Question

What is the inheritance pattern observed in the size of starch grains and seed shape of Pisum sativum? Workout the monohybrid cross showing the above traits. How does this pattern of inheritance deviate from that of Mendelian law of dominance?

Answer 1

The starch synthesis in pea plants is controlled by a single gene. It has two alleles B and b. BB homozygotes produced large starch grains as compared to that produced by bb homozygotes. After maturation it was observed that BB seeds were round and bb were wrinkled. When they were crossed the resultant progeny were intermediate-sized Bb seeds. The cross involved is

	B	B
b	Bb	Bb
b	Bb	Bb

Deviation from Mendel's law of dominance: If starch grain size is considered as the phenotype, then from this angle, the alleles show incomplete dominance. Thus, dominance is not an autonomous feature of a gene, it depends on gene product and production of particular phenotype from this product.