Five Mark Questions :
(a) How are Mendelian inheritance, polygenic inheritance and pleoitropy different from each or?
(b)Explain polygenic inheritance with a suitable example.

(a) Mendelian inheritance refers to the expression of monogenic traits, i.e. gene expression is controlled by one gene. In a pair of alleles, an expression of the recessive gene is always masked by the expression of a dominant gene. The ability of a gene to have multiple phenotypic effects because it influences several characters simultaneously is called pleiotropy. It is due to the effect of the gene on two or more inter-related metabolic pathways which contributes to the formation of different phenotypes. Polygenic inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have a cumulative effect with each dominant allele expressing a part or unit of the trait, the full being shown only when all the dominant alleles are present.
(b) Human skin colour is an example of polygenic inheritance. It is caused by a pigment called melanin. The quantity of melanin is due to three pairs of polygenes (A, B and C). Very dark or black colour is due to the presence of dominant alleles (AABBCC). Very light or dark colour is due to the presence of all recessive alleles (aabbcc). Intermediate colour is often shown in the F1 generation when a dark or a black colour individual marries a white colour individual. Such an offspring is called mulatto (AaBbCc). When two such individuals of intermediate colour marry, the skin colour of the children will vary from the very dark or black to the very light or white.