nineteenth century might seem an
unusual place to begin your searchfor the answer to why offspringresemble their parents. Yet, it wasin this community of scholars thatyoung Gregor Mendelbegan to breed gardenpea plants so that hecould study theinheritance of theircharacteristics.
SECTION PREVIEW
ObjectivesAnalyze the resultsobtained by GregorMendel in his experi-ments with gardenpeas.Predict the possible offspring of a geneticcross by using a Punnettsquare.
Vocabularyhereditygeneticstraitgametepollinationfertilizationhybridalleledominantrecessivelaw of segregationphenotypegenotypehomozygousheterozygouslaw of independent
assortment
10.1 Mendel’s Laws of Heredity
OriginWORDWORD
heredity From the Latinword hered-, mean-ing “heir.” Hereditydescribes thegenetic qualitiesyou receive fromyour ancestors.
Gregor Mendeland pea plants
when the male gamete in the pollengrain meets and fuses with the femalegamete in the ovule. After the ovuleis fertilized, it matures into a seed.
The reproductive parts of the peaflower are tightly enclosed in petals,preventing the pollen of other flow-ers from entering. As a result, peasnormally reproduce by self-pollina-tion; that is, the male and femalegametes come from the same plant.In many of Mendel’s experiments,this is exactly what he wanted. Whenhe needed to breed—or cross—oneplant with another, Mendel openedthe petals and removed the anthersfrom a flower, Figure 10.1a. He thendusted the pistil with pollen from theplant he wished to cross it with,Figure 10.1b, and covered the flowerwith a small bag to prevent pollen inthe air from landing on the pistil.This process is called cross-pollina-tion. By using this technique, Mendelcould be sure of the parents in hiscross. You can observe anthers andtheir pollen grains in the MiniLab onthis page.
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
260 MENDEL AND MEIOSIS
Looking at Pollen Pollengrains are formed within themale anthers of flowers. Whatis their role? Pollen containsthe male gametes or spermcells needed for fertilization.This means that pollen grainscarry the hereditary unitsfrom male parent plants tofemale parent plants. Thepollen grains that Mendeltransferred from the antherof one pea plant to the 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 sta-mens in each flower.
@ Remove one stamen and locate the enlarged end—theanther.
# 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. Provide an estimate of the number of pollen grains pres-
ent in an anther.2. Describe the appearance of a single pollen grain.3. Explain the role of pollen grains in plant reproduction.
MiniLab 10-1MiniLab 10-1 Observing and Inferring
Removeanthers
(male part)
TransferPollenPistil
(female part)
a b
Anthers
Cross-pollination
Pollengrains
Figure 10.1 In his experiments,Mendel often had totransfer pollen fromone plant to anotherplant with differenttraits. This is calledmaking a cross.
Pistil
Anther
Stamens
he worked with were from popula-tions of plants that had been tall formany generations and had always pro-duced tall offspring. Such plants aresaid to be true breeding for tallness.Likewise, the short plants he workedwith were true breeding for shortness.
Mendel’s MonohybridCrosses
What did Mendel do with the talland short pea plants he so carefullyselected? He crossed them to producenew plants. Mendel referred to theoffspring of this cross as hybrids. Ahybrid is the offspring of parents thathave different forms of a trait, such astall and short height. Mendel’s firstexperiments are called monohybridcrosses because mono means “one”and the two parent plants differed bya single trait—height.
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 a short pea plant thatwas less than two feet tall and whichcame from a population of pea plantsthat were all short. When he plantedthe seeds from this cross, he foundthat all of the offspring grew to be astall as the taller parent. In this firstgeneration, it was as if the shorterparent 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 genera-tion. Mendel found that three-fourths of the plants were as tall asthe tall plants in the parent and firstgenerations. He also found that one-
10.1 MENDEL’S LAWS OF HEREDITY 261
Figure 10.2 When Mendel crossed true-breeding tallpea plants with true-breeding short peaplants, all the offspring were tall. When heallowed first-generation tall plants to self-pollinate, three-fourths of the offspringwere tall and one-fourth were short.
fourth of the offspring were as shortas the short plants in the parent gen-eration. In other words, in the secondgeneration, tall and short plantsoccurred in a ratio of approximatelythree tall plants to one short plant,Figure 10.2. The short trait hadreappeared as if from nowhere!
The original parents, the true-breeding tall and short plants, areknown as the P1 generation. The Pstands for "parent." The offspring ofthe parent plants are known as the F1
Short pea plant
All tall pea plants
3 tall :1 short
Tall pea plant
F2
F1
P1
round yellow purpleaxial(side) green inflated tall
wrinkled green whiteterminal
(tips) yellow constricted short
Seedshape
Dominanttrait
Recessivetrait
Seedcolor
Flowercolor
Flowerposition
Podcolor
Podshape
Plantheight
generation. The F stands for “filial”—son or daughter. When you cross twoF1 plants with each other, their off-spring are called the F2 generation—the second filial generation. Youmight find it easier to understandthese terms if you look at your ownfamily. Your parents are the P1 gener-ation. You are the F1 generation, andany children you might have in thefuture would be the F2 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 10.3. In every case, hefound that one trait of a pair seemedto disappear in the F1 generation,only 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 that theyare located on chromosomes. Genesexist in alternative forms. We callthese different gene forms alleles (uh LEELZ). For example, each ofMendel’s pea plants had two alleles ofthe gene that determined its height.A plant could have two alleles fortallness, two alleles for shortness, orone allele for tallness and one forshortness. An organism’s two allelesare located on different copies of achromosome—one inherited fromthe female parent and one from themale parent.
The rule of dominance Remember what happened when
Mendel crossed a tall P1 plant with ashort P1 plant? The F1 offspringwere all tall. In other words, only onetrait was observed. In such crosses,Mendel called the observed traitdominant and the trait that disap-peared recessive. We now know that
262 MENDEL AND MEIOSIS
OriginWORDWORD
allele From the Greekword allelon, mean-ing "of each other."Genes exist in alter-native forms calledalleles.
Figure 10.3 Mendel chose seventraits of peas for hisexperiments. Eachtrait had two clearlydifferent forms; nointermediate formswere observed.
in Mendel’s pea plants, the allele fortall plants is dominant to the allelefor short plants. Plants that had oneallele for tallness and one for short-ness were tall because the allele fortallness is dominant to the allele forshortness. Expressed another way,the allele for short plants is recessiveto the allele for tall plants. Pea plantsthat had two alleles for tallness weretall, and those that had two alleles forshortness were short. You can see inFigure 10.4 how the rule of domi-nance explained the resulting F1 gen-eration.
