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5.2 Mutations:CreatingVariation
Let's look next at the causesof mutations and the different ways mutations can alter
DNA. There are many causesof mutations. Radioactiveparticles pass through our
bodies every day, for example, and if one of these particles strikes a molecule of DNA,
it can damage the molecule's structure. Ultraviolet solar radiation strikes our skin
cells and can causemutations to arise as these cells divide. Manv chemicalsalso can
interfere with DNA replication and lead to mutation. Whenever a cell copiesits DNA,
there is a small chance it may misread the sequenceand add the wrong nucleotide.
Our cells have proofreading proteins that can fix most of these errors, but they sometimes let mistakesslip bv.
Mutations alter DNA in severaldifferent ways:
. Point mutation: A single base changes from one nucleotide to another (also
known as a substitution).
' Insertion: A segmentof DNA is inserted into the middle of an existing sequence.
The insertion may be as short as a singlebaseor as long as thousandsof bases
(including entire genes).
. Deletion: A segment of DNA may be deleted accidentally.A small portion of a
gene may disappear,or an entire set of genesmay be removed.
' Duplication: A segmentof DNA is copieda secondtime. A small duplication can
produce an extra copy of a region inside a gene. Entire genescan be duplicated. In
somecases,even an entire genomecan be duplicated.
. Inversion: A segmentof DNA is flipped around and inserted backward into its
original position.
. Chromosome fusion: TWo chromosomes are joined together as one.
' Aneuploidy: Chromosomesare duplicatedor lost, leading to abnormal levelsof
ploidy.
One way that mutations can give rise to geneticvariation is by altering the DNA
within the coding region of a gene (the region that encodesa protein). An alteredcoding region may lead to a protein with a different sequenceof amino acids,which may
fold into a different shape.This changecould causethe protein to perform its original
activity at a faster or slower rate, or it may acquire a different activity.
Pointmutation
lnversion
TGCATTGCGTAGGC
Y
TGCATTCCGTAGGC
Insertion
TGCATTTA6GC
Chromosomefusion
TGCATTCCGTAGGC
A
CCGJ
Genomeduplication
Deletion
rGcArrckqrAGGc
Y
TGCATTTAGGC
Geneduplication
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Figure5.13 DNA canexperience
severaldifferentkindsof mutations,
suchas pointmutations,insertions,
deletions,
andduplications.
ARrATroN
t . 2 M U T A T T o NcsR: E A T T NvG
133
GenesandHeredity
in Bacteria
andArchaea
All livingthings use DNA or RNAto storegeneticinformation.But
eukaryotesare differentfrom bacteriaand archaeain the organizationof their DNA as well as in how they replicateDNA and pass
it down to their offspring.The name eukaryotepoints to one of the
most obviousthings that set eukaryotesapart from other forms of
life.In Greek,it means"true kernel,"referringto the nucleusin which
eukaryotes
keeptheirDNAtightlycoiled.Bacteriaand archaealacka
nucleus,and that trait has earnedthem the nameprokaryote,meaning"beforethe kernel."
fheword prokaryotecame into use beforethe dawn of molecular
systematics.
Basedon what is now knownaboutthe relationship
of
the threedomains,a numberof leadingmicrobiologists
argueagainst
using the term prokaryote.We know now that archaea are more
closelyrelatedto eukaryotes
than they areto bacteria,whichmeans
that if a cladeincludesarchaeaand bacteria*but not eukaryotesit wouldnot be monophyletic(Pace2OO9).In general,taxonomists
avoidgivingnamesto paraphyletic
taxa (Chapter4). Criticsargue
that usingthe word prokaryotemakesas much senseas mammalogistsgivinga namefor the cladethat includesall mammals-except
for rodents.So in this book,we'llavoidusingprokaryoteand simply
referto bacteria and archaea.
