Unit3:GeneticsCalculatingthe
probabilityofourgenetics
UnderstandinghowourgenesarepasseddownAndhowtocalculatetheprobabilitiesofourtraits.
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Leading questions:
1. What do Punnett Squares mean?
2. How do we calculate the probabilities of our traits?
3. What traits will pass down and by how much?
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Do you remember these?
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
What do the y and Y represent and where are they from?
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Recall from meiosis
• A pair of homologous chromosomes (chromosomes of similar traits) are crossed over and separated.
• The gene that results in the sex cells are the “letters” from Punnett Squares.
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
What does that mean?If the red chromosome containsthe genes for Brown eyes (B).
If yellow contains genes for Blueeyes (b)
After crossing over, the genesswitch chromosomes with theother.
bB
bb
B
B
Sisterchromatids
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
What does that mean?• At the end of
meiosis, all 4 ofthese sisterchromatids willseparate (segregate) into 4 sex cells.
• Because theprobability of gettingB and b is the same, that is why we useBb!
bb
B
B
B
bb
bB
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
When the genes ofthe homologouschromosomesseparate into separatesex cells, Mendelclassified this as the“Law of Segregation”.
Thiswillresultinasexcellwithtraitsthatareeitherfromyourmom(browneyes)ordad(blueeyes).
Weusesymbolstorepresentdominantandrecessivegenes.• Thedominantgene isalwaysrepresentedbyacapitalletter.
• E.g.A
• Therecessivegeneisalwaysrepresentedbythesameletterinthelowercase.
• E.g.a
Towritethecombinationofatrait,twolettersmustbeused.Ie.AA,aa,Aa
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Example
Brownhairisadominantgene
Blondehairisarecessivegene
Brownhair’sgenewillbeexpressed becauseitisthedominant gene
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Otherimportantterms• Alleles - Alternativeformsofageneforagiventrait(Bluevsbrowneyes)
• Genotype – theallelespossessedbyanindividualforaspecifictrait.
• Eg:BB,Bb,andbbarepossiblegenotypesforeyecolour.
• 2copiesofdominantallele=homozygousdominantgenotype(BB).
• 2copiesofrecessiveallele=homozygousrecessivegenotype (bb).
• 1copyofeach=heterozygous genotype(Bb).
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
• Phenotype – thephysicalappearanceofatrait(ie:whatthegenesactually“do”tous).
• Ex.Blueeyes,short,longtoenails,Extrahormones.
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Pforphysicaltraits!
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Howdowecalculatetheprobabilityofourgenes?
Monohybrid Crosses• Across(combinationofalleles) foronetrait =amonohybridcross.
• Eg:Ifaheterozygouswomanwithawidow’speakreproduceswithaman withthesame genotype,whatkindofhairlinewilltheirchildrenhave?
• (Note:widow’speakisdominant overstraighthairline).
Unit3:GeneticsMonohybridcrosses
• Youmayalsobeaskedtodeterminegenotypicratiosandphenotypicratios intheoffspring:
Genotypicratios:1WW:2Ww :1ww (or,moresimply,1:2:1)
Phenotypicratios:3widow’speak:1straighthairline(3:1)
**Phenotypicratioscanalsobewrittenaspercentages:25%chanceofstraighthairline75%chanceofwidow’speak
Unit3:GeneticsMonohybridcrosses
W w
W WW Ww
w Ww ww
Whatifsomeoneshowsthedominantphenotype,butwedonotknowiftheyarehomozygousdominant orheterozygous?• Wecandoatestcross:
• Crosstheindividualwiththeunknowngenotypewithonethatishomozygousrecessive.
• Whyhomozygousrecessive?
Unit3:GeneticsTestcross
Example:Akidhasbrowneyes(dominanttrait)butwedon’tknowifheisheterozygousorhomozygousdominant.Howcanwedeterminehisgenotype?
Unit3:GeneticsTestcross
B bb Bb bbb Bb bb
Kid’sgenotype
B Bb Bb Bbb Bb Bb
Kid’sgenotype
UseatestcrossThekid’sgenotypecouldbe2possibilities.IfALLoftheoffspringinthetestcrossshowthedominanttrait,thenthekid’sgenotypeishomozygousdominant.
InBorderCollies,blackcoat(B)isdominant toredcoat(b). Abreederhasablackmale thathaswonnumerousawards. ThebreederwouldliketousethedogforbreedingifheispurebredorBB. Tolearnthisinformation,shetestcrosseshimwitharedfemale(bb).