When recording the results ofcrosses, it is customary to use thesame letter for different alleles of thesame gene. An uppercase letter isused for the dominant allele, and alowercase letter for the recessiveallele. The dominant allele is alwayswritten first. So the allele for tallnessis written as T, and the allele forshortness as t, as it is in Figure 10.4.
The law of segregationNow recall the results of Mendel’s
cross between F1 tall plants, when thetrait 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. It received the allele fortallness from its tall parent and theallele for shortness from its shortparent in the P1 generation. Becauseeach F1 plant has two different al-leles, it can produce two differenttypes of gametes— “tall” gametesand “short” gametes. During fertil-ization, these gametes randomly pairto produce four combinations of alleles. This conclusion, illustrated inFigure 10.5 on the next page, iscalled the law of segregation.
Figure 10.4 The rule of dominance explains the resultsof Mendel’s cross between P1 tall and shortplants (a). Tall pea plants are about six feettall, whereas short plants are less than twofeet tall (b).
Tall plant
a
All tall plantsT t
Short plantT T t
tT
t
F1
b
Phenotypes andGenotypes
Mendel showed that tall plants arenot all the same. Some tall plants,when crossed with each other, yieldedonly tall offspring. These wereMendel’s original P1 true-breedingtall plants. Other tall plants, whencrossed with each other, yielded bothtall and short offspring. These werethe F1 tall plants in Figure 10.5 thatcame from a cross between a tallplant and a short plant.
Two organisms, therefore, canlook alike but have different underly-ing gene combinations. The way anorganism looks and behaves is calledits phenotype (FEE nuh tipe). Thephenotype of a tall plant is tall,whether it is TT or Tt. The gene
combination an organism contains isknown as its genotype (JEE nuh 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 you can’t alwaysknow an organism’s genotype simplyby looking at its phenotype. Anorganism is homozygous (hoh muhZI gus) for a trait if its two alleles forthe trait are the same. The true-breeding tall plant that had two al-leles for tallness (TT) would behomozygous for the trait of height.Because tallness is dominant, a TTindividual is homozygous dominantfor that trait. A short plant wouldalways have two alleles for shortness(tt). It would, therefore, always be
264 MENDEL AND MEIOSIS
OriginWORDWORD
phenotype From the Greekwords phainein,meaning “to show,”and typos, meaning“model.” The visi-ble characteristics ofan organism makeup its phenotype.
genotype From the Greekwords gen or geno,meaning “race,”and typos, meaning“model.” Thegenetic characteris-tics of an organismmake up its geno-type.
Tall plant
3 1
TallT T
TallT t
TallT t
Shortt t
Tall plant
F1
F2
Law of segregation Tt ×× Tt cross
T t T t
Figure 10.5 Mendel’s law of segrega-tion explains the results ofhis cross between F1 tallplants. He concluded thatthe two alleles for eachtrait must separate whengametes are formed. Aparent, therefore, passeson at random only oneallele for each trait to eachoffspring.
P1
F1
F2
Round yellow Wrinkled green
All roundyellow
9Roundyellow
3Roundgreen
3Wrinkledyellow
1Wrinkled
green
Dihybrid cross round yellow ×× wrinkled green
homozygous recessive for the trait ofheight.
An organism is heterozygous (hetuh 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 10.5 again.Can you identify the phenotype andgenotype of each plant? Is eachhomozygous or heterozygous? Youcan practice determining genotypesand phenotypes in the BioLab at theend of this chapter.
Mendel’s DihybridCrosses
Mendel performed another set ofcrosses in which he used peas thatdiffered from each other in two traitsrather than only one. Such a crossinvolving two different traits is calleda dihybrid cross because di means“two.” 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,when he crossed plants that producedyellow seeds with plants that pro-duced green seeds, all the plants
in the F1 generation produced yel-low seeds—yellow was dominant.Therefore, Mendel was not surprisedwhen he found that the F1 plants ofhis dihybrid cross all had the twodominant traits of round and yellowseeds, as Figure 10.6 shows.
The second generationMendel then let the F1 plants pol-
linate themselves. As you mightexpect, he found some plants thatproduced round yellow seeds andothers that produced wrinkled greenseeds. But that’s not all. He alsofound some plants with round greenseeds and others with wrinkled yel-low seeds. When Mendel sorted andcounted the plants of the F2 genera-tion, he found they appeared in a def-inite ratio of phenotypes—9 roundyellow: 3 round green: 3 wrinkledyellow: 1 wrinkled green. To explainthe results of this dihybrid cross,Mendel formulated his second law.
10.1 MENDEL’S LAWS OF HEREDITY 265
OriginWORDWORD
heterozygous From the Greekwords heteros,meaning “other,”and zygotos, mean-ing “joinedtogether.” A trait isheterozygous whenan individual hastwo different allelesfor that trait.
Figure 10.6 When Mendel crossed true-breeding plants with round yellowseeds and true-breeding plants with wrinkled green seeds, theseeds of all the offspring were round and yellow. When the F1plants were allowed to self-pollinate, they produced four different kinds of plants in the F2 generation.
Heterozygoustall parent
Heterozygoustall parent
T
T
T t
tt
T t
The law of independent assortment
Mendel’s second law states thatgenes for different traits—for exam-ple, seed shape and seed color—areinherited independently of eachother. This conclusion is known asthe law of independent assortment.When 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 ofindependent assortment), and viceversa. These alleles can then recom-
bine in four different ways. If thealleles for seed shape and color wereinherited together, only two kinds ofpea seeds would have been produced:round yellow and wrinkled green.
Punnett SquaresIn 1905, Reginald Punnett, an
English biologist, devised a short-hand way of finding the expectedproportions of possible genotypes inthe offspring of a cross. This methodis called a Punnett square. It takesaccount of the fact that fertilizationoccurs at random, as Mendel’s 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 ofeach parent would contain the Tallele, and the other half would con-tain the t allele. A Punnett square forthis cross is two boxes tall and twoboxes wide because each parent canproduce two kinds of gametes for thistrait. The two kinds of gametes fromone parent are listed on top of thesquare, and the two kinds of gametesfrom the other parent are listed onthe left side, Figure 10.7A. It doesn’tmatter which set of gametes is on topand which is on the side, that is,which parent contributes the T andwhich contributes the t. Refer to thePunnett square in Figure 10.7B todetermine the possible genotypes ofthe offspring. Each box is filled inwith the gametes above and to theleft side of that box. You can see thateach box then contains two alleles—one possible genotype.
