Answer:Variation in single and multiple genes.
Step-by-step explanation:
This is due to changes in the sequence of nucleotides in the GENE on specific loci located on the chromosomes of the DNA molecule.
The change may be due MUTATION. Therefore the expression of phenotype may be different from Madelia laws of independent assortments of genes, because the genes may be linked together and assort jointly,(linkage), or some genes may not have full dominance over the other(incomplete dominance and codominance). Some alleles of another gene may mask or conceal the expression of other allele
Example of these exceptions to the independent assortments are Incomplete dominance, codominance, sex –linkage, epistasis, complementary genes.