1. Beyond Dominant and Recessive Alleles
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

2. Codominance Heterozygous genotype has its own phenotype
both alleles contribute to the phenotype
a mixture of the two alleles
Example:
In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.

3. Co-dominant vs Incomplete Dominant

4. Multiple Alleles Genes that are controlled by more than two alleles
An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.
Example: Rabbit fur color
Human blood type
Example
A rabbit's coat color is determined by a single gene that has at least four different alleles, so is human blood type and eye color

5. Different combinations of alleles result in the colors shown here.
KEY
C = full color; dominant
to all other alleles
cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles
ch = Himalayan; color in certain parts of the
body; dominant to
c allele
c = albino; no color;
recessive to all other
alleles
Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc.
Example
A rabbit's coat color is determined by a single gene that has at least four different alleles, so is human blood type and eye color
Full color: CC, Ccch, Cch, or Cc
Chinchilla: cchch, cchcch, or cchc
Himalayan: chc, or chch
AIbino: cc

6. Human Blood Types There are four possible human blood types:
type A = IAIA or IAi
type B = IBIB or IBi
3. type AB = IAIB
4. type O = ii

7. What are the possible blood types of the children?
Homozygous Male Type B (IBIB) x
Heterozygous Female Type A (IAi) IA IB i
IAIB
IBi
1/2 = IAIB
1/2 = IBi

8. What are the possible blood types of the children?
male Type O (ii) x
female type AB (IAIB) i IA IB
IAi
IBi
1/2 = IAi
1/2 = IBi

9. Incomplete Dominance 
When one allele is not completely dominant over another
heterozygous genotype has its own phenotype
a blend of the two homozygous phenotypes

10. RR
A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).
RR = Red
WW = White
RW = Pink WW
Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring.

11. Polygenic Traits Traits controlled by two or more genes Example
Skin color in humans is controlled by more than eight different genes

12. Genetics and the Environment
Phenotypes can also be affected by the environment
Example:
During the summer the arctic fox’s fur is reddish brown to blend into the summer landscape. In the winter the coat is white to blend into the snowy background.