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Genetics: Dihybrid Crosses

Genetics: Dihybrid Crosses. Biology 12. Joke of the day…. What is a dihybrid cross?. These are crosses that involve genes for two different characteristics. Back to Mendel and his peas…. Mendel: Dihybrid Crosses.

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Genetics: Dihybrid Crosses

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  1. Genetics: Dihybrid Crosses Biology 12

  2. Joke of the day…

  3. What is a dihybrid cross? • These are crosses that involve genes for two different characteristics

  4. Back to Mendel and his peas…

  5. Mendel: Dihybrid Crosses • The natural progression for Mendel was to study two characteristics at the same time. • Thus, the study of 2 pairs of contrasting traits at the same time = a dihybrid cross • ex.. round yellow seeds X wrinkled green seeds

  6. Review: Mendel: • Principle of Independent Assortment • When more then one trait is studied in the same cross, the genes for each trait sort into gametes independently of the genes of the other traits

  7. Mendel’s peas: Let’s cross a homozygous round/yellow (AABB) with a homozygous wrinkled/green (aabb) plant….

  8. Steps for Dihybrid Crosses: • Determine Genotypes of Parents • Determine Genotypes of Gametes (remember gametes only have 1 copy of each gene) and think FOIL (from math class!) • Punnett Square: need 16 squares now, 4 gametes from each parent • Identify Phenotype Ratios in Offspring

  9. Example: • In snapdragons… • Tallness (T) is dominant to dwarfness(t) • Red color is due togene (R) and white to its corresponding allele (r). • The heterozygous condition resultsin pink (Rr) flower color. • A dwarf pink snapdragon is crossed with a planthomozygous for tallness and red flowers. Give the possible genotypes and corresponding phenotypes for all of the possible F1 generation.

  10. Step #1: Determine Genotypes of Parents “A dwarf pink snapdragon is crossed with a planthomozygous for tallness and red flowers. “ Parent 1:Dwarf pink = ttRr Parent 2:Homozygous tall, red = TTRR

  11. Step #2: Determine Genotypes of Gametes Dwarf pink = ttRr tR tr tr tR Homozygous tall, red = TTRR TR TR TR TR

  12. Step #3: Punnett Square The Punnett square determines the genotypes of the offspring Gametes from TALL red parent TR TR TR TR tR tr Gametes from dwarf pink parent tR tr

  13. Step #4: Identify Phenotype Ratios in Offspring Tall, red = 8/16 Tall, pink = 8/16 Tall, white = 0/16 Short, red = 0/16 Short, pink = 0/16 Short, white = 0/16 TtRR TtRR TtRR TtRR TtRr TtRr TtRr TtRr TtRR TtRR TtRR TtRR TtRr TtRr TtRr TtRr

  14. Try it : • Page 154 questions 1, 3 (and then try #2)

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