In bacteriaand archaea,a singlecjrcularchromosomefloats
within the cell. lt is not constrainedby a nucleus,nor is it tightly
spooledaroundhistones.That does not meanthis DNA is simplya
loosetangle.Bacteriaand archaeaproduceproteinsthat keepsections of DNA organizedin twisted loops,Likeeukaryotes,bacteria
and archaeacan regulategeneexpression
by unwindingand winding
t h e i rD N A .
Like eukaryotes,bacteria and archaea have genetic regulatory
regionsupstreamfrom their genes.Transcription
factorscan trigger dramaticchangesin gene expressionin bacteriaand archaea
throughregulatorycascades.Bacteriaand archaeacan alter therr
gene expressionin responseto signalsfrom their environment.
As
a result,some speciescan producesporeswhen conditionsturn
stressful.
Otherscan producetoxinswhentheysenseothermicrobes
competingfor resources.
generegulationis lesscomplexin bacteriaand
Overall,however,
archaeathan it rsin eukaryotes.
Bacteriaandarchaealackenhancers
(shortregionsof DNAthat helpin transcription),
for example,which
can be locatedthousandsof basepairsawayfrom genesthey control in eukaryotes.
Bacteriaandarchaeahaveself-splicing
intronsbut
lackthe abundantspliceosomal
intronsfound in eukaryotes,
which
requirea groupof proteinscalledthe splrceosome
to removethem
from transcripts.
Bacteriaandarchaeathus lackthe alternative
splicing found in eukaryotes.
As a result,they alwaysproducethe same
proteinfrom any givengene.
Replication
in bacteriaand archaeais alsosimpler.They do not
performmitosisor meiosis.
Theydo not havefull-blownsexualreproduction,in whichmalesand femalesproducegametesthat combine
in a new offspring.Instead,bacteriaand archaeatypicallygrow until
they are largeenoughto divide.They then build a secondcopy of
their circularchromosomeand then the two DNA moleculesare
draggedto eitherend of the dividingcell.The two daughtercellsare
identicalto the original,exceptfor mutationsthat ariseduringDNA
replication.
Bacteriaand archaeahavemanyof the same kindsof mutations
found in eukaryotes,such as point mutationsand insertions.But
theycannotacquiregeneticvariationas a consequence
of reproduction the way we see in eukaryotes(i.e.,independentassortmentof
As we'llseein Chapter10,this difference
chromosomes).
can havea
majoreffecton how mutationsspreadthroughpopulations.
Beneficialmutationsthat increasethe survivalor reproductive
rateof bacteriacan sweepquicklythrougha populationof microbes,
thanksto naturalselection.
As we'llsee in Chapter6, scientistshave
used microbesto perform important experimentson evolution,
observingnaturalselectionin action.And,as we'llsee in Chapter18,
bacteriacan rapidlyevolveresistance
antibiotics,
turningwhat were
once easilytreateddiseasesinto seriousthreatsto publichealth.
can spreadso quicklyisthat
Onereasonthat antibioticresistance
bacteriaare not limitedsimplyto passingdown their genesto their
(knownas verticalgenetransfer).lt'salsopossiblefor
descendants
one individualmicrobeto "donate"DNAto another,througha processcalledhorizontal gene transfer.
One way for genesto move from one microbe to another is via
plasmids,whicharesmallringletsof DNAthat areseparatefrom the
m a i nb a c t e r i a l c h r o m o s o m
Uen.d e rc e r t a i nc o n d i t i o n sa.m i c r o b ew i l l
translateplasmidgenesand assemblea tube calleda pilus,which
linksthe genesto a neighboring
cell.The donor cell can then pump
B
Terminator
Promoter
Ribosomes
NucleoidDNA
Plasma
membrane
DNA
template
I
I Transcription
Y
g sequence
Protein-codin
Capsule
C e lw
l all
RNAtranscript
tI Translation
_
I
Y-2
.;e€
Porypeptide@
BoxFigure5.1.1A: Bacteria
andarchaea
differfromeukaryotes,
like
humans,
in thatthegenetic
withinthecellisnotcontained
material
'134
withina nuclearmembrane.
