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

Hint: Pisum sativum is the scientific name of peas which belongs to the family Fabaceae. Let us first understand what is Mendelian law of dominance. Law of dominance states that for a pair of inherited traits, one of the factors will be dominant and the other pair will be recessive, unless both the genes are recessive.

Complete Answer:
In pea plants, a single gene controls the starch synthesis which has two alleles B and b. Effective starch is synthesized by BB homozygotes that results in large starch grains. Where as bb homozygotes synthesize lesser efficient starch compared to BB homozygotes therefore resulting in small starch grains.

After mutation of seeds it was observed that bb seeds were wrinkled and BB seeds were round in shape. When they were crossed,in case of heterozygotes, the resultant progeny Bb seeds were intermediate in size. The cross of the alleles is shown below;

	B	B
b	Bb	Bb
b	Bb	Bb

This pattern of inheritance deviates from the Mendelian law of dominance. Here, if the size of a starch grain is considered to be a phenotype, then the alleles of the starch grain shows incomplete dominance. Therefore dominance is not an independent feature of the gene. It is dependent on the gene product as well as production of a specific phenotype from this product.

Note: Mendelian proposed three laws of inheritance. They are Law of Dominance, Law of Segregation, and Law of independent assortment. The laws of inheritance play a vital role in determining the pattern of disease transmission.