Q. 254.0( 1 Vote )

How did Griffith

Answer :

Transformation occurs when one bacterium picks up free-floating DNA and incorporates it into its own genome.

Griffith’s experiment:

1. Fredrick Griffith carried out series of experiments with Streptococcus pneumoniae

2. According to him, when the bacteria is grown on a culture plate, some produce smooth shiny colonies (S), while some produce Rough (R) colonies.

3. This is because the S-strain bacteria have a mucous coat, while R-strain does not have.

4. Mice infected with S-strand from pneumonia but mice infected with R-strand do not develop pneumonia.

S-strain—injected————-Mice———-Mice die

R-strain—injected————Mice————Mice live

5. Griffith killed bacteria by heating and observed the heat-killed S-strain bacteria injected into mice did nt kill them.On injecting mixture of heat-killed S and live R bacteria, the mice died. He recovered living S-bacteria from died mice.

S-strain (heat killed)———-mice———-Mice live

S-strain (heat killed) + R-strain(live)————mice———-mice die

6. From this experiment, he concluded that the R-strain bacteria had been transformed by the heat-killed S-strain bacteria. Some transforming principle transferred from heat-killed S-strain, had enabled the R-strain to synthesise a smooth polysaccharide coat and become virulent. This must be due to transfer of the genetic material.



Mendel’s observation: one gene is responsible for a single phenotype.

Deviation: single gene exhibits multiple phenotypic expressions. E.g- Disease phenylketonuria, a single gene codes for enzyme phenylalanine hydroxylase which manifests in the form of mental retardation and a reduction in hair and skin pigmentation.


Mendel’s observation: only one of the parental character appear in a monohybrid cross in the F1 generation.

Deviation: in a mono-hybrid cross phenotype in the F1 generation resembles both the parents.

E.g- ABO blood group: ABO blood groups are controlled by the gene I. The gene I has three alleles IA, IB and i. In humans each person possesses any of the two of these three I gene alleles. IA and IB are completely dominant over i. When, IB and i are present only IB is expressed. Similar is the case of IA and i, but when IA and IB are present both express their own types, this is due to co-dominance.

Incomplete Dominance

Mendel’s observation: only one of the parental characters appear in a monohybrid cross in the F1 generation.

Deviation: in a monohybrid cross phenotype in the F1 generation did not resemble either of the two parents and was in between the two.

E.g- Snapdragon/Antirrhinum. There are two types pure breeding plants , red flowered and white flowered. On crossing the two F1 plants posses pink flowers, and on selfing them, F2 generation has 1 red:2pink:1white.

Polygeneic inheritance:

Mendel’s observation: all the traits have distinct alternate forms since one gene is responsible for a single phenotype.

Deviation: the occurrence of traits is spread across a gradient. Such traits and controlled by three or more genes. E’g- human height or skin colour.

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