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Monohybrid Cross Notes

Monohybrid Cross Notes. Looking at one trait. Dominant Vs. Recessive. Shows, if present Capital letter (A, B, C, …). Shows if both genes are recessive Lower case letter (a, b, c, …). Homozygous Vs. Heterozygous. “homo” = same

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Monohybrid Cross Notes

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  1. Monohybrid Cross Notes Looking at one trait

  2. Dominant Vs. Recessive • Shows, if present • Capital letter (A, B, C, …) • Shows if both genes are recessive • Lower case letter (a, b, c, …)

  3. Homozygous Vs. Heterozygous • “homo” = same • Alleles for the gene are the same • Can be dominant (AA) • Can be recessive (aa) • “hetero” = different • Alleles for gene are different (Aa) • Same letter, just different sizes

  4. Genotype Vs. Phenotype • Refers to genes • Can’t see genes • Ex. AA, Aa, aa • Like “physical appearance” • What do you see? • Ex. Tall/short, red/white

  5. Genotypic Ratio Vs. Phenotypic Ratio • How many individuals have each genotype? • Always AA:Aa:aa • How many individuals have the dominant trait? How many have the recessive trait? • Always dominant:recessive

  6. Example 1. In flowers, red petals are dominant over white petals. Show the cross between a homozygous dominant parent and a homozygous recessive parent. RR X rr R = red; r = white r r R R Note: R always comes before r. Genotype: Rr Phenotype: red Genotypic Ratio (GR = RR:Rr:rr): 0:4:0 Phenotypic Ratio (PR = red:white): 4:0

  7. Example 2. Show the cross between two heterozygous parents. Rr X Rr R = red; r = white R r R r Note: R always comes before r. Genotype: RR, Rr, rr Phenotype: red, white Genotypic Ratio (RR:Rr:rr): 1:2:1 Phenotypic Ratio (Red:white): 3:1

  8. Example 3. Show the cross between a heterozygous parent and a homozygous dominant parent. Rr X RR R = red; r = white R r R R Genotype: RR, Rr Phenotype: red Genotypic Ratio (RR:Rr:rr): 2:2:0 Phenotypic Ratio (Red: White): 4:0

  9. Example 4. Show the cross between a homozygous recessive parent and a heterozygous parent. rr X Rr R = red; r = white r r R r Genotype: Rr, rr Phenotype: red, white Genotypic Ratio (RR:Rr:rr): 0:2:2 Phenotypic Ratio (red:white): 2:2

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