A Single gene controls the size of the starch grain and the seed shape.
The trait of size of starch grain shows incomplete dominance. Hence in heterozygous condition the starch grain are of intermediate size.
(Deviation from Mendelian Law of Dominance) : The trait of seed shape follows Law of Dominance and the hybrid will show only dominant trait.
Detailed Answer :
The starch synthesis in pea plant is controlled by a single gene. This gene in pea plant shows some degree of pleiotropy as it controls the shape of the seed and in addition the size of starch grain too. This gene has two alleles B and b. The BB homozygotes produce large starch grains as compared to those produced by bb homozygotes. Mature homozygous BB seeds where round while bb seeds were wrinkled. The heterozygotes Bb form round seeds but the starch grains were of intermediate size. Thus if the size of starch grain is considered the Bb seeds show the phenomenon of incomplete dominance, but if seed shape is considered then allele B and b show dominant-recessive relationship. Thus in this case the pattern of inheritance deviates from the Mendelian Law of Inheritance in that here the dominance of an allele is not absolute but depends upon the product and the particular phenotype that it forms.