After the genotypes have beendetermined, you can determine the
266 MENDEL AND MEIOSIS
Figure 10.7 This Punnett square predicts the resultsof a monohybrid cross between twoheterozygous tall pea plants.
T
TT Tt
Tt
T
t
t tt
You can see thatthere are three different possiblegenotypes—TT, Tt,and tt—and that Tt can result fromtwo different combinations.
BB
The gametes thateach parent formsare listed on thetop and left sideof the Punnettsquare.
AA
phenotypes. Looking at the Punnettsquare for this cross in Figure 10.7B,you can see that three-fourths of theoffspring are expected to be tallbecause they have at least one domi-nant allele. One-fourth are expectedto be short because they lack a domi-nant allele. Of the tall offspring, one-third will be homozygous dominant(TT) and two-thirds will be heterozy-gous (Tt). Note that whereas thegenotype ratio is 1TT: 2Tt: 1tt, thephenotype ratio is 3 tall: 1 short. Youcan practice doing calculations suchas Mendel did in the Math Connectionat the end of this chapter.
Dihybrid crossesWhat happens in a Punnett square
when two traits are considered?Think again about Mendel’s crossbetween pea plants with round yel-low seeds and plants with wrinkledgreen seeds. All the F1 plants pro-duced seeds that were round and yel-low and were heterozygous for eachtrait (RrYy). What kind of gameteswill these F1 plants form?
Mendel explained that the traitsfor seed shape and seed color wouldbe inherited 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 10.8 shows.
ProbabilityPunnett squares are good for
showing all the possible combina-tions of gametes and the likelihoodthat each will occur. In reality, how-ever, you don’t get the exact ratio of
results shown in the square. That’sbecause, in some ways, genetics islike flipping a coin—it follows therules 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. So the probability of gettingheads when you toss a coin would beone in two chances, written as 1: 2 or1/2. A Punnett square can be used todetermine the probability of getting apea plant that produces round seedswhen two plants that are heterozygous(Rr) are crossed. Because this Punnett
10.1 MENDEL’S LAWS OF HEREDITY 267
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
Punnett Square of Dihybrid Cross
Gam
etes
from
RrY
y pa
rent
round yellow
round green
wrinkled yellow
wrinkled green
F1 cross: RrYy × RrYyFigure 10.8 A Punnett square for a dihybrid crossbetween heterozygous pea plants withround yellow seeds shows clearly thatthe offspring fulfill Mendel’s observedratio of 9 round yellow: 3 round green: 3wrinkled yellow: 1 wrinkled green.
How did Mendel analyze his data?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 3: 1 ratio of round-seeded plants to wrinkled-seeded plants in the F2 generation.
AnalysisMendel’s actual results
in the F2 generation are shown to the right.1. Calculate the actual
ratio of round-seeded plants to wrinkled-seeded plants. To do this, divide the number of round-seeded plants by the number of wrinkled-seeded plants (round to the nearest hundredth). Your answer tells you how manymore 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. What was the actual ratio Mendel observed for this cross? 2. How does Mendel’s observed ratio compare with the
expected 3: 1 ratio? 3. Why was the actual ratio different from the expected
ratio?
Problem-Solving Lab 10-1Problem-Solving Lab 10-1 AnalyzingInformation
square shows three plants with roundseeds out of four total plants, theprobability is 3/4, as Figure 10.9shows. Yet, if you calculate the frac-tion of round-seeded plants fromMendel’s actual data in the Problem-Solving Lab on this page, you will seethat slightly less than three-fourths ofthe plants were round-seeded. It isimportant to remember that theresults predicted by probability aremore likely to be seen when there is alarge number of offspring.
268 MENDEL AND MEIOSIS
Section AssessmentSection AssessmentUnderstanding Main Ideas1. What structural features of pea plant flowers
made them suitable for Mendel’s genetic studies?2. What are the genotypes of a homozygous and a
heterozygous tall pea plant?3. One parent is homozygous tall and the other
parent is heterozygous tall. Make a Punnettsquare to determine what fraction of their offspring is expected to 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
dominant to the allele for green peas. Suppose
you have a plant that produces yellow peas, but you don’t know whether it is homozygousdominant or heterozygous. What experimentcould you do to find out? Draw a Punnett squareto help you.
6. Observing and Inferring The offspring of across between a purple-flowered plant and awhite-flowered plant are 23 plants with purpleflowers and 26 plants with white flowers. Usethe letter P for purple and p for white. What arethe genotypes of the parent plants? Explain yourreasoning. For more help, refer to ThinkingCritically in the Skill Handbook.
SKILL REVIEWSKILL REVIEW
RR
R
R
RR
Rr
Rr
rr
r
r
Figure 10.9The probability that the offspring from amating of two heterozygotes will show adominant phenotype is 3 out of 4, or 3/4.
Kind of Plants
Number of Plants
Round-seeded
Wrinkled-seeded
5474
1850
Mendel’s results
rr
Section
10.2 MEIOSIS 269
Genes, Chromosomes,and Numbers
Organisms have tens of thousandsof genes that determine individualtraits. Genes do not exist free in thenucleus of a cell; they are lined up onchromosomes. Typically, a chromo-some can contain a thousand ormore genes.
Diploid and haploid cellsIf you examined the nucleus in a
cell of one of Mendel’s pea plants,you would find it had 14 chromo-somes—seven pairs. In the body cellsof animals and most plants, chromo-
somes occur in pairs. One chromo-some in each pair came from themale parent, and the other camefrom the female parent. A cell withtwo of each kind of chromosome iscalled a diploid cell and is said tocontain a diploid, or 2n, number ofchromosomes. This pairing supportsMendel’s conclusion that organismshave two factors—alleles—for eachtrait. One allele is located on each ofthe paired chromosomes.
Organisms produce gametes thatcontain one of each kind of chromo-some. A cell with one of each kind ofchromosome is called a haploid celland is said to contain a haploid, or n,
Mendel’s study of inheritancewas based on careful obser-vations of pea plants, but
pieces of the hereditary puzzle werestill missing. Modern technologiessuch as high-power microscopes allowus a glimpse of things that Mendelcould only imagine. You can now lookinside a cell to see the chromosomes onwhich the traits described by Mendelare carried. You can also examinethe process by which these traitsare transmitted to the nextgeneration.