B:Theprocess
of geneexpression
and
regulationis muchsimplerthan in eukaryotes.
c H A p r E RF t v ER A w M A T E R T A LH: E R I T A B L E
vaRtATtoN AMoNG tNDrvrDUALs
i :opy of the plasmidthroughthe pilus,and oftenpumpsa copy of
s:'ne of its chromosomalDNAas well.In effect,plasmidsaregenetic
::-asites,usingbacteriaand archaeaas theirhostsandspreadingto
-=,vhoststhroughthe piliencodedin theirowngenes.However,
they
:r also carry genesencodingbeneficialtraits,such as antibiotic
-:srstance,
whichcan provideadvantages
to their hosts,
Virusescan alsocarry out horizontalgenetransfer.As they rep:3te, some virusescan accidentallyincorporatehost genes into
'-.rr owngenome.Whenthey infecta newhost,theycan insertthose
r:res intotheir newhost'schromosome.
Inmanycases,
genetransferisa deadend.Thedonated
horizontal
a:res areharmful to the recipientcell,whichdiesor growstoo slowly
': competewith other individuals.
But if a microbeacquiresa useful
g:re, naturalselectioncan favorit. Evidence
for successful
horizon:a genetransfercan be found in studieson the spreadof antibiotic
':srstance:the samegeneoftenturnsup in differentspecies.lt'salso
:':ssibleto identifycasesof horizontalgenetransferthat occurred
- llionsof yearsago by performinglarge-scalecomparisonsof DNA
- bacteriaandarchaea,In a numberof cases,scientistshavefound
genesin species
thatareunlike
theircloserelatives
butarehomologousto genesfoundin distantly
relatedclades.
Thesestudiesindicatethathorizontalgene
transfer
hasbeena majorelement
ofevolution.InE.coli,Iorexample,
80 percent
of allthegenesin itsgenome
genetransferat somepointsincethe
showevidence
of horizontal
(Dagan
lastcommonancestor
of bacteria
andMartin2007).As we
genetransferis promptingscientists
sawin Chapter4,horizonlal
to
revise
theirconcepts
of species
andof theoverall
shapeof thetree
of life.
Compared
to bacteria
andarchaea,
eukaryotes
appearto have
genetransfer.
experienced
relatively
littlehorizontal
Thereare a
numberof possible
explanations
for thisdifference.
Oneis opportunity:thecomplexity
DNAreplication
of eukaryotic
maynotafford
theopportunity
to takeup foreigngenes.In bacteriaandarchaea,
a
contiguous
setofgenesmayforma functional
unit,called
anoperon,
inwhichtheyareallcontrolled
bythesameupstream
regulatory
elements.Theentireoperoncanbe inserted
intoa newhost,whereit
maybe ableto providea usefulfunction.Eukaryotes
lackoperons,
genesarelesslikely
however,
sothatforeign
to beusefulina newcell.
Vertical gene transfer: The process
of receivinggeneticmaterialfrom an
parentce,@
ancestor.
A
Horizontal genetransfer: Any process
in whichgeneticmaterialis transferred
o'n"!'-l',=t="j.1-1
W
to anotherorganismwithoutdescent.
Plasmids:Molecules
of DNA,found
mostoftenin bacteria,
that canreplicateindependently
of chromosomal
DNA.
Y
@@
/\
Plasmid
ldentical
daughter
cells
Horizontal
9ene
transfer
/
\
ffi@
Cellsaregenetically
identical
to
ancestors,
exceptfor an acquired
pointmutation.
Box Figure 5.1.2 Bacteriareproduceby dividingin two. As they
preparefor the division,they separate
the two strandsof DNA
CellscarryDNAof ancestors
aswellasDNAacquired
genetransfer.
by horizontal
and add a new strandto eachone,creatingtwo new DNA molecules,whicharetypicallyidenticalto the originalone.On rare
occasions,however,genescan be passedfrom one bacteriumto
another,througha processknownashorizontalgenetransfer.
