Crosses of pure-breeding lines can result in progeny phenotypes that don't appear to follow mendel's rules. explanations for this include:
a) no definitively dominant or recessive allele
b) more than two alleles exist
c) multiple genes involved
d) gene-environment interactions

b) More than two alleles exist

Explanation:

There are cases where more than two alleles determine a character in the population. This type of inheritance is called multiple alleles (or polyalelia). Classical examples of polyalelia are hair color determination in rabbits and blood groups of the ABO system in humans.  

In the attached figure we can see another example of polyalelia where a flower is self-fertilized generating different phenotypes in its offspring.

step 1: initiation

initiation is the beginning of transcription. it occurs when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

step 2: elongation

elongation is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

step 3: termination

termination is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.