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12th Biology 2012 Set1 Outside Delhi Board Paper Solution

Question 30

Describe Frederick GriffithÂ’s experiment on Streptococcus pneumonia. Discuss the conclusion he arrived at.
(a) Explain a monohybrid cross taking seed coat colour as a trait in Pisum sativum. Work out the cross upto F₂ generation.
(b) State the laws of inheritance that can be derived from such a cross.
(c) How is the phenotypic ratio of F₂ generation different in a dihybrid cross?

Frederick Griffith performed experiments on Streptococcus pneumonia which is responsible for causing pneumonia and mice. He observed that the bacteria produce two colonies:
(i) R strain: or rough colonies which are non-virulent.
(ii) S strain: or smooth colonies which are virulent and caused death of mice.
He observed that S cells are virulent due to production of a smooth polysaccharide coat which is absent in R- cells. He performed the following experiment:
Mice + R-cell bacteria  ----> Mice lived
Mice + S- cell bacteria ------> Mice died
Then he killed S- celled bacteria by heating which cause denaturation of its genetic material and then injected in mice.
Mice + Heat killed S- cells ------> Mice lived
Now he injected mice with heat killed S- cells and non-virulent R- cells.
Mice + Heat killed S- cells + R-cells ----->Mice died.
Then he recovered living S-cells from the dead mice.
Then he concluded that some transforming principle had been transferred from heat killed S- cell to living R- cells which enables R- cells to secrete polysaccharide coat and become virulent.

Pisum sativum

(b)The law of inheritance is the law of segregation which is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F₂ generation.
(c)The phenotypic ratio of F₂ generation in a monohybrid cross is 1:2:1 whereas the ratios of F₂ generation in a dihybrid cross is 9:3:3:1.