Answer:
A. The extension of Mendelian genetics comprises the codominance, incomplete dominance, polygenic inheritance, multiple alleles, lethal genes, sex lined genes, gene mapping, and others.
B. The phenomenon of incomplete dominance takes place when of both the recessive and dominant forms, no one gets expressed in the F1 generation, however, an intermediate expression is witnessed. For example, crossing red flower with the white one generates pink flowers.
On the other hand, in codominance, both the alleles get expressed, like in the blood group AB. The association of genes results in epistasis, it changes the normal dihybrid phenotypic ratio, that is, 9: 3: 3: 1 to 12: 3: 1. The genes, which suppress the others are called epistatic, while the ones, which get suppressed are known as hypostatic.