Question

Starch synthesis in pea is multiple alleles/ not

Answer 1

Starch synthesis in pea is not controlled by Multiple alleles.

It is controlled by a single gene having two alleles. Starch synthesis in pea plants (Pisum sativum)  is associated with another interesting phenomenon of pleiotropy.
 
The gene associated with starch synthesis, controls the size of the grain formed, as well as its shape. When we look at the cross of F2 generation, where heterozygous individuals are crossed, we observe interesting results in the phenotypes of offsprings in the two characters it controls (size and shape).

Shape:-                              Size:-

RR - Round                        RR - Large
Rr  - Round                        Rr   - Intermediate
rr   - Wrinkled                     rr    - Small

Here we see that the same gene 'r' affects the two characters in different ways. The R gene shows Dominance over r when the shape is being decided. It (R) is however, incompletely dominant over r when it comes to size. 

The difference in size of the pea grains can be understood as the gene product i.e starch is not synthesised in the same quantity in all the three genotypes. The heterozygous(Rr) offspring, produces somewhat less starch than the homozygous (RR) does, thus resulting in the difference in size of the grain.

The cross involved here is:-

Answer 2

Yes It is. Any gene contains different alleles and so is the one that controls starch synthesis in pea plant. The starch synthesis in the pea plant is controlled by a single gene which has 2 alleles - B and b. It is an example of incomplete dominance as far as the phenotype of grain size is considered.

The BB alleles will produce starch grains that are big and bb will produce smaller starch grains.