1. Incomplete dominance Co dominance
Inheritance Patterns
Incomplete dominance
Co dominance

2. Incomplete Dominance
Snapdragon plants
What happens when a true breeding red snapdragon (RR)
crosses with a
True breeding white snapdragon (R'R')?

3. Result? The F1 generation are all heterozygous (RR') Pink
What happened to these traits? Did they blend?
p. 143

4. F2 generation yielded… F1 F2
1 red: 2 pink: 1 white (different than that of Mendel’s F2 monohybrid cross ratio of 3:1)

5. Conclusion: traits do not blend
Explanation:
Genes code for specific proteins
Most are enzymes with different degrees of catalytic activity
In this case,
One allele (R) codes for an enzyme that produces red pigment

6. Conclusion: traits do not blend
The other allele (R') codes for an enzyme that does not produce pigment
So, heterozygous plants (RR'), with only one allele for the functional enzyme produces just enough red pigment to display pink. (not completely dominant!)
2 alleles with functional enzymes (RR) produce enough pigment to show red

7. Incomplete dominance
A condition in which the heterozygous phenotype is intermediate between the 2 homozygous phenotypes
We use RR' (not Rr) to indicate incomplete dominance

8. Sample problem The F1 generation?
In four O’clock plants, red flowers are incompletely dominant over white flowers. The heterozygous flowers are pink. If a red-flowered four o’clock plant is crossed with a white-flowered four o’clock plant, what will be the flower colour of
The F1 generation?

9. Solution What is required? What is given?
The flower colour of F1 generation?
What is given?
Red flowers (RR) incompletely dominant
White flowers (R'R')
Heterozygous flowers (RR') are pink

10. Carry out solution… R' R'
RR' R R
The F1 generation is all pink

11. Your turn… The F1 generation crossed with its red parent?
In four O’clock plants, red flowers are incompletely dominant over white flowers. The heterozygous flowers are pink. If a red-flowered four o’clock plant is crossed with a white-flowered four o’clock plant, what will be the flower colour of
The F1 generation crossed with its red parent?
The F1 generation crossed with its white parent?

12. Solution What is required? What is given? F1 (RR') X red parent (RR)
F1 (RR') X white parent (R'R')
What is given?
Red flowers (RR) incompletely dominant
White flowers (R'R')
Heterozygous flowers (RR') are pink

13. Results: R R' R R' RR RR' RR' R'R' R' R R R' 2 red, 2 pink
2 white, 2 pink

14. Co dominance
Sometimes, both alleles of a given characteristic are dominant
Therefore, in heterozygotes, both alleles are expressed.

15. I.e. Black rooster is crossed with a white hen
Result?
Black and white plumage
Both colours were expressed

16. Multiple alleles In addition to co dominance,
Many genes may have multiple alleles:
More than two alleles available to determine the phenotype of an organism
I.e. co dominance & multiple alleles

17. I.e. ABO blood group A is dominant to O B is dominant to O
There are 3 different alleles for blood type: IA IB i
Each type indicates a specific glycoprotein present on blood cells
(People with type O have no glycoproteins on their blood cells)
(People with AB have both types present)
A is dominant to O
B is dominant to O
AB are both co dominant

19. Sample problem
If a woman has blood type AB, and a man has blood type A, what possible blood types will their children have?

20. What is required? What is given? Possible blood types of children?
Woman AB IAIB
Man A IAIA or IAi

21. Solution: IAIA IAIB IAIA IAIB IAi IBi IA IB IA IB IA IA i
2 type A, 2 type AB
2 type A, 1 type AB, 1 type B

22. Your turn…
Suppose a man with blood type B marries a woman with type AB blood. What blood types would you expect to find among their children?
What is required?
Possible blood types of children
What is given?
Man – IBIBor IBi
Woman - IAIB

23. Solution: IAIB IBIB IAIB IAi IBIB IBi IB IB IB i IA IA IB IB
2 type AB, 2 type B
1 type AB, 2 type B, 1 type A