Unit3:GeneticsTestcross
1.Iftheblackmaleis BB,whatkindofgamete(sperm)canheproduce?
2.Iftheredfemaleis bb,whatkindofgamete(eggs)cansheproduce?
B
b
InBorderCollies,blackcoat(B)isdominant toredcoat(b). Abreederhasablackmale thathaswonnumerousawards. ThebreederwouldliketousethedogforbreedingifheispurebredorBB. Tolearnthisinformation,shetestcrosseshimwitharedfemale(bb).
Unit3:GeneticsTestcross
2.Ifanyofthepuppiesarered,whatisthefather'sgenotype?
Blackmalecouldbe:
BBorBb
Testwithbb
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Trysomequestionsnow!
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
PunnettSquareswithBloodtypes• Thereare4mainbloodtypesin
humans:A,B,AB,andO
• BloodtypeAandBaredominant,TypeOisrecessive.
• WeuseIA toindicateadominantAallelegeneandIBforalleleBblood.
• iirepresentsrecessiveforOblood.
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
PunnettSquareswithBloodtypes
• InordertogettypeABblood,bothdominant allelesofAandBmustbeexpressedtogether.Theyareco-dominant(seelater).
• Ex.IAIB
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
BloodtypeExample• CouldamanwithtypeBbloodandawomanwithtype
ABproduceachildwithtypeOblood?
• 2possibilitiesfortypeBblood:IBIB orIBi
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
BloodtypeExample• CouldamanwithtypeBbloodandawomanwithtype
ABproduceachildwithtypeOblood?
• Sincethere’snohomozygousrecessive(ii),TypeObloodisnotpossible.
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
BloodtypeExample1.WhatifthemotheristypeOandthefatherisA?Whatwouldtheoffspring'sbloodtypebe?
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
BloodtypeExample2. A woman with Type O blood and a man who is Type AB have are expecting a child. What are the possible blood types of the kid?
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Sex-linkedproblemswithPunnettSquares• Traitscontrolledbygenesonthesexchromosomesare
calledsex-linkedtraits.• WeusuallyfocusontraitscontrolledbytheX
chromosome• ThisisbecauseithasmanymoregenesthantheY
chromosome!
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Sex-linkedproblemswithPunnettSquares• Notationforsex-linkedgenes:• GenescarriedontheXchromosome:
• UsetheletterXtoindicatethatthegeneisontheXchromosome
• Useasuperscript letterforthetraititself.• Eg:XB =dominantgenefornormalcolourvision
Xb =recessivegeneforcolour-blindness
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Sex-linkedproblemswithPunnettSquaresExamples:Red-greencolour-blindness(X-linked)
XB =normal;Xb =colour-blind
Genotype Phenotype
XBXB Female; normalvision
XBXb Female;normalvision(carrier)
XbXb Female; colourblind
XBY Male;normalvision
XbY Male;colourblind
Noticehowthegenotypesarewrittenandwhatthephenotypesare!
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Sex-linkedproblemswithPunnettSquares• Note:Becausemalesonlyhaveone Xchromosome,
theycannotbecarriers ofsex-linkeddisordersorconditions.
• Theyeitherhavethegeneandshowthecondition,ordon’thaveitanddon’tshowthecondition.
Genotype Phenotype
XBXB Female; normalvision
XBXb Female;normalvision(carrier)
XbXb Female; colourblind
XBY Male;normalvision
XbY Male;colourblind
Cannotbeacarrier!!!
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Sex-linkedproblemswithPunnettSquaresExamplescont’d:• Hemophilia(deficiency
inbloodclottingfactors)• Duchennemuscular
dystrophy• (Theseareallrecessive
conditions)
Sex-LinkedInheritancePracticeProblems• Boththemotherandthefatherofacolorblindmaleappeartobenormal.Colourblindness isrecessive.Fromwhomdidthesoninheritthealleleforcolorblindness?Whatarethegenotypesofthemother,father,andtheson?
Sex-LinkedInheritancePracticeProblems• Awomaniscolorblind.Whatarethechancesthathersonwillbecolorblind?Ifsheismarriedtoamanwithnormalvision,whatarethechancesthatherdaughterswillbecolorblind?Willbecarriers?
