Punnett Squares: Genetic Crosses

Punnett Squares:Genetic CrossesMaking Genetic PredictionsMendel found that you could predict thetraits of a percentage of the offspring. He invented and used the tool of a Punnettsquare to illustrate these percentages.Punnett SquaresThe Law of Dominance Traits are shown with a Capital letter for dominant and a lower case for recessive Brown Eyes (B) Blue Eyes (b)Punnett SquaresEye Color Brown eyes B Blue eyes bPurebred individual with brown eyes (BB) HomozygousPurebred individual with blue eyes (bb) HomozygousHybrid individual with brown eyes (Bb) HeterozygousPunnett SquaresEye Color Father is (BB) Mother is (bb)Their Genes Are Crossed BB x bb Since we are looking at only 1 gene, it is called a monohybrid cross.Law Of SegregationLaw of Segregation Alleles for the same trait must be in separate sex cells. Father is (BB) as his diploid genotype Mother is (bb) as her diploid genotypeSomatic Cells BB bb Sex cells B B b bMonohybrid CrossesMonohybrid cross deals with one gene We are working with only eye color Setting up a punnet squareThe sex cells go on the outside of the squarePunnett Squares BBb

b

Bb = BrownAll of theoffspring willhave brown eyes,but they will beheterozygousindividuals.Punnett SquaresEye Color Father has Brown eyes and is heterozygous (Bb). Mother has Brown eyes and is heterozygous (Bb).Punnett SquaresBBBbBbbbB

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BbBB = BrownBb = Brownbb = BlueRatiosGenotype Ratio 1 BB to 2 Bb to 1 bbPhenotype Ratio 3 Brown eyes to 1 blue EyesWhat is the chance of this couple having a child with blue eyes? 1/ 4 or a 25 % chancePunnett SquaresOne parent is heterozygous for the trait the other parent is homozygous Eye Color Father (Bb) Brown Mother (bb) bluePunnett SquaresBbbbBbbbb

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BbRatiosGenotype Ratio 0 BB to 2 Bb to 2 bb (reduce) 0 BB to 1 Bb to 1 bbPhenotype Ratio 2 Brown eyes to 2 blue eyes (reduce) 1 Brown eye to 1 blue eye What is the chance of this couple having a child with blue eyes? or 50%Mendels WorkPea Color:Yellow: YGreen: yP GenerationYY x yyMendels WorkCrossed two of the F1 tosee what the F2 wouldlook like.Multiple CharacteristicsPunnett Squares work with multiple characteristics.Crosses can be done with an assortment of genes.Dihybrid CrossesThe Parents are heterozygous for both hair and eye color (BbEe: Brown hair and Brown Eyes). B: Brown hair - E: Brown eyes b: Blonde hair - e : Blue eyes Cross = BbEe x BbEeMendels LawsThe law of segregation You cant have the same letters in a Sex cellThe law of independent assortment The two genes will have their alleles placed into the same sex cells randomly.Work with one parent at a time BbEe (B b) (E e) Possible sex cells (BE) (Be) (bE) (be)BE Be bE be BE

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Phenotype RatiosPhenotypes Brown Hair Brown Eyes _________ Brown Hair Blue Eyes _________ Blonde Hair Brown Eyes _________ Blonde Hair Blue Eyes _________Phenotype RatiosPhenotypes Brown Hair Brown Eyes ____9____ Brown Hair Blue Eyes ____3____ Blonde Hair Brown Eyes ____3____ Blonde Hair Blue Eyes ____1____Phenotype PercentagesPhenotypes Brown Hair Brown Eyes ____56%___ Brown Hair Blue Eyes ____19%___Blonde Hair Brown Eyes ____19%___ Blonde Hair Blue Eyes ____6%____