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Polygenic Inheritance and Epistasis Monohybrid Cross P: Round vs Wrinkled Peas RR rr F1: All Round Rr 3/4 Round: 1/4 Wrinkled 1/4RR 2/4Rr 1/4rr F2: Dihybrid Cross P: Round, Green vs Wrinkled ,Yellow RRyy rrYY F1: All Round,Yellow F2: 9/16 Round,Yellow, 3/16 Round Green, 3/16 Wrinkled Yellow, 1/16 Wrinkled Green RrYy Punnett Square RRYY RRYy RRyy RrYY RrYy Rryy rrYY rrYy rryy 1/16 2/16 1/16 2/16 4/16 2/16 1/16 2/16 1/16 RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy Genotype/Phenotype RRYY 1/16 RRYy 2/16 RRyy 1/16 F2: RrYY 2/16 RrYy 4/16 Rryy 2/16 9/16 Round,Yellow, 3/16 Round Green, 3/16 Wrinkled Yellow, 1/16 Wrinkled Green rrYY 1/16 rrYy 2/16 rryy 1/16 9/16 R_Y_ 3/16 R_yy 3/16 rrY_ 1/16 rryy Epistasis •2 (or more) gene pairs cause 1 trait •One gene pair modifies or hides the effect of another Epistasis Example 1 P: Purple Sweet Pea vs White Sweet Pea F1: F2: All Purple 9/16 purple 7/16 white precursor intermediate anthocyanin molecule enzyme molecule enzyme pigment C P no color no color purple Alleles C_P_ C-enzyme made c - no C_pp P-enzyme made p - no ccP_ ccpp Epistasis Example 1 P: Purple Sweet Pea vs White Sweet Pea F1: F2: All Purple 9/16 purple 7/16 white 9/16 C_P_ purple 3/16 C_pp white 3/16 ccP_ white 1/16 ccpp white Need 1 dominant at each locus for expression Epistasis Example 2 agouti precursor enzyme intermediate enzyme no color black A B Alleles AAbb aaBB A-enzyme a - no B-enzyme b - no Homozygous recessive at one locus prevents expression at other Epistasis Example 2 P: Agouti F1: F2: 9/16 A_B_ 3/16 A_bb 3/16 aaB_ 1/16 aabb vs All Agouti White Bombay Phenotype A or B protein precursor substance H blood type H-enzyme A,B h - no i - no type O type A HHAB HHii hhAA hhBi type AB Summer Squash Color Dominant at one locus prevents expression other locus B-yellow, b-green unless A - white P: AABB x aabb F1: AaBb F2 : 9/16 A_B_ 3/16 A_bb 3/16 aaB_ 1/16 aabb aaB_ yellow aabb green A ___ white Summer Squash Shape One dominant has different effect than 2 dominants ccdd long no dominants - long Ccdd sphere 1 dominant - sphere CcDD disk 2 dominants - disk P: CCDD x ccdd F1: CcDd F2 : 9/16 C_D_ 3/16 C_dd 3/16 ccD_ 1/16 ccdd Polygenic Traits •2 (or more genes) determine trait • Each dominant allele contributes quantitatively Polygenic Cross P: F1: F2: Red Wheat vs White Wheat All Pink Wheat 1/16 red 4/16 light red 6/16 pink 4/16 light pink 1/16 white Two gene pairs Alleles A,a,B,b Each dominant allele contributes to color Genotype AABB 1/16 AABb 2/16 AAbb 1/16 AaBB 2/16 AaBb 4/16 Aabb 2/16 4 dominant alleles? 3 dominant alleles? 2 dominant alleles? 1 dominant alleles? 0 dominant alleles? aaBB 1/16 aaBb 2/16 aabb 1/16 Other Polygenic Traits • Height in plants • Weight • Human skin and eye color AABBCC - very dark skin aabbcc - caucasian AABbcc or AaBbCc - medium skin color The End
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