Sex-LinkedInheritancePracticeProblems• Boththehusbandandthewifehavenormalvision.Thewifegivesbirthtoacolorblinddaughter.Isitmorelikelythefatherhadnormalvisionorwascolorblind?Whatdoesthisleadyoutodeduceaboutthegirl’sparentage?
Sex-LinkedInheritancePracticeProblems• Whatisthegenotypeofacolorblindmalewithlongfingersiss=longfingers?Ifallhischildrenhavenormalvisionandshortfingers,whatisthelikelygenotypeofthemother?
Unit3:GeneticsTracingx-linkedwithpedigree
PedigreeCharts!
PedigreeChartstrackingx-linkeddisorders
Unit3:GeneticsTracingx-linkedwithpedigree
PedigreeCharts!Mydog’sPurebredPedigree:http://www.k9data.com/pedigree.asp?ID=116679
Pedigreechartstracksthegenetictraitsovergenerationssimilartoafamilytree.
SquaresindicateMALE
CirclesindicateFEMALES
Unit3:GeneticsTracingx-linkedwithpedigree
Example1:Leopold’sfamily(QueenVictoria’sson)
(a)Whatistheprobabilitythatherothersonwashemophilic?
(b)Whatistheprobabilitythatherdaughterwasacarrier?Hemophilic?
(c)Whatistheprobabilitythatbothchildrenwerenormal?
Unit3:GeneticsTracingx-linkedwithpedigree
Example1:Victoria’syoungestdaughterBeatrice’sfamily
(a)Lookingatthepedigreeoftheroyalfamily,identifywhichofBeatrice’schildrenreceivedthehemophilicgene;whycanyoumakethisconclusion?
(b)NoticethatBeatrice’sdaughter,Eugenie,marriedKingAlfonsoXIIIofSpainandhadsixchildren,oneofwhomwasthefatherofJuanCarlos,thecurrentKingofSpain.WouldyoupredictthatJuanCarloswasnormal,acarrier,orahemophilic?
Unit3:GeneticsTracingx-linkedwithpedigree
DrawingExample
Amanandwomanmarry.Theyhavefivechildren,2girlsand3boys.Themotherisacarrierofhemophilia,anX-linkeddisorder.Shepassesthegeneontotwooftheboyswhodiedinchildhoodandoneofthedaughtersisalsoacarrier.Bothdaughtersmarrymenwithouthemophiliaandhave3children(2boysandagirl).Thecarrierdaughterhasonesonwithhemophilia.Oneofthenon-carrierdaughter’ssonsmarriesawomanwhoisacarrierandtheyhavetwindaughters.
Whatisthepercentchancethateachdaughter(twins)willalsobeacarrier?
Unit3:GeneticsDihybridCrosses
DihybridCrosses(2alleles)!
Mendel’s2nd LawistheLawofIndependentAssortment:
• TheLawofIndependentAssortmentiswhenthereare2differenttraits onthehomologouschromosome(eyecolourandhaircolour)
• Eachofthesetraitswillindependently separate.
• Forexample:Bb(eyes)andHh (hair)willseparateintoBHBh Hb andhb.
• Theimportantpartisthatthetraitsdonot“sticktogether”
Unit3:GeneticsDihybridCrosses
Unit3:GeneticsDihybridCrosses
Thisleadsustodihybridcrosses
Trackingtheinheritanceof2alleles
Ex.ApersonwhoisWwSs (widow’speakandshortfingers)andapersonwhoisalsoWwSs havechildren:
1. Figureoutthegametesforeachparent• WS,Ws,wS,andws forboth• PlacethesecombinationsontheedgesofaPunnettsquare
2. Figureoutthegenotypesoftheoffspring(useaPunnettsquare)
Unit3:GeneticsDihybridCrosses
FromthePunnettsquare:• Phenotypicratio - 9widow’speak,shortfingers;
• 3widow’speak,longfingers;
• 3straighthairline,shortfingers;
• 1straighthairline,longfingers(9:3:3:1)
Tocalculatethechancesofhavingtwotraits,youmustmultiply theprobabilitiesofhavingeachindividualtraitandreporttheresult.
• Eg:Fromthepreviousproblem,whatarethechancesofhavingachildwithshortfingers andawidow’speak?
• Chanceofshortfingers=¾• Chanceofwidow’speak=¾• Chanceofhavingboth=¾x¾=9/16
Unit3:GeneticsDihybridCrosses
Unit3:GeneticsCalculatingtheprobabilityofourgenetics
Trysomequestionsnow!