A race of inbred plant has a mean petal length of 12 mm, a second race from another location has the same mean petal length. when the two races are crossed, the F1 also has the mean petal length of 12mm. however the F2 generation derived from crossing the F1 to one another shows a very wide spread in plant length. About 3 out of 770 have a length as small as 8 mm. and about 3 out of 770 have a length of 16 mm. How many allelic pairs seem to be involved in the different petal length? How much does each effective allele contribute to the length of petal?