Chapter 5: Evolution
InheritancePatternsand Mendel'sLaws In the mid-1800s,an Austrianmonk namedGregor Mendet producedlawsof heredity thatestablished the fieldof modern genetics. Mendelbredthousands of peaplantsin order to studyinheritedtraits.By applyingstatisticalanalysisto his carefullyrecordeddata.Mendel demonstrated how geneticvariationsoccurin observable patterns.
To answerquestionsabout heredityon the Florida Biology I End-of-Course Assessprobability. An eventthat is certainhas a probability ment,you must know the basicsof probability l; has a of 0. All otherprobabilitieslie between0 of an eventthat cannotoccur and l. Probabilitycannotpredictthe outcomeof a singleevent.Instead,it is usedin large How likely is it that a samplesto predictan averageoutcomefrom certaincircumstances. motheranda brorvn-eyed fatherwill havea blue-eyed blue-eyed child'l Probabilityprovides the answcr. The Rule of Multiplication The probabilityof two independent eventsoccurringis: of first eventx probabilityof secondevent For example.the probabilityof gettingheadson two coin flips involvestwo independentevents.The chanceof the first coin flip beingheadsis I :2. The chance of the secondcoin flip alsobeingheadsis also I : 2. Thereforetheprobabilitythat two coin flipswill bothbe headsis 112x ll2: l14. The Rule ofAddition The probabilityof eitherof two mutuallyexclusiveeventsoccurring probabilityof one event+
ofthe otherevent
For example,the probabilityof rolling a die and gettingeithera 3 or a 4 l l 6 + l 1 6 : 2 1 6o r Figure 5.3: Probability Mendelcarriedout his experiments on peaplants,for whichthe allelefor purpleflowers is dominant (P) andthe allelefor whiteflowersis recessive(p).lf the offspringof crossed peaplantsis homozygousdominant.it inheritstwo dominantalleles(PP) andthedominant trait (purpleflowers)is cxhibited.If it is homozygousrecessive,it inheritstwo recessive trait(whiteflowers)is exprcssed. If it is heterozygous,it inheralleles(pp)andtherecessive (Pp) the its onedominantalleleanclonerecessive allcle andonly dominanttrait is expressed. The genotype refersto an organism's entiresetof alleles.The phenotypeis how a geneis in physicalappearance, expressed suchasa purplcfloweror blueeyes.Think ofthe genotype asthe blueprintandthe phenotype asthe finalvisibleproduct. In heredityexperiments, the parentsare calledthe P generation.The offspringfrom the The offspringfrom crossingsamongthe crossingof the parentorganismsis the F, generation. F i s c a l l e dt h e F . g e n e r a t i o n .
Chapter 5: Evolution
Mendel'slawsof heredityand othergeneticprinciplesincludethe following. '
The [aw of dorninar':i:{:i saysthat whentwo organisms,eachof which is homozygous for two differenttraits (suchas purple flowersand white flowers),are crossed.the offspringproducedwill be !-:'yhrt!(with two differentalleles)but will expressonly the dominantallele.Thus,Mendelobservedthat hybridplantsdid not resultin a blending of traits(suchas mottledpurple-and-white flowers),but insteadthe plantsinherited trait each intact.The recessivetrait that is presentbut not expressedis said to be latent.
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The law of segregatton statesthat the alternateallelesfor a trait segregate (or separate)from eachotherduringthe formationof gametesand thus remaindistinct.This law is bestdemonstrated i.i{):::;. by the ilri.rni'nrih:r/-r This is a crossbctweentwo organismsthat eachare hybrid forone trait, a situationshownas (Ppx Pp). Mendelfound thatthephenotvperatio for thistypeof crossis always3 dominantto I recessive. He alsofoundthat the genotvpe ratio is alwaysI to 2 to I or in percentage terms,25 percenthomozygousdominantto 50 percentheterozygous to 25 percenthomozygous recessive. The monohybridcrosscan be illustratedby a Punnettsquare,as in Figure 5.4.A Punnettsquareshowsall thepossibleoutcomesof a geneticcrossandhelpsin figuringthe probabilityof a particularoutcome.