SECTION PREVIEW
ObjectivesAnalyze how meiosismaintains a constantnumber of chromo-somes within a species.Infer how meiosis leadsto variation in a species.Relate Mendel’s laws ofheredity to the eventsof meiosis.
Vocabularydiploidhaploidhomologous
chromosomemeiosisspermeggzygotesexual reproductioncrossing overgenetic recombinationnondisjunction
10.2 Meiosis
Metaphase chromosomes(top) and a plant cell inearly anaphase of meiosis
Magnification:1100�
Magnification: 2300�
number of chromosomes. This factsupports Mendel’s conclusion thatparent organisms give one factor, orallele, for each trait to each of theiroffspring.
Each species of organism containsa characteristic number of chromo-somes. Table 10.1 shows the diploidand haploid numbers of chromo-somes of some species. Note thelarge range of chromosome numbers.Note also that the chromosomenumber of a species is not related tothe complexity of the organism.
Homologous chromosomesThe two chromosomes of each
pair in a diploid cell help determinewhat the individual organism lookslike. These paired chromosomes arecalled homologous chromosomes(huh MAHL uh gus). Each of a pair of homologous chromosomes hasgenes for the same traits, such as podshape. On homologous chromo-somes, these genes are arranged inthe same order, but because there aredifferent possible alleles for the samegene, the two chromosomes in ahomologous pair are not always iden-tical to each other. Identify thehomologous chromosomes in theProblem-Solving Lab.
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
Adder’s tongue fern 1260 630
Table 10.1 Chromosome Numbers of Some Common Organisms
Can you identify homologous chromosomes?Homologous chromosomes are paired chromosomes havinggenes for the same trait located at the same place on thechromosome. The gene itself, however, may have differentalleles, producing different forms of the trait.
AnalysisThe diagram below shows chromosome 1 with four
different genes present. These genes are represented by theletters F, g, h, and J. Possible homologous chromosomes ofchromosome 1 are labeled 2-5. Examine the five chromo-somes and the genes they contain to determine which ofchromosomes 2-5 are homologous with chromosome 1.
Thinking Critically1. Could chromosome 2 be homologous with chromosome
1? Explain why.2. Could chromosome 3 be homologous with chromosome 1?
Explain why.3. Could chromosome 4 be homologous with chromosome
1? Explain why.4. Could chromosome 5 be homologous with chromosome 1?
Explain why.
Problem-Solving Lab 10-2Problem-Solving Lab 10-2 Interpreting ScientificIllustrations
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-
270 MENDEL AND MEIOSIS
Leopardfrog
Adder’stongue fern
Corn
Let’s look at the seven pairs ofhomologous chromosomes inMendel’s peas. These chromosomepairs are numbered 1 through 7. Eachpair contains certain genes located atspecific places on the chromosome.Chromosome 4 contains the genesfor three of the traits that Mendelstudied. Many other genes can befound on this chromosome as well.
Every pea plant has two copies ofchromosome 4. It received one fromeach of its parents and will give oneat random to each of its offspring.Remember, however, that the twocopies of chromosome 4 in a peaplant may not necessarily have iden-tical alleles. Each can have one of thedifferent alleles possible for eachgene. The homologous chromo-somes diagrammed in Figure 10.10show both alleles for each of threetraits. Thus, the plant represented bythese chromosomes is heterozygousfor each of the traits.
Why meiosis?When 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 14chromosomes. That means that eachoffspring formed by fertilization ofgametes would have twice the num-ber of chromosomes as each of itsparents. The F1 pea plants wouldhave cell nuclei with 28 chromo-somes, and the F2 plants would havecell nuclei with 56 chromosomes.The nuclei would certainly becrowded! What do you think theseplants might look like?
Clearly, there must be anotherform of cell division that allows off-spring to have the same number of
chromosomes as their parents. Thiskind of cell division, which producesgametes containing half the numberof chromosomes as a parent’s bodycell, is called meiosis (mi OH sus).Meiosis occurs in the specializedbody cells of each parent that pro-duce gametes.
Meiosis consists of two separatedivisions, known as meiosis I andmeiosis II. Meiosis I begins with onediploid (2n) cell. By the end of meio-sis II, there are four haploid (n) cells.These haploid cells are called sexcells—gametes. Male gametes arecalled sperm. Female gametes arecalled eggs. When a sperm fertilizesan egg, the resulting cell, called azygote (ZI goht), once again has thediploid number of chromosomes.
10.2 MEIOSIS 271
Chromosome 4
a
Terminal
Tall
Inflated
Axial
Short
Constricted
T
l
t
i
A
Homologous chromosome 4
Figure 10.10Each chromosome 4 in garden peas contains genes forflower position, height, and pod shape. Flower positioncan be either axial (flowers located along the stems) orterminal (flowers clustered at the top of the plant).Plant height can be either tall or short. Pod shape canbe either inflated or constricted.
OriginWORDWORD
meiosis From the Greekword meioun,meaning “to dimin-ish.” Meiosis is cell division thatresults in a gametecontaining half thenumber of chromo-somes of its parent.
The zygote then can develop bymitosis into a multicellular organism.The pattern of reproduction thatinvolves the production and subse-quent fusion of haploid sex cells iscalled sexual reproduction. Thisreproductive pattern is illustrated inFigure 10.11.
The Phases of MeiosisDuring meiosis, a spindle forms
and the cytoplasm divides in thesame ways they do during mitosis.However, what happens to the chro-mosomes in meiosis is very different.Figure 10.12 illustrates interphaseand the phases of meiosis. Examinethe diagram and photo of each phaseas you read about it.
InterphaseRecall from Chapter 8 that, during
interphase, the cell replicates its chro-mosomes. During interphase thatprecedes meiosis I, the cell also repli-cates its chromosomes. After replica-tion, each chromosome consists of
two identical sister chromatids, heldtogether by a centromere.
Prophase I A cell entering prophase I behaves
in a similar way to one enteringprophase of mitosis. The chromo-somes coil up and a spindle forms.Then, in a step unique to meiosis,each pair of homologous chromo-somes comes together, matched geneby gene, to form a four-part structurecalled a tetrad. A tetrad consists of twohomologous chromosomes, each made up of two sister chromatids. Thechromatids in a tetrad pair tightly. Infact, they pair so tightly that nonsis-ter chromatids from homologouschromosomes sometimes actuallyexchange 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. It isestimated that during prophase I ofmeiosis in humans, there is an averageof two to three crossovers for eachpair of homologous chromosomes.
