2. Genetics: Incomplete Dominance & Codominance Biology 12

3. Joke of the day:

4. 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.

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

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

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

8. 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

9. Incomplete Dominance: C R C R = redC r C r = whiteC R C r = pink Rr = pink

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

11. 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.

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

13. 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. Example:

14. What about the F 2 generation? Phenotype?  1 red, 2 roan and 1 white  1:2:1 Genotype?  1H r H r, 2 H r H w and 1H w H w

15. Which type of dominance… Codominance!

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