GA R t A T t O N
5 . 2 M U T A T T O N SC: R E A T T N V
135
.t
t1
i1
14
I
tl.
!.frl
r
genescanhavephenotypic
Figure5.14 Pointmutationsin humanprotein-coding
effectsthat
rangefrom the benign(e.g.,eyecolor)to the severe.
Shownherearea varietyof striking(but
genethat replaces
rare)mutations.Forexample,
a mutationin the FGFR3
the aminoacidproline
with serineat position380 is responsible
for albinism(A;Oettingand King1993);a singleC-to-T
(B;Wanget a1.2007);
transitionin the IMBRl geneleadsto triphalangeal
thumb polydactyly
a
m i s s e n sm
e u t a t i o ni n e x o nX I Vo f t h e G l 1 3g e n e r, e p l a c i ntgh e a m i n oa c i dp r o l i n ew i t h s e r i n el ,e a d :
(C);a missense
to Greig'scephalopolysyndactyly
substitutionin the K/f proto-oncogene
replacing
t h e a m i n oa c i da r g i n i n w
e i t hg l y c i n e
at position
7 9 5l e a d st o p i e b a l d i s (mD ;S d n c h e z - M a ret itna l .
2 0 0 3 ) a; m u t a t i o ni n t h eA C V Rg1e n es u b s t i t u t i nt g
e i t h h i s t i d i n ae t p o s i t i o '
h e a m i n oa c i da r g i n i nw
2 0 6 r e s u l t isn f i b r o d y s p l a soisas i f i c a npsr o g r e s s i (vEa ;S h o r ee t a 1 . 2 0 0 6a) ;n da p o i n tm u t a t i o ni n
progeriasyndrome(F;Eriksson
the LaminA genecausesHutchinson-Gilford
et al.2OO3).
Cis-actingelements:Stretchesof
D N Al o c a t e dn e a ra g e n e - e i t h e r
( a d j a c e nt ot
i m m e d i a t eu
l yp s t r e a m
the promoterregion),downstream,
or
i n s i d ea n i n t r o n - t h a ti n f l u e n cteh e
expression
of that gene.Cisregions
oftencodefor bindingsitesfor oneor
moretransposable
factors.
Trans-acting
elements:Sequences
of
DNAthat arelocatedawayfrom the
focalgene(e.g.,on anotherchromosome).Thesestretches
of DNAgener, i c r o R N Ao,r
a l l yc o d ef o r a p r o t e i nm
Mutations can also have important effects without altering the product of a gt'n'
Instead, they can simply change how much of a protein is made, or they can chan,.
the timing or location of its production. These changes in levels o/gene expres\r,,
can alter the behavior of cells or tissues and can have profound consequences 1,
evolution as an additional component of heritable variation. Mutations can alter ger .
expression by affecting where, when, or how much a gene is transcribed. For exanrpl.
mutations may cause a transcription factor to bind more strongly than it did befor
Or they may prevent a specific transcription activator protein from binding, so tl)..
the gene no longer can be expressed in a particular kind of tissue.
Transcription factors, hormones, and other regulatory molecules are themseh'
o t h e rd i f f u s i b lm
e o l e c u lteh a tt h e n
encoded in genes,which means mutations that alter their genes ultimately can a11,.
the genes they regulate. As a result of these interactions, a gene can be affecteil :
a mutation that is far away from the gene itself. (Nearby elements that affect gt.:
expression are cis-acting elements; faraway ones are trans-acting elements.)
Mutations, as we'll see in later chapters, are required for evolution to occur. I.]
it's important to bear in mind that they're extremely rare. To measure just how olt,
\nfluencesexpress\onof \he foca\ gene
mutations occur, scientists can erther run experiments onpopu\ations
136
c H A p r E RF r v ER A w M A T E R t a L H
: E R I T a B L vEA R l a r r o N A M o N G r N D r v r D U A L s
o{ ce\\s or ma,
Table5.3 Themanywaysmutations
caninfluence
expression
of a gene.
