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Chapter 11 (Part 2)

Chapter 11 (Part 2). Punnett Squares Honors Genetics Ms. Gaynor. What is “Probability”?. Why did Mendel get his results? What is the probability that Mendel would get his results EACH time? PROBABILITY = likelihood (chance) that an event will happen . Probability Rules.

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Chapter 11 (Part 2)

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  1. Chapter 11 (Part 2) Punnett Squares Honors Genetics Ms. Gaynor

  2. What is “Probability”? • Why did Mendel get his results? • What is the probability that Mendel would get his results EACH time? • PROBABILITY = likelihood (chance) that an event will happen

  3. Probability Rules Probability Rule #1 The multiplication rule (“and”) • States that the probability that 2 or more independent events will occur together is the product (x ing) of their individual probabilities

  4. Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm   Sperm r R 1⁄2 1⁄2 R R r R R 1⁄2 1⁄4 1⁄4 Eggs r r R r r 1⁄2 1⁄4 1⁄4

  5. Probability Rule #2 • The rule of addition (“or”) • States that the probability that any 1 of 2 or more exclusive events will occur is calculated by adding (+ ing) together their individual probabilities

  6. IN SUMMARY:2 Probability Rules Multiplication Rule • What are the chances of _?_AND_?_ happening at the same time? • MULTIPLY probability of EACH event happening TOGETHER Addition Rule • What are the chances of _?_ OR_?_ happening at same time? • ADD the probability of EACH event happening TOGETHER

  7. Probability Example Problems • Ms. Gaynor has 4 sisters. What is the probability that this will happen (her mother having 5 girls in a row)? • ½ x ½ x ½ x ½ x ½ =?? • 0.03125 or 3.125 % chance

  8. Mendel’s Results • Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses? • We can answer this question using a Punnett square, which shows a genetic cross

  9. What is a Punnett Square? • a diagram (box) used to predict the probabilities of the possible outcomes for offspring that will result from a cross between 2 parents • SHOWS EXPECTED RESULTS (not necessarily actual)

  10. Monohybrid Cross • a cross between 2 individuals that looks at 1 trait • There are 4 different types • Ex: freckles OR dimples • a cross between 2 individuals that looks at the possibilities of inheriting 2 DIFFERENT traits at one time • Ex: Freckles AND dimples Dihybrid Cross

  11. Monohybrid Punnett Square Mom’s genotype (Hh) x Dad’s genotype (hh) Tall Short Mom’s allele #1 Mom’s allele #2 H h Dad’s Allele #1 h Dad’s Allele #2 h

  12. Genotypic Ratio vs. Phenotypic Ratio • Genotypic Ratio = the GENOTYPE probabilities (expected results) of offspring • Ex: 50 % AA and 50 % Aa • Phenotypic Ratio = the PHENOTYPE probabilities (expected results) of offspring • Ex: 100% Tall

  13. Ratios • Can be expressed as a: • DECIMAL • Ex: 0.75 and 0.25 • FRACTION • Ex: ¾ and ¼ • PRECENTAGE • Ex: 75% and 25% • RATIO • Ex: 3: 1

  14. Punnett Square Genotypic ratio = 50 % Hh, 50% hh Phenotypic ratio = 50 % Tall, 50% short

  15. COMPLETE DOMINANCE: 4 different types of monohybrid crosses

  16. #1: Homozygous Dominant x Homozygous Recessive Mom’s genotype (HH) x Dad’s genotype (hh) Tall Short H H h h Genotypic ratio = 100% Hh (4:0) Phenotypic ratio = 100% Tall (4:0)

  17. #2: Homozygous Recessive x Homozygous Recessive Mom’s genotype (hh) x Dad’s genotype (hh) short Short h h h h Genotypic ratio = 100% hh (4:0) Phenotypic ratio = 100% short (4:0)

  18. #3: Homozygous Dominant x Heterozygous Mom’s genotype (HH) x Dad’s genotype (Hh) Tall Tall H H H h Genotypic ratio = 50% HH, 50% Hh (2:2) Phenotypic ratio = 100% Tall (4:0)

