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Class 12th Biology 2012 Set1 Delhi Board Paper Solution

Question 30

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?
State the aim and describe Messelson and Stahl’s experiment.


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


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.


Messelson and Stahl in 1958 aimed at proving that the DNA replicates in a semi-conservative fashion. The semi-conservative DNA replication suggests that after the completion of replication, each DNA molecule will have one parental and one newly-synthesised strand.

Experimental proof

(1)  E.coli was grown in a medium containing 15NH4 Cl (15 N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. As a result, 15 N  was incorporated into the newly-synthesised DNA. This heavy DNA could be distinguished by centrifugations in CsCl density gradient.

(2) Then, these E. coli cells were transferred to a medium with normal 14NH4 Cl and the DNA was extracted as double-stranded helix. The various samples were separated on CsCl gradients for measuring the density of DNA.

(3)  After 40 minutes, the DNA of the second generation was extracted from the 14NH4  Cl medium and was found to have equal amounts of hybrid and light DNA.