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Genetics
- 1. GENETICS Presentation by Karthi.M I MSc -BIOCHEMISTRY
- 2. Extra-Nuclear Inheritance by Endosymbionts • Certain intra-cellular parasites such as bacteria and virus particles maintain symbiotic relationship with host cells. • They are self-reproducing and look like the cytoplasmic inclusions. • Sometimes they exhibit an infection like transmission with a hereditary continuity of their own. • Generally such symbionts are coined by letters of the Greek alphabets (sigma,kappa,mu,etc.).
- 3. CO2 sensitivity in Drosophila • L.Heritier and Teissier found that a certain strain of drosophila melanogaster shows a high degree of sensitivity to CO2. • Where as the wild type strain can be exposed for long periods to pure CO2 without permanent damage. • The sensitive strain quickly becomes unco-ordinated in even brief exposure to low concentrations. • This trait (extra-sensitivity) is transmitted primarily, but not exclusively, through the maternal parent.
- 4. • Tests have disclosed that CO2 sensitivity is dependent upon an infectious DNA virus called sigma. • It is found in cytoplasm of CO2 sensitives Drosophila. • These infective particles are transmitted normally via the egg’s larger amount of cytoplasm but occasionally through the sperms as well. • CO2 sensitivity may even be induced in normal flies by injections of cell free extracts of sigma particles from CO2 sensitive flies.
- 5. Kappa Particles in Paramecium • In 1938, T.M.Sonneborn reported that some races (known as “killers” or killer strain) of the common ciliate protozoan, Paramecium aurelia produce a poisonous substances, called paramecin. • which is lethal to other individuals called “sensitives”. • The paramecin is water soluble, diffusible and depends for its production upon cytoplasmically located particles called kappa.
- 6. • Electron microscopic observations -kappa particles are about-0.4 micron long symbiotic bacteria, caedobacter taeniospiralis. • 20% of kappa bacteria of the killer strain contain a refractile protein containing “R body” and are called “brights”. • They are infected with a virus that controls the synthesis of toxic viral protein, the paramecin. • A killer Paramecium may contain hundreds(eg:400) of kappa particles.
- 7. • The presence of kappa particles in the killer Paramecium is dependent for their maintenance and replication on the chromosomal dominant gene K. • Paramecia with nuclear genotype kk are unable to harbour kappa particles. • When a Paramecium of killer strain having the genotype KK or (K+) conjugates with the Paramecium of nonkiller strain having the genotype kk, the exoconjugants are all heterozygous Kk genes.
- 9. Milk Factor In Mice • Bittner found that females of certain lines of mice are highly susceptible to mammary cancer and this trait was found to be maternally transmitted trait. • Results of reciprocal crosses between these and animals of low- cancer incidence strain depend on the characteristic of the female parent. • When the young mice of a low-incidence strain are allowed to be nursed by susceptible foster mothers produces a high rate of cancer in them.
- 10. • Apparently this is a case of infective agent transmitted in the milk. • This so called milk factor resembles in many respects with a virus and has been discovered to be transmissible also by saliva and semen. • The presence of milk factor also depends on nuclear genes.
- 11. Thank u all