  19. #4: Heterozygous x Heterozygous Mom’s genotype (Hh) x Dad’s genotype (Hh) Tall Tall H h H h Genotypic ratio = 25% HH, 50% Hh, 25% hh (1:2:1) Phenotypic ratio = 75% Tall, 25% short (3:1)

  20. Phenotype Genotype Purple PP (homozygous) 1 Pp (heterozygous) 3 Purple 2 Pp (heterozygous) Purple 1 pp (homozygous) White 1 Ratio 3:1 Ratio 1:2:1 Figure 14.6

  21. The Testcross • Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype • Genotype is not immediately obvious • Cross an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

  22. Conduct a test cross, where the unknown dominant individual is crossed with the known recessive . • H _ ?_ x hh

  23. Test Cross Mom’s genotype (H?) x Dad’s genotype (hh) Tall Short H ? h h Genetypic ratio = Phenotypic ratio =

  24. Mendel’s 2nd Law:Law of Independent Assortment • says that the genes are inherited independently of other genes • Occurs during Metaphase I • Mendel assumed traits occur on different chromosomes!

  25. Dihybrid Crosses • How are two characters transmitted from parents to offspring? • As a package? • Independently?

  26. Dihybrid Crosses • crosses involveing crossing 2 DIFFERENT traits at one time • Example: Mate 2 parents and look at the probability of seeing 2 traits, such as: • eye color AND hair color • freckles AND dimples • Produces four possible phenotypes in the F2 generation

  27. Using the information from a dihybrid cross, Mendel developed the law of independent assortment • Each pair of alleles segregates independently during gamete formation

  28. For the following genotype, how many DIFFERENT gametes can be formed? • AaBBCcDdeeFf • 10 different • A, a, B, C, c, D, d, e, F, f

  29. How do You Do Dihybrid Crosses? 2 ways 1. one BIG Punnett Square (16 boxes instead of 4) 2. Make 2 separate Punnett Squares • 1 square for EACH trait • Then use PROBABILITY RULES and MULTIPLY

  30. One BIG Punnett Square 1. Find the possible gametes using F.O.I.L method AaBb x AABb AB AB Ab Ab aB AB ab Ab

  31. 2. Place the gametes in the Punnett Square & fill in boxes AB Ab aB ab AB Ab AB Ab

  32. Genotypic Ratio AAbB = 8/16 or 1/2 AaBB = 2/16 or 1/8 Aabb = 2/16 or 1/8 AABB = 2/16 or 1/8 AAbb= 2/16 or 1/8

  33. Phenotypic Ratio A= freckles, a= no freckles B = Dimples, b= no dimples AAbB = 8/16 (50%) freckles, dimples AaBB = 2/16 (12.5%) freckles, dimples Aabb = 2/16 (12.5%) freckles, no dimples AABB= 2/16 (12.5%) freckles, dimples AAbb = 2/16 (12.5%) freckles, no dimples So…12/16 (75%) freckles AND dimples 4/16 (25%) freckles AND no dimples

  34. Now…the easier way! • Write out a key for EACH trait • AA, Aa= Freckles, aa= No freckles • BB, Bb= Dimples, bb= No dimples • Figure out the genotypes of EACH parent and put them together • Draw 2 Punnett squares & figure out the FRACTION ratio of possible genotype and phenotypes • Draw punnett square #1 for trait #1 • Draw punnett square #2 for trait #2 • Figure out genotype/phenotype possibilities of offspring by multiplying fractions

  35. The Laws of Probability Control Mendelian Inheritance • Mendel’s laws of segregation and independent assortment • Reflect the rules of probability

  36. Probability Rules and Dihybrid Crosses • A dihybrid or other multicharacter cross • Is equivalent to two or more independent monohybrid crosses occurring simultaneously • In calculating the chances for various genotypes from such crosses • Each character first is considered separately and then the individual probabilities are multiplied together

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