Which idea is depicted by a cross in which the $F_1$ generation resembles both the parents?

$A$ gene showing codominance has

In shorthorn cattle,genes for red $(R_1)$ and white $(R_2)$ coat colour occur. Crosses between red $(R_1R_1)$ and white $(R_2R_2)$ produced $(R_1R_2)$ roan. This is an example of

In codominance,the phenotypic ratio is......

When both dominant and recessive alleles express their traits simultaneously,it is called.....

What is codominance? Explain it in detail with the example of human blood groups.