Exceptions to Mendel’s Laws Mendel was confused by some of his crosses but there was not enough information available for him to “get it”

Single gene inheritance that does not follow the rules of the Principle of Dominance

Incomplete Dominance Wavy Hair Cream colored guinea pigs Pink Flowers

Snapdragons show Incomplete Dominance X WW = white RR = red WR = pink Blending of Alleles - Both alleles contribute equally to the phenotype, no dominance

Codominance Sickle Cell Disease Roan Coat Color Speckled Chickens

Codominance X BB = black BW = speckled WW = white Both alleles contribute equally to the phenotype (no blending), white feathers and black feathers

Multiple Alleles Human ABO Blood Types 4 Blood Types: A, B, AB, O 1 Gene with 2 alleles, single gene inheritance BUT in the population there are other alleles possible

Beyond Single Gene Inheritance Polygenic - many genes that may or may not follow the rules of the Principle of Dominance

Polygenic Traits Many genes with many different alleles contribute to the phenotype Hair color, eye color, height, skin color