A single gene controlling various phenotypes is called pleiotropy. Phenylketonuria is a genetic disorder in which a base substitution occurs in chromosome 12 is affected. But it affects multiple systems such as brain development, decreased pigmentaton and eczema. This is an example of pleiotropy. If a single phenotype is controlled by several genes, then that type of inheritance is called polygenic inheritance. For example - the colour of the skin is controlled by several genes. When the dominant allele fails to completely dominate the recessive allele, the phenomenon is called incomplete dominance. Example: Pink flower colour in snapdragon. The phenomenon in which a gene which has more than two types of alleles is called multiple allelism. For example: The blood group has three different alleles (IA,IB and i ).