272 MENDEL AND MEIOSIS
Figure 10.11 In sexual reproduction, the doubling of the chromosomenumber that results from fertil-ization is balanced by the halv-ing of the chromosome numberthat results from meiosis.
Fertilization
Meiosis
Mitosis andDevelopment
Diploid zygote(2n = 46)
Haploid gametes(n = 23)
sperm cell
Multicellular diploid adults
(2n = 46)
Meiosis
egg cell
10.2 MEIOSIS 273
Figure 10.12Follow the diagrams showing interphase and meiosis as youread about each phase. Compare these diagrams with thoseof mitosis in Chapter 8. Note that after telophase II, meiosisis finished and gametes form. In whatother ways are mitosis and meiosisdifferent?
Meiosis I
Metaphase I
Metaphase II
Prophase I
Prophase II
Interphase
Telophase II
Telophase IAnaphase II
Anaphase I
Meiosis II
Magnification: 450�
Magnification: 900�
Magnification: 900�
Magnification: 940�
Magnification: 640�Magnification: 470�
Magnification: 800�
Magnification: 540�
Magnification: 380�
This exchange of genetic materialis diagrammed as an X-shaped config-uration in Figure 10.13b. Crossingover results in new combinations ofalleles on a chromosome, as shown inFigure 10.13c. You can practicemodeling crossing over in theMiniLab at the left.
Metaphase IAs prophase I ends, the cen-
tromere of each chromosomebecomes attached to a spindle fiber.The spindle fibers pull the tetradsinto the middle, or equator, of thespindle. This is an important stepunique to meiosis. Note that homol-ogous chromosomes are lined up sideby side as tetrads. In mitosis, on theother hand, homologous chromo-somes line up on the equator inde-pendently of each other.
Anaphase I Anaphase I begins as homologous
chromosomes, each with its twochromatids, separate and move toopposite ends of the cell. This occursbecause the centromeres holding thesister chromatids together do notsplit as they do during anaphase inmitosis. This critical step ensuresthat each new cell will receive onlyone chromosome from each homolo-gous 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. Eachcell has only half the genetic infor-mation of the original cell because ithas only one chromosome from eachhomologous pair. However, anothercell division is needed because eachchromosome is still doubled, consist-ing of two sister chromatids.
274 MENDEL AND MEIOSIS
Modeling CrossingOver Crossing overoccurs during meiosisand involves only thenonsister chromatidsthat are present duringtetrad formation. Theprocess is responsiblefor the appearance ofnew combinations ofalleles in gamete 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 in 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 four gamete cells that will result at theend of meiosis. Record your model’s appearance by draw-ing the gametes’ chromosomes and their genes in yourdata table.
% Next, repeat steps 2-4. This time, however, assume thatcrossing over occurs between genes B and C.
Analysis1. Predict and diagram the appearance of the chromosomes
prior to replication.2. Define crossing over and explain when it occurs.3. Compare any differences in the appearance of genes on
chromosomes in gamete cells when crossing over occursand when it does not occur.
4. Crossing over has been compared to “shuffling the deck”in cards. Explain what this means.
5. What would be accomplished if crossing over occurredbetween sister chromatids? Explain your answer.
MiniLab 10-2MiniLab 10-2 Formulating Models
2 Chromosomes with chromatids
Nonsister chromatids
Twist tie
Mark geneswith a pencilpoint
No crossing over Crossing over
Appearance of gamete cells Appearance of gamete cells
Data Table
The phases of meiosis IIThe newly formed cells in some
organisms undergo a short interphasein which the chromosomes do notreplicate. In other organisms, how-ever, the cells go from late anaphaseof meiosis I directly to metaphase ofmeiosis II, skipping telophase I,interphase, and prophase II.
The second division in meiosisconsists of prophase II, metaphase II,anaphase II, and telophase II. Duringprophase II, a spindle forms in eachof the two new cells and the spindlefibers attach to the chromosomes.The chromosomes, still made up ofsister chromatids, are pulled to thecenter of the cell and line up ran-domly at the equator duringmetaphase II, just as they do in mito-sis. Anaphase II begins as the cen-tromere of each chromosome splits,allowing the sister chromatids to sep-arate and move to opposite poles.Finally, nuclei re-form, the spindlesbreak down, and the cytoplasmdivides during telophase II. Theevents of meiosis II are identical tothose you studied for mitosis.
At the end of meiosis II, four hap-loid sex cells have been formed fromone original diploid cell. Each hap-loid cell contains one chromosomefrom each homologous pair. Thesehaploid cells will become gametes,transmitting the genes they containto offspring.
Meiosis Provides forGenetic Variation
Cells that are formed by mitosis areidentical to each other and to the par-ent cell. Meiosis, however, provides amechanism for shuffling the chromo-somes and the genetic informationthey carry. By shuffling the chromo-somes, genetic variation is produced.
Genetic recombinationHow many different kinds of
sperm can a pea plant produce? Eachcell undergoing meiosis has sevenpairs of chromosomes. Because eachof the seven pairs of chromosomescan line up at the cell’s equator in twodifferent ways, 128 different kinds ofsperm are possible (27 = 128).
10.2 MEIOSIS 275
OriginWORDWORD
pro- From the Greekword pro, meaning“before.”
meta- From the Greekword meta, meaning“after.”
ana- From the Greekword ana, meaning“away, onward.”
telo- From the Greektelos, meaning“end.”
The four phases ofcell division areprophase,metaphase,anaphase, andtelophase.
Figure 10.13 Late in prophase I, the homologous chromosomes come together to formtetrads (a). Arms of nonsister chromatidswind around each other (b), and geneticmaterial may be exchanged (c).
Homologous chromosomesCrossing over in tetrad
Gametes
Tetrad
A A
Sister chromatids Nonsister chromatids
a b
c
B B
a a
b b
A A
B Bb
a
A A a a
B Bb b
b
a
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).Figure 10.14a shows a simple exam-ple of how genetic recombinationoccurs. You can see that the genecombinations in the gametes varydepending on how each pair ofhomologous chromosomes lines upduring metaphase I, a random process.
