Bio 321 F13 lecture 2 v2

Ra7os of a monohybrid cross th postulate mendels

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Unformatted text preview: ent assortment •  each chromosome is inherited independently from other chromosomes –  among the A gametes, 50% are B and 50% are b –  among the a gametes, 50% are B and 50% are b –  etc. •  unliked loci are inherited independently of each other –  we will discuss linkage in a future lecture One of Mendel’s dihybrid crosses YYRR yyrr YYrr yyRR YyRr YyRr YYRR, YYRr, YyRR, YyRr YYrr, Yyrr YyRr yyRR, yyRr yyrr this is a Punnett square for a dihibrid cross parent 1 gamete types parent 2 gamete types YR 25% Yr 25% yR 25% yr 25% YR 25% 1/16 YRYR 1/16 YRYr 1/16 YRyR 1/16 YRyr Yr 25% 1/16 YrYR 1/16 YrYr 1/16 YryR 1/16 Yryr yR 25% 1/16 yRYR 1/16 yRYr 1/16 yRyR 1/16 yRyr yr 25% 1/16 yrYR 1/16 yrYr 1/16 yryR 1/16 yryr parent 1 gamete types YR 25% Yr 25% yR 25% yr 25% 1/16 YYRR 1/16 YYRr 1/16 YyRR 1/16 YyRr Yr 25% 1/16 YYRr 1/16 YYrr 1/16 YyRr 1/16 Yyrr yR 25% 1/16 YyRR 1/16 YyRr 1/16 yyRR 1/16 yyRr yr 25% parent 2 gamete types YR 25% 1/16 YyRr 1/16 Yyrr 1/16 yyRr 1/16 yyrr where do the ratios come from? count up the squares you can also calculate the ra5os this way a trihybrid cross (#s are phenotype frequencies) In class exercise 2 •  In the following trihybrid cross: –  grandparent 1: DDYYRR •  tall plant, yellow seeds, round seeds –  grandparent 2: ddyyrr •  short plant, green seeds, wrinkled seeds •  What propor7on of F2 offspring would be tall, green seeds, round seeds 3 loci, 2 alleles => 8 gamete types •  •  •  •  •  •  •  •  •  DYR DYr DyR Dyr dYR dYr dyR dyr => 64 cell PunneR square using the method for phenotypic ra7os from 2 slides ago •  (genotypic ra7os could be done in a trihybrid PunneR square; 8 gamete types; 64 cells) •  ¾ are tall (DD or Dd) •  ¼ are green (yy) •  ¾ are round (RR or Rr) •  ¾ x ¼ x ¾ = 9/64 when you have cross data •  How do you test whether the underlying gene7c basis is a Mendelian locus? •  e.g. in a monohybrid cross for the plant height locus DD, Dd = tall, dd = short •  say we recovered: •  66 tall plants, 20 short plants in the F2 •  does this fit a Mendelian locus with dominance? •  need a “goodness of fit” sta7s7cal test we are formally tes7ng a hypothesis •  the Mendalian ra7o can be t...
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This note was uploaded on 10/01/2013 for the course BIO 321 taught by Professor John during the Spring '12 term at SUNY Stony Brook.

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