How did Griffith prove transforming principle in Genetics. Explain the procedure.

OR

How do “Pleiotropy”, “incomplete dominance”, “co-dominance” and “polygenic inheritance” deviate from the observation made by Mendel ? Explain with the help of one example for each.

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.

OR

Pleiotropy:

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.

Co-dominance:

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 I^A, I^B and i. In humans each person possesses any of the two of these three I gene alleles. I^A and I^B are completely dominant over i. When, I^B and i are present only I^B is expressed. Similar is the case of I^A and i, but when I^A and I^B 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.