These numbers increase greatly asthe number of chromosomes in thespecies increases. In humans, n = 23,so the number of different kinds ofeggs or sperm a person can produceis more than 8 million (223). Whenfertilization occurs, 223 � 223, or 70trillion, different zygotes are possi-ble! It’s no wonder that each individ-ual is unique.
In addition, crossing over canoccur anywhere at random on a chro-mosome. Typically, two or threecrossovers per chromosome occur dur-ing meiosis. This means that an almostendless number of different possiblechromosomes can be produced bycrossing over, providing additionalvariation to that already produced by
the random assortment of chromo-somes. This reassortment of chromo-somes and the genetic informationthey carry, either by crossing over orby independent segregation ofhomologous chromosomes, is calledgenetic recombination. It is a majorsource of variation among organisms.Variation is important to a speciesbecause it is the raw material thatforms the basis for evolution. Howdoes crossing over increase geneticvariability? To answer this question,read the Inside Story on the next page.
Meiosis explains Mendel’s resultsMeiosis provides the physical basis
for explaining Mendel’s results. Thesegregation of chromosomes inanaphase I of meiosis explainsMendel’s 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 explainsMendel’s observation that factors, orgenes, for different traits are inheritedindependently of each other. Today,Mendel’s laws of heredity form thefoundation of modern genetics.
276 MENDEL AND MEIOSIS
Figure 10.14 If a cell has two pairsof chromosomes (n = 2), four kinds ofgametes (22 ) are pos-sible, depending onhow the homologouschromosomes line up at the equatorduring meiosis I (a).This event is a matterof chance. Whenzygotes are formedby the union of these gametes, 4 � 4 or 16 possiblecombinations mayoccur (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
arr
ange
men
t of
chro
mos
omes
in e
ggs
Possible arrangement ofchromosomes in sperm
Ab aB ab
ba
Possible gametes
b A b
b
A a
B
B Ba
aA
b
A
B
a
Possible gametes
b bAA a
B
B B a
aA
b
A
B
a
b
Sister chromatids
Nonsister chromatids
Genetic Recombination
One source of genetic recombination is a processknown as crossing over, the exchange of genetic
material by nonsister chromatids during meiosis.
Critical Thinking How can the frequency of crossing over be used to map the location of genes on chromosomes?
New allele combinations Crossing overcauses a shuffling of allele combinations, just as shuffling a deck of cards producesnew combinations of cards dealt in a hand.Rather than the alleles from each parentstaying together on their homologous chro-mosome, new combinations of alleles canform. Thus, variability is increased.
33
44
10.2 MEIOSIS 277
Chiasmata The pair-ing of homologouschromosomes duringsynapsis is precise—they line up with eachother allele by allele.The nonsister chro-matids twist aroundeach other, forming X-shaped regions calledchiasmata (ki az MAH
tuh). The two nonsisterchromatids break andexchange geneticmaterial.
22
Tetrads Duringlate prophase I,homologous chro-mosomes cometogether to form atetrad. This pairingof homologouschromosomes iscalled synapsis.
11
Mapping Geneticists have used the fre-quency of crossing over to map the relativelocation of alleles on chromosomes. Allelesthat are further apart on the chromosomesare more likely to have chiasmata betweenthem than alleles that are close together.
The X-shaped regions are called chiasmata.
Chiasmata
INSIDESSTORTORYY
INSIDE
Mistakes in MeiosisAlthough the events of meiosis
usually proceed accurately, sometimesan accident occurs and chromosomesfail to separate correctly. The failureof homologous chromosomes to sepa-rate properly during meiosis is callednondisjunction. Recall that duringmeiosis I, one chromosome fromeach homologous pair moves to eachpole of the cell. Occasionally, bothchromosomes of a homologous pairmove to the same pole of the cell.
Trisomy, monosomy, and triploidyIn one form of nondisjunction, two
kinds of gametes result. One has anextra chromosome, and the other ismissing a chromosome. The effectsof nondisjunction 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 tri-somy (TRI soh mee). In humans, if agamete with an extra chromosomenumber 21 is fertilized by a normalgamete, the resulting zygote has 47chromosomes instead of 46. This
zygote will develop into a baby withDown 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 survive,the resulting organism usually doesnot. An example of monosomy that isnot lethal is Turner syndrome, inwhich human females have only asingle X chromosome instead of two.
Another form of nondisjunctioninvolves a total lack of separation ofhomologous chromosomes. Whenthis happens, a gamete inherits acomplete diploid set of chromo-somes, as shown in Figure 10.15.When a gamete with an extra set ofchromosomes is fertilized by a nor-mal haploid gamete, the offspring hasthree sets of chromosomes and istriploid. The fusion of two gametes,each with an extra set of chromo-somes, produces offspring with four
278 MENDEL AND MEIOSIS
Male parent (2n)
Meiosis
Abnormalgamete (2n)
Nondisjunction
Zygote(4n)
Female parent (2n)
Meiosis
Abnormalgamete (2n)
Nondisjunction
Figure 10.15 Follow the steps tosee how a tetraploidplant such as thischrysanthemum isproduced.
sets of chromosomes—a tetraploid.This can be seen in Figure 10.15
Organisms with more than theusual number of chromosome setsare called polyploids. Polyploidy israre in animals and almost alwayscauses death of the zygote. However,polyploidy frequently occurs inplants. Often, the flowers and fruitsof these plants are larger than nor-mal, and the plants are healthier.Many polyploid plants, such as thesterile banana plant and the day lilyshown in Figure 10.16, are of greatcommercial value.
Meiosis is a complex process, andthe results of an error occurring aresometimes unfortunate. However,mistakes in meiosis can be beneficial,such as those that have occurred inagriculture. Tetraploid (4n) wheat,triploid (3n) apples, and polyploidchrysanthemums all are availablecommercially. You can see that athorough understanding of meiosisand genetics would be very helpful toplant breeders. In fact, plant breedershave learned to artificially producepolyploid plants using chemicals thatcause nondisjunction.
10.2 MEIOSIS 279
Section AssessmentSection AssessmentUnderstanding 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 role of meiosis in maintaining a con-stant number of chromosomes in a species?
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 of meiosis explain Mendel’s
law of independent assortment?
6. Interpreting Scientific IllustrationsCompare Figures 10.12 and 8.12. Explain whycrossing over between nonsister chromatids ofhomologous chromosomes cannot occur duringmitosis. For more help, refer to ThinkingCritically in the Skill Handbook.
