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?
OR
State the aim and describe Messelson and Stahl’s experiment.
• A Single gene controls the size of the starch grain and the seed shape.
• 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 trait of size of starch grain shows incomplete dominance.
• Hence in heterozygous condition the starch grain are of intermediate size.
• Deviation from Mendel's law of dominance that is observed is that if starch grain size is considered as the phenotype, then the alleles show incomplete dominance as we get intermediate sizes but in case of Mendel’s peas this wasn’t observed.
OR
The aim of Messelson and Stahl's experiment was to prove that DNA replicates semi conservatively.
The experiment is as follows:
(i) They grew E. coli in a medium containing 15NH4Cl as the only nitrogen source for many bacterial generations. The result was that 15N was incorporated into newly synthesised DNA. This could be distinguished from the normal DNA by CsCl density gradient centrifugation.
(ii) Then the cells were transferred into a medium with normal 14NH4Cl and then samples were taken at various time intervals and eventually dsDNA was extracted.
(iii) It was seen that the DNA that was extracted from the culture one generation after the transfer from 15N to 14N medium had a hybrid or intermediate density.
iv) DNA extracted from the culture after another generation was composed of equal amounts of this hybrid DNA and of ‘light - 14N’ DNA.
The experiment proved that the DNA in chromosomes replicate semi-conservatively.
The experiment is shown by a schematic representation below:
