1. BEYOND MENDEL’S LAWS Heredity

2. Incomplete Dominance Review:  Inherited traits were either dominant or recessive  The dominant allele in an individual was always expressed, even if the recessive allele was present However......there are exceptions to Mendel’s work

3. In some cases, neither allele is completely dominant Therefore, in a heterozygous individual, there is a blending of 2 traits This is called Incomplete Dominance

4. Example R = Red R’ = White (R’ is used rather than “r” to indicate incomplete dominance) A pure red snapdragon flower (RR) is crossed with a pure white snapdragon flower (R’R’). Neither trait is dominant. RR R’ RR’ R’ RR’ RR’ is neither red nor white, but rather, pink

5. Co-dominance In other cases, both alleles may be dominant and are both expressed This is called Co-dominance  Ex. Feathers of black birds and white birds

6. Multiple Alleles Many genes have more than 2 different alleles, or multiple alleles Example: Human blood types 3 different possible alleles for blood type I A I B I O I A and I B are co-dominant and are dominant over I O There can only be two alleles in blood type, but there are three alleles to choose from Possible genotypes: I A I A, I B I B, I A I B, I A I O, I B I O, I O I O Possible phenotypes: Type A, Type B, Type AB, Type O