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Mendel’s 3 rd Experiment

Mendel’s 3 rd Experiment. Dihybrid Crosses. Byhybrid Crosses. Crossbreeding of two characteritics Mendel found pure breeding lines of pea plants with contrasting traits for 2 characteristics

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Mendel’s 3 rd Experiment

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  1. Mendel’s 3rd Experiment Dihybrid Crosses

  2. Byhybrid Crosses • Crossbreeding of two characteritics • Mendel found pure breeding lines of pea plants with contrasting traits for 2 characteristics • E.g. pure dominant round-seeded plants have a genotype of RR and pure dominance of yellow-seed plants have a genotype of YY

  3. Note: colour of seed is not linked to shape of seed, know as independent assortment • Mendel found plants with RRYY and rryy genotypes (P generation) and crossbred them • RRYY is purebred dominant round, yellow seeds • Rryy is purebred recessive wrinkled, green seeds

  4. Let’s take a look… • Each RRYY plant will produce gametes with RY • Each rryy plant will produce gamets with ry

  5. Interpretation of Mendel’s 3rd Experiment • 100% RrYy (hybrid round, yellow seeds) • According to the Principle of Dominance, Mendel expected to find 100% of the F1 generation to express both dominant traits (all plants would have round, yellow seeds) and that is exactly what happened.

  6. In the F2 generation when he crossed 2 of the hybrid offspring (RrYy) from the F1 he found the following:

  7. Phenotypic Ratio • 9/16 - round/yellow (dominant, dominant) • 3/16 – round/green (dominant, recessive) • 3/16 – wrinkled/yellow (recessive, dominant) • 1/16 – wrinkled/green (recessive, recessive) • 9:3:3:1 Phenotype ratio of a dyhybrid cross

  8. Genotypic Ratio • In the F2 generation when he crossed 2 of the hybrid offspring from F1 he found the following phenotype ratios: • ¾ or 75% round seeds, ¼ or 25% Wrinkled • ¾ or 75% yellow seeds, ¼ or 25% green • ¾ x ¾ = 9/16

  9. Principle of Independent Assortment • Only genes of different chromosomes can assort independently during meiosis

  10. Linkage – traits that are usually inherited together • In fruit flies, wing shape and body colour are inherited together • In pea plants seed coat colour and flower colour are linked • Linkage occurs because the gene locations are carried on the same chromosomes

  11. Sex Linked • Traits that are inherited with the chromosome for sex (male or female) • i.e. colour-blindness in Male offspring

  12. Colour-blindness in Male offspring • Mother carries red-green colour blindness trait (XCXc) • Father has normal vision (XCY) Results: Genotypic ratio: ½ girls are homozygous XCXC, ½ girls are hybrid XCXc ½ boys are XCY, ½ boys are XcY Phenotypes: all girls are normal (no colour blindness) ½ boys are colour blind

  13. Incomplete Dominance • Occurs when different alleles control a charatcterisitic, but NEITHER is dominant • The resulting phenotype shows a “blending” of the two traits coded by the alleles. • For example: A red snapdragon (flower) with the allele genotype RR is crossed with a white snapdragon with the allele genotype WW

  14. All of the resulting F1 generation phenotypes are PINK Let’s try the F2 generation

  15. Resulting phenotypes are: 1 red: 2 pink:1 White

  16. Multiple Alleles • Are multiple gene locations or more than 2 alleles for a trait • i.e. blood types

  17. Blood Types These charts show the possible blood type results for offspring.

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