© K. Malone, 2005 Exceptions to Mendel’s Rules Some alleles are neither dominant or recessive – what???? Some traits are controlled by multiple alleles or multiple genes Incomplete Dominance = cases when one allele is not completely dominant over another

© K. Malone, 2005 Incomplete Dominance Four o’clock plants Red should be dominant over white, or vise versa, but neither is completely dominant, thus, incomplete dominance. When you cross Red and White Four O’clock Flowers, you get PINK flowers!!!! Box 31

© K. Malone, 2005 Codominance Codominance = both alleles contribute equally to the phenotype The offspring is a mixture of both alleles Red cow White cow X Roan cow Neither white or red Red fur and white fur

© K. Malone, 2005 Multiple Alleles Multiple Alleles = when a gene has more than two alleles –The trait itself has more than 2 forms (alleles) Show a cross between a female MM duck and a male M R M d duck. Duck Patterns M = mallard (most common) M R = restricted M d = dusky M M MRMR MdMd MM R MM d

© K. Malone, 2005 Polygenic Traits Polygenic traits = traits controlled by more than one gene Poly = many, Genic = genes If you see a graph like this on my test, or the TAKS test, then you know one gene can’t control the trait! Polygenic! This range of heights and skin colors tells us more than 2 genes control both traits. If 2 genes controlled height and 2 genes controlled skin color, we would see either tall or short people, light or dark skin, and nothing in between. Is that what we see?