(a) Polygenic inheritance is an inheritance pattern controlled by three or more genes (multiple genes) and the graded phenotypes are due to the additive or cumulative effect of all the different genes of the trait. An example of human skin colour to understand the phenomenon of polygenic inheritance. Skin colour in humans is caused by a pigment called melanin. The quantity of melanin is due to three pairs of polygenes (A, B and C).
Black or very dark is represented by (AA BB CC) and
White or very light is represented by (aa bb cc).

Individuals with the genetic constitution (Aa Bb Cc) have an intermediate colour often called mulatto .
A total of eight allele combinations is possible in the gamets forming 27 distinct genotypes distributed into 7 phenotypes, i.e. very dark, dark, fairly dark, intermediate,fairly light and very light.

(b) In Mendelian inheritance, only one gene controls one trait. Ogenegen may have two different forms of an allele. However, in Polygenic there is more than one gene controlling one trait. 
For example, the gene of tallness in the case of Mendelian character has two form of the allele, both of which affect the height of the plant. 
In the case of skin colour three genes A, B and C control the skin colour. 

In Pleiotropy one gene affects more than one phenotype or trait. However in polygenic inheritance  more than one gene controls or affects one phenotype. 

In case of  the amino acid tyrosine. It is needed for general protein synthesis, and it is also a precursor for several neurotransmitters (e.g., dopamine, norepinephrine), the hormone thyroxine, and the pigment melanin. Thus, mutations in any one of the genes that affect tyrosine synthesis or metabolism may affect multiple body systems. These and other instances in which a single gene affects multiple systems and therefore has widespread phenotypic effects.