8. Complex Inheritance
Some alleles are neither recessive or dominant, and many traits are controlled by multiple alleles or multiple genes.
Codominance (co- means share)
Incomplete Dominance (Incomplete means in-between)

9. Incomplete Dominance
When one allele is not completely dominant over another it is called incomplete dominance.
In incomplete dominance, the offspring's phenotype is between the two parents phenotypes.

10. Incomplete Dominance
A cross between red (RR) and white (WW) four o’clock plants produces pink colored flowers (RW).

11. Codominance Codominance (Co - means share)
In Codominance, both alleles contribute to the phenotype.
In certain varieties of chicken, the allele for white (WW) feathers is codominant with the alleles black (BB) feathers.
(BW) chickens are speckled with both black and white feathers.
The black and white colors combine to form a new color, but appear separately.

12. Multiple Alleles
Genes that are controlled by more than two alleles are said to have multiple alleles.
A Genotype can’t have more than two alleles. However, more that two possible alleles can exist in a gene pool.
A rabbit’s coat color is determined by a multiple allele gene that has at least 4 different alleles.

13. Multiple Alleles

14. Polygenic Traits
Traits controlled by two or more genes are said to be polygenic traits.
Eye color in humans is a polygenic trait controlled by more than four different genes.

15. Pleiotropy
Most single genes affect more than one observable trait, a phenomenon we know as pleiotropy.
For example, the alleles for hair pigment affect skin pigments.

16. Gene Interactions
An epistatic gene is one that covers up alleles of another, hypostatic gene. The gene whose expression is affected by the epistatic gene is said to be hypostatic.
Epistatic genes, each with more than one allele, can interact to produce recessive phenotypes due to epistatic interactions.
“BOSSY GENES”
The BE gene controls the color of
melanin pigment: BE is black; bb E is brown