Location of Mutation
Type of Mutation
Consequencefor Gene Action
Coding Region
insertion,
Substitution,
deletion,duplication.
Altersthe product of the gene,
andthus its functionor activity.
cis-Regulatory
insertion,
Substitution,
deletion,duplicationthat
altersthe bindingaffnity
of promoters,activators,
repressors,
erc.
Altersthe timing,location,
or
levelof expression
of the gene.
Regions
trans-Regulatory
Regions
Mutationto codingregions Altersthe bindingaffnity and
factor.
thus the activityof a promoter,
of trans-acting
activator,repressor,
etc.
Mutationto cis- or transregulatory regionsof
factors.
trans-acting
Physiological
Pathways(e.g.,
hormones)
Altersthe developmental
or
environmental
contextin which
the geneis expressed.
Alterswhere,when,or to what
extentinhibitory,
activating,
or
facothertrans-acting
regulatory
tors areexpressed.
Altersthe timing,location,
or
Mutationsalterwhere,
levelof expression
when,or how muchan enof the gene.
docrinesignalis produced.
Altersthe developmental
or
environmental
contextin which
the geneis expressed.
surveysof living populations.In 2008,for example,Michael Lynch and his colleagues
at Indiana University rearedcoloniesofyeast (Lynchet al. 2008).From a singleancestor, Lynch and his colleaguesrearedhundreds of geneticallyidentical populations of
yeast.They then allowed these lines to reproducefor 4800 generations.After selecting someof the descendants,
the scientistssequencedall 12 million basepairs of DNA
in eachcell'sgenome.
Each time a yeast cell divides, the scientistsfound, each site in its DNA has a
0.00000003percent chanceof undergoing a point mutation. This probability is so low
that a typical yeast cell may not acquirea single point mutation in its whole genome.
But in a population of millions of yeast cells,point mutations will arise in thousands
of individuals in eachnew generation.
Lynch'sexperiments also showed that different kinds of mutations occur at different rates.The investigatorsfound that each gene has a roughly one-in-a-million
chanceof being lost or duplicatedeachtime a cell divides.Duplicationsand deletions
are rare, in other words, but they're also about a thousand times more likely than
point mutations.
Estimating mutation ratesin multicellular organismslike humans is a more complicated matter, for severalreasons.First, all of our genes are present in duplicate
tdiploidy), and this meansthat many mutations that arisein one of the chromosomes
rvill be hidden or masked by a functional copy of the same gene on the sister chromosome. Second,not all mutations that arise in our bodies are transmitted to our
offspring.Any individual cell in our body has a chanceof mutating as it divides.If it's
a skin cell,the skin cellsthat descendfrom it will continue to carry that mutation. But
this lineageof cells will come to an end when we die. Such mutations are known as
somatic mutations becausethey occur in the "soma," or body.
If, on the other hand, a mutation arisesin the line of cells that gives rise to sperm
or egg cells, it may be passedon to ofispring. And those offspring, in turn, may pass
the mutation down to their own descendants.These mutations are known as germ-
Somaticmutations:Mutationsthat
affectcellsin the body("soma")of
an organism.
Thesemutationsaffect
all the daughtercellsproducedby
the affectedcell and can affectthe
phenotypeof the individual.
In animals,
somaticmutationsarenot oassed
downto offspring.
ln plants,somatic
mutationscanbe passeddownduring
vegetative
reproduction.
Germ-linemutations:Mutationsthat
affectthe gametes(eggs,sperm)of an
individual
andcanbetransmittedfrom
parentsto offspring.Becausethey can
be passedon, germ-linemutationscreate the heritablegeneticvariationthat
is relevantto evolution.
5 . 2 M U T A T T o N sc:R E A T T NvGA R r A T r o N
137
line mutations. Even though somatic mutations sometimes drastically reduce the
performance and fitness of an individual (e.g.,many cancers,as we seein Chapter 18,
are the result of somatic mutations), they are not heritable. Heritable variation within
populations arises becauseof the gradual accumulation of germ-line mutations.
