Section 12.3Complex Inheritance of

Human Traits

Codominance Example in Blood

Sickle Cell - Homozygous Recessive

Normal RBC - Homozygous Dominant

Sickle Cell Anemia CauseHeterozygotes are said to have sickle cell trait.Defective Hemoglobin molecule (caused by 1 amino acid change) forms crystal-like structures that changes the shape of the RBC (sickle shape).

Blood Typing LabHuman Blood type exhibits Multiple Alleles.Why Blood Type?

Incompatible blood types could cause Blood clotting and ultimately Death.

Use for Identification Purposes.Paternity Tests (still require DNA testing)

Genetics BackgroundGene for Human Blood Type = Gene I

This gene codes for a molecule (antigen) that attaches to a membrane protein on the surface of RBCs.

Blood Antibodies

Polygenic Inheritance

Multiple genes are responsible for the phenotypes that we “see.”

Example:Skin color (3 - 4 Genes)