SKILL REVIEWSKILL REVIEW
Figure 10.16The banana plant isan example of atriploid plant (a). Thisday lily is a tetraploidplant (b).
a
b
How can phenotypes and genotypes of plants be determined?
I t’s difficult to predict the traits of plants if all that you see is theirseeds. But if these seeds are planted and allowed to grow, certain traits
will appear. By observing these traits, you might be able to determine thepossible phenotypes and genotypes of the parent plants that produced theseseeds. In this lab, you will determine the genotypes of plants that growfrom two groups of tobacco seeds. Each group of seeds came from differentparents. Plants will be either green or albino (white) in color. Use the fol-lowing genotypes for this cross. CC = green, Cc = green, and cc = albino
INTERNETINTERNET
ProblemCan the phenotypes and genotypes
of the parent plants that producedtwo groups of seeds be determinedfrom the phenotypes of the plantsgrown from the seeds?
HypothesesHave your group agree on a
hypothesis to be tested that willanswer the problem question. Recordyour hypothesis.
ObjectivesIn this BioLab, you will:■ Analyze the results of growing two
groups of seeds.■ Draw conclusions about phenotypes
and genotypes based on thoseresults.
■ Use the Internet to collect and com-pare data from other students.
Possible Materialspotting soilsmall flowerpots or seedling flatstwo groups of tobacco seedshand lenslight sourcethermometerplant-watering bottle
Safety PrecautionsAlways wash your hands after han-
dling plant materials. Always weargoggles in the lab.
Skill HandbookUse the Skill Handbook if you need
additional help with this lab.
PREPARATIONPREPARATION
Sharing Your DataSharing Your Data
Find this BioLab on the Glencoe Science Web site
at science.glencoe.com. Briefly describe yourexperimental design. Post your results in thetable provided.
10.2 MEIOSIS 281
1. Thinking Critically Why was itnecessary to grow plants from theseeds in order to determine thephenotypes of the plants thatformed the seeds?
2. Drawing Conclusions Using theinformation in the introduction,describe how the gene for greencolor (C) is inherited.
3. Making Inferences For the groupof seeds that yielded all greenplants, are you able to determineexactly the genotypes of the par-ents that formed these seeds? Canyou determine the genotype ofeach plant observed? Explain.
4. Making Inferences For thegroup of seeds that yielded somegreen and some albino plants, are
ANALYZE AND CONCLUDEANALYZE AND CONCLUDE
PLAN THE EXPERIMENTPLAN THE EXPERIMENT
1. Examine the materials providedby your teacher. As a group,make a list of the possible waysyou might test your hypothesis.
2. Agree on one way that yourgroup could investigate yourhypothesis.
3. Design an experiment that willallow you to collect quantita-tive data. For example, howmany plants do you think youwill need to examine?
4. Prepare a numbered list of direc-tions. Include a list of materialsand the quantities you will need.
5. Make a data table for recordingyour observations.
Check the Plan1. Carefully determine what data
you are going to collect. How many seeds do you think you will need? How long will you carry out the experiment?
2. What variables, if any, will have to be controlled? (Hint: Think about the growing conditions for the plants.)
3. Make sure your teacher has approved your experimental plan before you proceed.
4. Carry out your experiment. Make any needed observa-tions, such as the numbers of green and albino plants in each group, and complete your data table.
5. Go to the Glencoe ScienceWeb Site at the address shownbelow to post your data.
you able to determine exactly thegenotypes of the plants thatformed these seeds? Can youdetermine the genotype of eachplant observed? Explain.
5. Using the Internet Compareyour experimental design withthat of other students. Were yourresults similar? What mightaccount for the differences?
BIOLOGY
Gregor Mendel was an Austrian monk who experimented with garden peas.
In 1866, he published the results of eight years of experiments. His work was ignored until
1900, when it was rediscovered.
Mendel had three qualities that led to hisdiscovery of the laws of heredity. First, he
was curious, impelled to find out why things hap-pened. Second, he was a keen observer. Third, hewas a skilled mathematician. Mendel was the firstbiologist who relied heavily on statistics for solu-tions to how traits are inherited.
Darwin missed his chance About the same time that Mendel was carry-
ing out his experiments with pea plants, CharlesDarwin was gathering data on snapdragon flow-ers. When Darwin crossed plants that had nor-mal-shaped flowers with plants that had odd-shaped flowers, all the offspring had normal-shaped flowers. He thought the two traits hadblended. When he allowed the F1 plants to self-pollinate, his results were 88 plants with normal-shaped flowers and 37 plants with odd-shapedflowers. Darwin was puzzled by the results anddid not continue his studies with these plants.Lacking Mendel’s statistical skills, Darwin failedto see the significance of the ratio of normal-shaped flowers to odd-shaped flowers in the F2generation. What was this ratio? Was this ratiosimilar to Mendel’s ratio of dominant to recessivetraits in pea plants?
Finding the ratios for four other traits Figure 10.3 on page 262 shows seven traits
that Mendel studied in pea plants. You havealready looked at Mendel’s data for plant heightand seed shape. Now use the data for seed color,flower position, pod color, and pod shape to findthe ratios of dominant to recessive for these traitsin the F2 generation.
Draw Table B in your notebook or journal.Calculate the ratios for the data in Table A andcomplete Table B by following these steps:• Step 1 Divide the larger number by the smaller
number. • Step 2 Round to the nearest hundredth.• Step 3 To express your answer as a ratio, write
the number from step 2 followed by a colonand the number 1.
282 MENDEL AND MEIOSIS
ConnectionMathMath
Connection A Solution fromRatios
Why were ratios so important in understandinghow dominant and recessive traits are inherited?
To find out more about Mendel’s work, visit the
Glencoe Science Web site.science.glencoe.com
CONNECTION TO BIOLOGYCONNECTION TO BIOLOGY
Seed Flower Pod Pod Color Position Color Shape
Yellow Lateral Green Inflated6022 651 428 882
Green Terminal Yellow Constricted2001 207 152 299
Table A Mendel’s Results
Seed Flower Pod PodColor Position Color Shape
Calculation
Ratio 3:1yellow:green
Table B Calculating Ratios for Mendel’s Results
6022 2001 = 3.00
BIOLOGY
Chapter 10 AssessmentChapter 10 Assessment
SUMMARYSUMMARY
Section 10.1
Section 10.2
Main Ideas■ Genes are located on chromosomes and exist in
alternative forms called alleles. A dominant allelecan mask the expression of a recessive allele.
