Dihybrid Crosses Involve 2 traits Eg. Crossing tall
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Dihybrid Crosses • Involve 2 traits • Eg. Crossing tall yellow pea plants with short green pea plants • Procedure is same as with monohybrids • Must first find gametes • Then cross in punnett square • Then show gametes coming together, making offspring! Working out Gametes in Dihybrid Crosses TTYY TY TY TY TY Working out Gametes in Dihybrid Crosses TtYY TY TY tY tY Working out Gametes in Dihybrid Crosses TtYy TY Ty tY ty TtYy X TtYy TY Ty tY ty TY TTYY TTYy TtYY TtYy Ty TTYy TTyy TtYy Ttyy tY TtYY TtYy ttYY ttYy ty TtYy Ttyy ttYy ttyy Dihybrid Cross 9:3:3:1 Pheno Ratio Dihybrid crosses involve 2 traits P Generation P Gametes F1 Generation Predict the phenotypic outcome if a hetero tall, homozygous yellow pea plant is crossed with TTYY TY tY TY TTYY TtYY TY TTYY TtYY Phenotypes Genotypes All Tall &Yellow 2 TTYY & 2 TtYY A dwarf, red snapdragon is crossed with a homozygous tall white plant. Determine the pheno and geno of the F1’s. (Tall is dom, red incompletely dom to white) Genotype of Dwarf, red snapdragon ttRR Genotype of Homozygous Tall white snapdragon TTWW Gametes from Parents tR X TW tR TW TtRW Phenotype of offspring: all Tall & Pink Genotype of offspring: TtRW Summary of Dihybrid Cross • Most complex punnett square involves 2 parents heterozygous for both traits • • • • eg. RrBb X RrBb 4 possible gametes from each parent Punnett square is 4 X 4 Classic 9:3:3:1 phenotypic ratio in offspring • If parents are RRBb and RrBB….. • Only 2 possible gametes from each parent (RB or Rb) and (RB or rB) • Punnett square becomes 2 X 2 instead of 4 X 4! RB Rb RB RRBB RRBb rB RrBB RrBb In poultry, a crested head is produced by a dom. allele and a plain head by it’s rec allele. Black feather is dom to red. A homozygous red feathered, hetero. crest headed bird is mated to a hetero. black, plain headed bird! Find the pheno’s and geno’s of the F 1’s. C = crested c = plain B = black b = red bC bc Bc BbCc Bbcc bc bbCc bbcc Phenotypic Ratio 1:1:1:1 Parental Genotypes bbCc & Bbcc Genotypic Ratio 1:1:1:1 Gametes would be abc & ABC All offspring: AaBbCc Gametes would be ABC, AbC, aBC, abC ABc, Abc, aBc, abc Trihybrid Cross YIKES!! Working out Gametes!! n 2 where n = number of different sets of alleles n= 1 for mono n= 2 for dihybrid n= 3 for trihybrid Lethal Alleles Dominant allele is Lethal! Mexican hairless dog!! Hh Gene Interaction 1.Polygenic Inheritance – Where a trait is controlled by more than one gene • Skin color • Eye color • Height • Example: Coat • B = black coat • b = brown coat color in dogs ALSO W = prevents pigment w = does not prevent color W is a separate gene located on a separate chromosome W allele masks the effect of the B gene 2. Epistatic Interaction Genes that mask the expression of other genes Dog example continued black phenotype • wwBb genotype = ______ white phenotype • WwBb genotype = ______ brown phenotype • wwbb genotype = _______ 3. Complementary Interaction • Occurs when two genotypes combine to create a phenotype that neither can produce alone Genotype A + Genotype B Chicken Example R = rose comb P = pea comb R + P = walnut r + p = single comb = Phenotype C Walnut Rose Pea Single Question on Complimentary Interaction • Find the F1’s and the F2’s for the following parents: • RRpp x rrPP • Gametes would be Rp x rP • All offspring would be RrPp • Phenotype: all walnut • Must use punnett square for second cross!! RrPp X RrPp RP Rp rP rp RP RRPP RRPp RrPP RrPp Rp RRPp RRpp RrPp Rrpp rP RrPP RrPp rrPP rrPp rp RrPp Rrpp rrPp rrpp Dihybrid Cross 9:3:3:1 Pheno Ratio Walnut Rose Pea Single Pleiotropic Genes • Affect many characteristics throughout body • Sickle Cell Anemia – mutations can lead to fatigue and weakness, enlarged spleen, rheumatism, pneumonia • Sickle cell disease (easy to follow info!) Notice sickle shape of cells…can’t carry enough oxygen & get stuck in caps Blood smear showing sickle cell anemia Normal & Abnormal Red Blood Cells Pleiotropic Genes • Eg. Marfan’s Syndrome – inability to produce connective tissue – eye, skeleton and cardiovascular defects show up Marfan’s Syndrome Abraham Lincoln Symptoms of Marfan’s?? Marfan’s Syndrome Long thin limbs & Abnormal joints Environment and Phenotype • Primrose flowers • Red if raised at room temp • White when raised above 30 0 C Effect of Environment on Phenotype Submerged leaves are finely divided, Compared with the ones Growing in air Sun lightens hair & Darkens freckles Genetics Problems 1:Dihybrid crosses Practice Genetics Problems More Practice!