PP
Pp
Pp
pp
Figure 5.4: Prinnett Squarefor a Monohybrid Cross The testcross is a methodof findingthegenotypeof an organismshowingthe dontinant trait. For example,a purple-floweredpea plant can be crossedwith a whitefloweredpeaplant,a plantknownto be homozygousrecessive. If all theoffspringhave purple flowers.then the originalplant must be homozygousdominant.If half of the offspringhavepurpleflowersand half havewhite flowers,thenthe originalplantmust be heterozygous dominant. The law of indepenclentas:,,crtmentappliesto a crossbetweentwo organismsthat arehybrid for two or moretraits.Forexample,peaplantseedscanbe yellow ( )') or green (1,)andtheirtexturecanbe round(R) or wrinkled(r). This situationis calleda d!h1,L:rid cross.Thephenotype ratiofor the l6 possibleoutcomesof thedihybridcrossis 9:3:3:1. In otherwords,for two pea-plantseedtraitsthat behaveindependently, the possibleoutcomesare9 round-yellow,3 round-green,3 wrinkled-yellow,and I wrinkled-green.The Punnettsquarefor this dihybrid crossis shownin Figure5.5.
RY
Ry
rY
ry
RY
RR YY
RR Yy
Rr YY
Rr Yy
Ry
RR Il
RR y l '
Rr Yy
Rl lt
rY
Rr YY
Rt'Yt'
rr YY
rr Yy
ry
Rr Yt,
Rr yy
rr Yv
ff 1:1:
Figure5.5: PunnettSquarefor a Dihybrid Cross patternin whichbothinheritedallelesareexpressed. Codominanceis an inheritance A good exampleof codominance arethe MN blood groupsfound in humans. Incomplete dominanceis whena gene'sallelesarenot dominantand recessive but insteadproducea blendingof the individualtraits.Usingsnapdragons as an example. (R/ crossedwith a white snapdragon (r'l producesa pink snapdragon a red snapdragon /Rr), which indicatesthat neitherthe red nor white color was dominant.Thus.the three kinds of inheritanceform a continuumfrom complctedominance(dominant (bothallelesexpressed) alleleoverrecessive allele)at oneendto codominance at the (blendingof two alleles)lyingbetweenthetwo otherend with incomplete dominance extremes. Multiple alleles is a situationin whichtherearemorethantwo possibleallelesfor a gene.(Mostgenesin a populationhavetwo alleles,onedominantandone recessive.) Forthe ABO bloodgroLlpin humans,therearethreealleles.A, B, andO. whichproto form 4 bloodtypes.TheA and B allelesarecodominant. duce6 possiblegenotypes This situationn.leans thatonly the O bloodtype is a "universaldonor"thateveryperson'.s immunesystemcantolerate. A personwith bloodtypeA, B, or AB mustreceive the sameblood type in transfusionor elsehis or her immunesystemwill respondby attackingthe antigensof the foreign blood type. This resultsin agglutination, or clumping.whichcanbe fhtal. Humanshave46 chromoSex-linkagerefersto linkedgeneson the X chromosome. somes.44 of whichare autosomesarid2 of which are sex chromosomes.The sex chromosomes areX andY,with mostof the genescarriedon the X chromosome, also called the sex chromosome.Femaleshavetwo copiesof the X chromosome(XX), (XY). Sex-linked traits while maleshaveone X chromosome andoneY chromosome A sex-linkedgeneticdefectsuchas hemophilia(ft/, arecarriedon the X chromosome. a diseasein which blood doesnot clot properly,occursonly in a femalewho hastwo copiesof the defectivegene(XX). A femalewith only onecopyof the defectivegene (XtX', with H a healthygene)will not havehemophiliabut is a carrierand can pass on the defectivegeneto offspring.Hemophiliaoccursin maleswho inheritonly one
Chapter 5: Evolution
copy of the defectivegene(XnY). For this reason,sex-linkedgeneticdefectsappear muchmore frequentlyin males.Othersuchconditionsdueto genemutationarecystic fibrosis,Huntington's disease, color-blindness, andsicklecell disease. Polygenic inheritance is the interactionof multiple genesto fbrm a singlephenotype.For example,heightin humansis not limitedto beingeithershortor tall but is insteada continuous variation, or wide range,from very shortto extremelytall. The oppositeof polygenicinheritanceis pleiotropy, in which one generesultsin several phenotypes.
