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Genetics: Incomplete Dominance & Codominance

Genetics: Incomplete Dominance & Codominance. Biology 12. Joke of the day:. Review of Mendel’s Principles. Genes are passed parents  offspring; get one allele from each parent During Meiosis, the alleles for a gene segregate from each other.

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Genetics: Incomplete Dominance & Codominance

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  1. Genetics: Incomplete Dominance & Codominance Biology 12

  2. Joke of the day:

  3. Review of Mendel’s Principles • Genes are passed parents  offspring; get one allele from each parent • During Meiosis, the alleles for a gene segregate from each other. • During Meiosis, genes independently assort with each other.

  4. PP = purple Pp = purple pp = white Review: Dominant/Recessive • One allele is dominant over the other (capable of masking the recessive allele)

  5. PP Pp Pp pp Review Problem: • In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants. P p GENOTYPES: - PP (1); Pp (2); pp (1) - ratio 1:2:1 P p PHENOTYPES: - purple (3); white (1) - ratio 3:1

  6. Exceptions to Mendel’s principles • Some alleles are neither dominant nor recessive. • Many traits are controlled by more than one gene (polygenic traits)

  7. Incomplete dominance: • A situation in which neither allele is dominant. • When both alleles are present a “new” phenotype appears that is a blend of each allele. • The heterozygous phenotype is somewhere in between homozygous phenotype. • 2 alleles produce 3 phenotypes

  8. CRCR = red CrCr = white CRCr = pink Incomplete Dominance: Rr = pink

  9. R r r r Rr Rr rr rr Problem: Incomplete Dominance • Show the cross between a pink and a white flower. GENOTYPES: - Rr (2); rr (2) - ratio 1:1 PHENOTYPES: - pink (2); white (2) - ratio 1:1

  10. Codominance: • Neither allele are dominant; both are expressed. • A cross between organisms with two different phenotypes produces offspring with has both phenotypes of the parental traits shown.

  11. Example: codominance • In some chickens: Black Chicken x White  Speckled Chicken

  12. Example: • Roan coat color in horses and cattle are codominant for the alleles R and r. • Homozygous R individuals are red (or bay) • Homozygous r individuals are white • While heterozygous Rr individuals are roan (which means they have both white and red hair, giving them a lightened appearance.

  13. What about the F2 generation? • Phenotype? • 1 red, 2 roan and 1 white • 1:2:1 • Genotype? • 1Hr Hr, 2 HrHw and 1Hw Hw

  14. Which type of dominance… Codominance!

  15. To do: • Page 145: questions 2, 3, 4, 5 • Practise sheet: Codominance and Incomplete dominance • Colouring sheet

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