A Multi-Celled organism has many cells (more than one). Examples would be plants, humans and animals/mammals.
A Single-Celled organism only has one cell that makes up it's entire self. Examples would be bacteria and archaea.
Answer: The genotype of the generation resulting from the given P ( parental generation) is Aa that is all offsprings in the first generation will be heterozygous dominant.
The genotype of the parents is AA and aa and the gametes produced by these parents are A and a respectively.
When these gametes fuse, they result in the offsprings with genotype Aa. This represents a dominant phenotype due to the presence of dominant gene ( a gene that masks the expression of recessive gene and expresses itself), which is A in this case.
They are both made up of monomers called nucleotides.