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9-2: Genetic Crosses

9-2: Genetic Crosses. Help to predict the genetic makeup + appearance of offspring. Genotype & Phenotype. Genotype – genetic makeup of an organism Consists of alleles inherited from parents Ex. PP or Pp = purple flower; pp = white flower

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9-2: Genetic Crosses

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  1. 9-2: GeneticCrosses Help to predict the genetic makeup + appearance of offspring

  2. Genotype & Phenotype • Genotype – genetic makeup of an organism • Consists of alleles inherited from parents • Ex. PP or Pp = purple flower; pp = white flower • Phenotype – the appearance of an organism as a result of its genotype • Ex. PP or Pp = purpleflower; pp = white flower

  3. More words to know… • Homozygous – both alleles in a pair are alike • Also known as a PUREBRED • Homozygous dominant = PP • Homozygous recessive =pp • Heterozygous – alleles are different • Also known as a HYBRID • Heterozygous = Pp

  4. Probability • The likelihood that a specific event will occur • May be expressed as a decimal, percentage, fraction, or ratio • Equation: • # times an event is expected to happen # opportunities for it to happen

  5. Probability (cont) • Examples: • Mendel observed that yellow seed color in F2 appeared 6,022 times and green seed color appeared 2,001 times • Total number of seeds = 8,023 • Probability of dominant trait appearing • 6,022/8,023 = .75, 75%, or ¾ • Probability of recessive trait appearing • 2,001/8,023 = .25, 25%, or ¼

  6. Probability (cont) • What does probability tell us about the offspring of 2 heterozygous individuals? • There are 3 out of 4 chances the offspring will have the dominant trait • There is 1 out of 4 chances the offspring will have the recessive trait • *The results of probability are more likely to occur if you have more trials*

  7. Probability (cont) • Ex. Flipping a coin • What is the probability a coin will land on heads? • 50% • What is the probability a coin will land on tails? • 50%

  8. Monohybrid Crosses • A cross between individuals involving one pair of contrasting traits is called a monohybrid cross • There are 4 possible outcomes (22) • Results are created with the use of Punnett Square • A diagram that helps biologist predict the probability that certain traits will be inherited by offspring

  9. Example 1: Homozygous (Dominant) x Homozygous (recessive) • P – purpleflower; p – white flower • Cross = PP x pp • Genotypes = 4 Pp • Phenotypes = 4 purpleflowers P P p p

  10. Example 2a: Homozygous (Dominant) x Heterozygous • B – blackcoat; b – brown coat • Cross = BB x Bb • Genotypes = 2 BB, 2Bb • Phenotypes = 4 black coat B b B B

  11. Example 2b: Homozygous (recessive) x Heterozygous • B – blackcoat; b – brown coat • Cross = bb x Bb • Genotypes = 2 bb, 2 Bb • Phenotypes = 2 brown coat, 2 black coat B b b b

  12. Example 3: Heterozygous x Heterozygous • B – black coat; b – brown coat • Cross = Bb x Bb • Genotypes = 1 BB, 2Bb, 1bb • Phenotypes = 3 black, 1 brown B b B b

  13. Gentoypic vs. Phenotypic Ratio • Genotypic ratio for the last cross • 1BB : 2Bb : 1bb • Phenotypic ratio for the last cross • 3 black :1 brown

  14. Example #4: Testcross • A testcross is used to find an unknown genotype • Can help determine genotype of any individual whose phenotype is dominant • cross individual w/ homozygous recessive • Examples:

  15. Complete vs. Incomplete Dominance • Complete dominance • When one allele is completely dominant over the other • Ex. Bb • Incomplete dominance • No dominant or recessive allele; a heterozygous individual has an intermediate phenotype • Ex. Bb

  16. Example #5: Incomplete Dominance • Japanese 4 o’clock flowers (snapdragons) • R = red • r = white • Cross: RR x rr • Genotypes: 4 Rr • Phenotypes: 4 pink flowers

  17. Cross 2 heterozygous flowers • Cross: Rr x Rr • What is the probable genotypic ratio? • 1 RR : 2 Rr : 1 rr • What is the probable phenotypic ratio? • 1 Red : 2 Pink : 1 white

  18. CODOMINANCE • When both alleles are dominant and influence the phenotype of the individual • Ex. Blood type B B A A

  19. Dihybrid Crosses • A dihybrid cross involves crossing 2 contrasting traits • There are 16 possible outcomes (24) • Punnett square:

  20. Example #1: Homozygous Dominant x Homozygous Recessive • R = round seeds • r = wrinkled seeds • Y = yellow seed color • y = green seed color • Cross = RRYY x rryy • How do we determine the alleles given off by each parent? • The “FOIL” method!!!

  21. FOIL (First-Outer-Inner-Last) • Parent 1 RRYY • Alleles : RY,RY,RY,RY

  22. FOIL (First-Outer-Inner-Last) • Parent 2 rryy • Alleles: ry,ry,ry,ry

  23. ry ry ry ry RY • Genotypes: 16 RrYy • Phenotypes: 16 round, yellow RY RY RY

  24. Heterozygous x Heterozygous • Cross: RrYy x RrYy • Alleles from each parent: • Parent 1 = RY, Ry, rY, ry • Parent 2 = RY, Ry, rY, ry

  25. RrYy x RrYy RY Ry rY ry RY Ry rY ry

  26. Results of the cross • Genotypes – • 1 RRYY, 2RRYy, 1 RRyy, 2 RrYY, 4 RrYy, 2 Rryy, 1 rrYY, 2 rrYy, 1 rryy • Phenoytpes– • 9 round, yellow; 3 round, green; 3 wrinkled, yellow; 1 wrinkled, green

  27. Multiple Alleles + Polygenic Traits • Multiple Alleles – 3 or more alleles • Blood types • A, B, O • Polygenic – 2 or more genes • Height • Weight • Eye color • Intelligence • Skin color

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