for the tall trait (TT) and heterozygous plant (Tt) would have the phenotype of being tall
plants. The recessive phenotype only occurs if the dominant allele is absent, which is when
an individual is homozygous recessive (tt).
Incomplete Dominance and Codominance
In all of Mendel’s experiments, he worked with traits where a single gene controlled the trait
and where one allele was always dominant to the other. Although the rules that Mendel
derived from his experiments explain many inheritance patterns, the rules do not explain
them all. There are in fact exceptions to Mendel’s rules, and these exceptions usually have
something to do with the dominant allele.
One exception to Mendel’s rules is that one allele is always completely dominant over a
recessive allele. Sometimes an individual has an intermediate phenotype between the two
parents, as there is no dominant allele. This pattern of inheritance is calledincomplete
dominance.
An example of incomplete dominance is the color of snapdragon flowers. One of the genes
for flower color in snapdragons has two alleles, one for red flowers and one for white flowers.
A plant that is homozygous for the red allele will have red flowers, while a plant that is
homozygous for the white allele will have white flowers. On the other hand, the heterozygote
will have pink flowers (Figure6.4). Neither the red nor the white allele is dominant, so the
phenotype of the offspring is a blend of the two parents.
Figure 6.4: Pink snapdragons are an example of incomplete dominance. ( 15 )Another example of incomplete dominance is sickle cell anemia, a disease in which the
hemoglobin protein is produced incorrectly and the red blood cells have a sickle shape. A
person that is homozygous recessive for the sickle cell trait will have red blood cells that all