Chapter 6: Introduction to Genetics Section 6-4: Different Types of Inheritance Incomplete Dominance Occurs when the heterozygous phenotype is in between both homozygous phenotypes Occurs when the heterozygous phenotype is in between both homozygous phenotypes Example: Snapdragons (Red is dominant) Example: Snapdragons (Red is dominant) RR = red flowersrr = white flowers RR = red flowersrr = white flowers Rr = pink flowers Rr = pink flowers Sometimes 2 different letters are used to show incomplete dominance Codominance Occurs when both alleles are equally expressed in the heterozygous phenotype Occurs when both alleles are equally expressed in the heterozygous phenotype Ex. Erminette chickens Ex. Erminette chickens BB = black feathersbb = white feahters BB = black feathersbb = white feahters Bb = black and white checkered feathers Bb = black and white checkered feathers (some feathers black, some feathers white) (some feathers black, some feathers white) Roan Cows show CODOMINANCE Multiple Alleles Occurs when there are more than two alleles for a gene Occurs when there are more than two alleles for a gene Example: Human blood groups Example: Human blood groups Type A, Type B, Type AB, Type O Type A, Type B, Type AB, Type O Three alleles: I A, I B, i Three alleles: I A, I B, i Blood Type Possible Genotypes Type A I A I A or I A i Type B I B I B or I B i Type AB I A I B (also shows codominance) Type O ii Polygenic Traits Traits controlled by more than one gene Traits controlled by more than one gene Many human traits such as body shape, facial appearance, mouth and eye position, height, and skin color are polygenic Many human traits such as body shape, facial appearance, mouth and eye position, height, and skin color are polygenic Also.feather color in parakeets Also.feather color in parakeets Polygenic Traits Sam