1. Test crosses, Incomplete Dominance, and Codominance
Genetic Exceptions
Test crosses, Incomplete Dominance, and Codominance

2. Test Cross Used to determine an unknown dominant genotype.
Offspring that show a dominant phenotype could have either a homozygous dominant (RR) or heterozygous (Rr) genotype…
How can the second allele be determined?

3. Test cross example In mice, black (R) is dominant to white (r)
Take a black mouse with an unknown dominant genotype and cross it with a homozygous recessive (rr) white mouse, then observe the color of the offspring. R ?
R ? Rr
r r Rr
R r r r
If any offspring are white, the unknown allele is ______.
No white offspring then unknown allele is _____. R r

4. Incomplete Dominance
According to Mendel’s Law of Dominance; if dominant allele is present then the dominant trait will be expressed.
However...there are always exceptions to the rules!!!
In some organisms; both the dominant(R) and recessive(r)determine phenotype
Phenotype that results is a “blending” of proteins.
Alleles themselves do not combine, only the characteristics(proteins) they code for do.

5. NOT MIXING ALLELES, MIXING COLORS OF PROTEINS !
Example: Flower Color
Red (R) Blue (r)
FLASHLIGHT R R
R r Rr Rr R r RR Rr r r Rr Rr Rr rr
All F1= Heterozygous (Rr)
F2 = 1 RR : 2 Rr : 1 rr
All Heterozygous genotypes will show the blending of phenotypes.
NOT MIXING ALLELES, MIXING COLORS OF PROTEINS !

6. SNAPDRAGONS ILLUSTRATE INCOMPLETE DOMINANCE
GENERALLY; MOST ‘BLENDED’ FLOWER COLORS ILLUSTRATE INCOMPLETE DOMINANCE!

7. CO-DOMINANCE aka: Shared Dominance.
Two dominant alleles code two different proteins.
R’ = the codominant allele
No blending of characteristics.
All dominant proteins are coded.

8. Example 1: Coat color
R= brown R’= black
R R’ R R’ RR
RR’
RR’
R’R’

9. How are they similar? How are they different?
They are both codominant for black and white….
The panda has a gene that codes for color placement.

10. Example 2: Sickle Cell R R’ R R’
R= Round
R’= Sickle
Persons with RR’ suffer from milder symptoms than R’R’ because they have some normal cells
R R’ R R’ RR
RR’
RR’
R’R’

11. NORMAL RR
SICKLE CELL RR’

12. Example 3: Blood Types
3 alleles code for blood type: IA IB and i(type O)
A is codominant to B and both are expressed: Type AB
A is dominant to O : Type AA or AO
B is dominant to O : Type BB or BO
O is recessive: Type OO

13. Taste Receptor: Phenylthiocarbamide (PTC)
The “taster” gene is incompletely dominant to the “non-taster” gene.
People with two copies of the taster gene taste the chemical compound more intensely than those with only one copy.
Chemicals similar to PTC are found in plants of the mustard family. (Broccoli, mustard, etc.)
This gene is found among many other taster genes on chromosome 7.