■ When Mendel crossed pea plants differing inone trait, one form of the trait disappeared until the second generation of offspring. Toexplain his results, Mendel formulated the law of segregation.
■ Mendel formulated the law of indepen-dent assortment to explain that twotraits are inherited independently.
■ Events in genetics are governed by thelaws of probability.
Vocabularyallele (p. 262)dominant (p. 262)fertilization (p. 259)gamete (p. 259)genetics (p. 259)genotype (p. 264)heredity (p. 259)heterozygous (p. 265)homozygous (p. 264)hybrid (p. 261)law of independent
assortment (p. 266)law of segregation
(p. 263)phenotype (p. 264)pollination (p. 259)recessive (p. 262)trait (p. 259)
Mendel’s Lawsof Heredity
Main Ideas■ In meiosis, one diploid (2n) cell produces four
haploid (n) cells, providing a way for offspringto have the same number of chromosomes astheir parents.
■ Mendel’s results can be explained by the distri-bution of chromosomes during meiosis.
■ Random assortment and crossing over duringmeiosis provide for genetic variation among themembers of a species.
■ Mistakes in meiosis may result from nondisjunc-tion, the failure of chromosomes to separateproperly during cell division.
Vocabularycrossing over (p. 272)diploid (p. 269)egg (p. 271)genetic recombination
(p. 276)haploid (p. 269)homologous chromo-
some (p. 270)meiosis (p. 271)nondisjunction (p. 278)sexual reproduction
(p. 272)sperm (p. 271)zygote (p. 271)
Meiosis
CHAPTER 10 ASSESSMENT 283
1. An organism that is true breeding for a traitis said to be ________.a. homozygous c. a monohybridb. heterozygous d. a dihybrid
2. At the end of meiosis, how many haploidcells have been formed from the original cell?a. one c. threeb. two d. four
UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 3. When Mendel transferred pollen from onepea plant to another, he was ________ theplants.a. self-pollinating c. self-fertilizingb. cross-pollinating d. cross-fertilizing
4. A short pea plant is ________.a. homozygous recessiveb. homozygous dominantc. heterozygousd. a dihybrid
Chapter 10 AssessmentChapter 10 Assessment
5. Which of these shows a dominant trait ingarden peas?a. b. c. d.
6. During what phase of meiosis do sister chromatids separate?a. prophase I c. anaphase IIb. telophase I d. telophase II
7. During what phase of meiosis do homolo-gous chromosomes cross over?a. prophase I c. telophase Ib. anaphase I d. telophase II
8. Recessive traits appear only when an organ-ism is ________.a. matureb. different from its parentsc. heterozygousd. homozygous
9. Mendel’s use of peas was a good choice forgenetic study because ________.a. they produce many offspringb. they are easy to growc. they can be self-pollinatedd. all of the above
10. A dihybrid cross between two heterozygousparents produces a phenotypic ratio of________.a. 3: 1 c. 9: 3: 3: 1b. 1: 2: 1 d. 1: 6: 9
11. If two heterozygous organisms for a singledominant trait mate, the ratio of their off-spring should be about ________.
12. A trait that is hidden in the heterozygouscondition is said to be a ________ trait.
13. An organism that has two different alleles fora trait is called ________.
14. The process that results in Down syndromeis called ________.
15. If a species normally has 46 chromosomes,the cells it produces by meiosis will each have________ chromosomes.
16. Metaphase I of meiosis occurs when________ line up next to each other at thecell’s equator.
17. The stage of meiosis shown here is ________.
18. In the first generation of Mendel’s experi-ments with a single trait, the ________ traitdisappeared, only to reappear in the nextgeneration.
19. A cell that has successfully completed meiosishas a chromosome number called ________.
20. Meiosis results in the direct production of________.
21. Why do you think Mendel’s results are alsovalid for humans?
22. On the average, each human has about sixrecessive alleles that would be lethal ifexpressed. Why do you think that humancultures have laws against marriage betweenclose relatives?
23. Assume that a couple has four children whoare all boys. What are the chances that theirnext child will also be a boy? Explain youranswer.
24. How does separation of homologous chro-mosomes during anaphase I of meiosisincrease variation among offspring?
APPLYING MAIN IDEASAPPLYING MAIN IDEAS
284 CHAPTER 10 ASSESSMENT
TEST–TAKING TIPTEST–TAKING TIP
Use the Buddy System Study in a group. A small gathering of peopleworks well because it allows you to draw from abroader base of skills and expertise. Keep it smalland keep on target.
Chapter 10 AssessmentChapter 10 Assessment
CHAPTER 10 ASSESSMENT 285
25. Relating to the methods of science, why doyou think it was important for Mendel tostudy only one trait at a time during hisexperiments?
26. Observing and Inferring Why is it possibleto have a family of six girls and no boys, butextremely unlikely that there will be a publicschool with 500 girls and no boys?
27. Comparing and Contrasting Comparemetaphase of mitosis with metaphase I ofmeiosis.
28. Recognizing Cause and Effect Why is itsometimes impossible to determine the genotype of an organism that has a dominantphenotype?
29. Observing and Inferring While examining acell in prophase I of meiosis, you observe apair of homologous chromosomes pairingtightly. What is the significance of the placesat which the chromosomes are joined?
30. Concept Mapping Complete the conceptmap by using the following vocabulary terms:recessive, zygote, homozygous, fertilization,heterozygous.
THINKING CRITICALLYTHINKING CRITICALLY
ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS
In fruit flies, the allele for long wings isdominant to the allele for short wings.
Interpreting Data Study the Punnettsquare and answer the following questions.1. What term is given to the parent fly
whose genotype is shown?a. heterozygous c. recessiveb. homozygous d. haploid
2. What is the phenotype of each parent?a. both dominantb. both recessivec. one dominant and one recessived. unable to tell
3. What is the genotype of each parent?a. WW—Ww c. Ww—Wwb. Ww—ww d. WW—WW
4. What are the phenotypes of the offspring?a. all long wingsb. all short wingsc. mostly long wingsd. half short and half long
5. Interpreting Data Suppose the fruit flyparents in the Punnett square abovewere both heterozygous for an eye colortrait in which R is red and r is white.What genotypes appear in the offspring?What fraction of the offspring will haveshort wings and white eyes?
W
Ww ww
Ww
?
w
? ww
produces a
1.
that can be
2.
3.
dominant 5.
4.or
or
For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web site.science.glencoe.com
CD-ROM