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Dihybrid Crosses and Law of Independent Assortment Monohybrid cross • Mendel’s experiments followed the inheritance of single characters – E.g. flower color, seed color , etc. – monohybrid crosses Law of Segregation Review – During gamete formation, alleles segregate (separate) • only one of two allelles for a gene is put into a gamete. – At fertilization, offspring receive one allele for each gene from each parent. – alleles are on different chromosomes! Dihybrid cross • Other of Mendel’s experiments followed the inheritance of 2 different characters – seed color and seed shape – dihybrid crosses Mendel was working out many of the genetic rules! Dihybrid cross P true-breeding yellow, round peas Y = yellow R = round x YYRR yyrr true-breeding green, wrinkled peas y = green r = wrinkled yellow, round peas F1 100% generation (hybrids) YyRr self-pollinate F2 generation 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas What’s going on here? • If alleles (different characteristics) are on different chromosomes… – Are alleles transmitted from parents to offspring together or independently? – Will the Y and R alleles always stay together, generation after generation? Is it this? YyRr YR Y and R alleles transmitted together yr Or this? YR YyRr Yr Which system explains the data? yR Y and R alleles transmitted independently yr Is this the way it works? YyRr YyRr YR x YyRr or yr What was actually observed YR YR yr yr YYRR YyRr YyRr yyrr Well, that’s NOT right! YR YyRr Yr yR yr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled YyRr Dihybrid cross YyRr x YyRr YR YR YR YYRR Yr YYRr yR YyRR YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr Yyrr yyRr YR yyrr Yr yR yr 9/16 yellow round 3/16 green round YyRr YYRr YyRr yr yr Yr yr or YyRr BINGO! 3/16 yellow wrinkled 1/16 green wrinkled Can you think of an exception to this? Mendel’s 2nd law of heredity • Law of independent assortment yellow green round wrinkled – Each pair of alleles (different characteristics) segregate independently • non-homologous chromosomes align independently • only true if alleles (different characteristics) are on separate chromosomes (e.g. gene for colour on separate chromosome as gene for texture) YyRr Yr Yr yR yR YR YR yr yr Law of Independent Assortment Which stage of meiosis creates the law of independent assortment? Remember Mendel didn’t even know DNA —or genes— existed! Metaphase 1 EXCEPTION If genes are on same chromosome & close together will usually be inherited together rarely crossover separately “linked” Sample Problem In pea plants, the green colour allele (G) is dominant over the yellow colour allele (g) for seed colour and tall (T) is the dominant plant height. A pea plant that is heterozygous for both seed colour and height is crossed with a plant that is homozygous recessive for both. Determine the genotypic ratio for their offspring. Solution GgTt 1:1:1:1 x ggtt GT Gt gT gt gt GgTt Ggtt ggTt ggtt gt GgTt Ggtt ggTt ggtt gt GgTt Ggtt ggTt ggtt gt GgTt Ggtt ggTt ggtt 4/16 = 1/4 green tall 4/16 = 1/4 green short 4/16 = 1/4 yellow tall 4/16 = 1/4 yellow short • What is the probability of getting an offspring that are heterozygous for both characters? A: They must be GgTt 4/16 = ¼ or 25% Any Questions? 1. Bikini Bottom 2 WS 2. Pg. 135 # 10-14 (Mono) 3. Pg. 141 #1,2,4-9 (Di) Homework Quiz next class on Mono and Dihybrid Crosses! 2005-2006