Until recently scientists could make only very indirect estimatesof the germ-line
mutation rate in humans. One common method was to study the rate of diseasesthat
are caused by a mutation of a single gene. But improvements in DNA sequencing
have made it possible for scientists to make far more precise estimates.In 2010,Leroy
Hood of the Institute for SystemsBiology in Seattleand his colleaguessequencedthe
entire genomes of tvvo human siblings and their parents. They searchedthe genomes
of the children for mutations that their parents did not carry. All told, they identified
70 new mutations in eachchild {Roachet al. 2010).
KeyConcepts
for evolution
byadding
in levels
of geneexpression
canhaveprofound
consequences
Changes
genetic
another
component
to heritable
variation.
Geneticchangesin geneexpression
arisewhen mutationsoutsideof the codingregionsaffectwhere,
when,or how mucha geneis transcribed.
i
5.3 Heredity
Allele:One of severalalternative
formsof the DNA sequence
of the
samelocus.
Meiosis:A form of celldivisionthat
in whichthe
occursonly in eukaryotes,
numberof chromosomes
is cut in half.
Meiosisgivesriseto gametesor
sporesand is essential
for sexual
reproduction.
Genetic recombination:Theexchange
ofgeneticmaterialbetweenpaired
chromosomes
duringmeiosis.
Recombinationcanform newcombinations
of allelesand is an importantsourceof
heritablevariation.
138
Once new mutations arise,organismscan passthem down, along with their unmutated DNA, to their offspring. Bacteria and archaeareproduce by dividing and making
a new copy of their genomefor eachdaughtercell (Box 5.1).In sexuallyreproducing,
multicellular eukaryotes,on the other hand, heredity is more complex.For one thing,
only germJine cells can pass on genes to offspring. For another, the development of
sex cells introduces new genetic variation, so that sexually reproducing organisms
producegeneticallyunique offspring insteadof clones.
One reasonthat parentsdo not producefamilies of identical children is that each
parent'sown paired chromosomesare not identical.One chromosomemay have one
version of a gene (an allele) while the other chromosomehas an allelewith a slightly
different DNA sequence.Differencesbetweenthe chromosomesin eachpair generate
variation among the gamete cells that are produced, so that no tvvo sperm or egg cells
are identical.
Gametesof sexually reproducing organismsare produced through a distinctive
Meiosis can generatea stunning
kind of cell division known as meiosis (Figure5.'15).
amount of geneticvariation. During the final stageof meiosis,each pair of chromo'
somes separates,so that only a single copy ends up in each daughter cell. Which copy'
endsup in eachcell occursrandomly,and it occursrandomly for eachof the different
chromosomal pairs. This means that a single sperm cell may inherit the maternal
copy of one chromosome,but the paternal copy of another. Humans have 23 pairs
of chromosomes,and the independentassortmentof eachseparatechromosomecan
result in many different combinationsof maternally inherited and paternally inher
ited genes.
In addition, each pair of chromosomes may cross over during meiosis and
exchange segments of DNA in a process known as genetic recombination. By the
time these chromosomes are packed into gamete cells, many of them have alreadr'
been rearranged so that they differ from either of the parental chromosomes.Recom
bination also can creategeneticvariation among the different gametesproduced br
an individual. Consequently,the processesof independent assortment and recombi
nation can, through the rearrangement of alleles, generate a staggering number of
possiblegametegenotypes.
This rich mixing of alleles occurs in the formation of gametes of both the male
and the female. Sexualreproduction brings the chromosomalforms of each parenr
c H A p r E RF r v ER A w M A T E R T a LH: E R I T a B L vEa R t a r l o N A M o N G l N D t v t D U A L S
Father
Chromosomes
areduplicated.
