According to the text:

1. Pleiotropy occurs when a single gene exhibits multiple phenotypic expressions. The text explicitly states, "An example of this is the disease phenylketonuria... This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation" .
2. Skin color is an example of polygenic inheritance (controlled by three or more genes) .
3. ABO Blood group is an example of multiple alleles and co-dominance .
4. Colour Blindness is a sex-linked recessive disorder .