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

5. Review Problem: P P p PP Pp Pp pp GENOTYPES: - PP (1); Pp (2); pp (1)
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 PP Pp
PHENOTYPES: Pp pp
- 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. Incomplete Dominance:
Rr = pink
CRCR = red
CrCr = white
CRCr = pink

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