Chromosomes
are duplicated
crossover
Chromosomes
of DNA.
segments
andexchange
of homologous
Segregation
chromosomes
of
Segregation
sisterchromatids
Haploid
gametes
f
\
likehumans,
organisms,
Figure5.15 Amongsexuallyreproducing
Duringthe
malesandfemalescombinetheir gametesto reproduce.
crossoverancl
eachpairof chromosomes
productionof gametes,
onlyone copyfrom
of DNA.Eachgametereceives
segments
exchange
a unique
carries
each
child
a
result,
As
eachoairof chromosomes.
parents.
her
of
his
or
DNA
of the
combination
5.3 HEREDTTY 139
lrlr
Figure5.16 Independent
assortment
occursduringmeiosisin eukaryotic
gametes
and produces
organisms
with a mixtureof maternaIand
paternalchromosomes.
Alongwith
independent
chromosomalcrossover,
geneticdiversity
increases
assortment
of
by producingnovelcombinations
alleles.
together, creating yet another combination of alleles. Consequently,when
Maternalcopies
a human sperm cell fertilizes an egg,
the chromosomescombine to produce
a new set of 23 pairs. But this new set
is drawn from a rich pool of genetic
variants reflecting millions of possible
combinations of allelesinherited from
both the father and the mother. The
particular combination that fuses to
form the new diploid offspring indi'
vidual is literally one in a million, and
it is not likely to happen twice. This is
why sibling offspring from the same
parents always differ in their inherited
characteristics.(An exception to that
rule, of course,is identical twins who
developfrom a single fertilized egg.)
The best estimate of the differences between our paired chromo.
somes comes from Craig Venter, a
genome-sequencingpioneer. In 2007
he and his colleaguespublished the
complete sequenceof his own genome
(Levy et aL.2007).They compared each
pair of chromosomes,tallying up the
differences.The researchersidentified
Novelcombinations
of alleles
3.2 million placeswhere a single nucle
otide in one chromosomedid not match the correspondingnucleotidein its partner.
The scientistsalso found about a million segmentsof DNA on one chromosomethat
were missing from its partner,or that had been inserted.
@@ @)@
KeyConcept
recombination,
meiosis
Because
of chromosomes
andgenetic
can
of theindependent
assortment
generate
genetic
extraordinary
diversity
amonggametes.
Genotype:Thegeneticmakeupof an
individual.
Althougha genotypeinc l u d e sa l lt h e a l l e l e os f a l lt h e g e n e si n
that individual,
the term is oftenused
to referto the specificallelescarriedby
gene.
an individual
for any particular
Phenotype:An observable,
measurablecharacteristic
of an organism.
A
phenotypemaybe a morphological
structure(e.g.,antlers,muscles),
a
(e.g.,learning),
process
developmental
process
a physiological
or performance
trait (e.g.,runningspeed),
or a behavior
(e.g.,matingdisplay).
Phenotypes
can
producedby
evenbe the molecules
genes(e.9.,hemoglobin).
f4O
5.4 TheLinkbetweenMostPhenotypes
ls Complex
andGenotypes
Scientistsdraw a distinction between the genetic material in an organism and the
traits that the geneticmaterial encodes.The geneticmakeup of an organism is knor.r'n
as its genotype, and the manifestation of the genotype is known as the phenotype.
Organismsdo not inherit a phenotype; they inherit genes,which together constitute
a genotype,which gives rise to a phenotype.
Understanding how phenotypesemergefrom genotypesis no easytask. A trait
does not come with a label on it, detailing all the genesthat helped to build it and
the specificrole played by each of the genes.Instead,scientistsrely on a variety of
methods to explore how genesand gene expressioncontribute to the formation oi
organismalphenotypes.Thesemethods range from controlled breeding experiment'
to detailed genetic mapping studies-even to perturbations of expressionof focal
developmentalgenes(Chapter10 describesmany of thesemethods in greaterdetailr
The traits that Mendel studied in his peas (Box5.2) have relatively simple, dis
crete, alternative phenotypic states.The peas were either wrinkled or smooth, for
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