CYTOPLASMIC INHERITANCE

CYTOPLASMIC INHERITANCE

• Discovered by Correns.
• Inheritance of characters which are controlled by cytogene or cytoplasm is called cytoplasmic inheritance.

There are 3 types of cytoplasmic inheritance:-
1. Maternal inheritance.
2. Organellar Inheritance.
3. Inheritance involving infectious particles.

• Genes which are present in cytoplasm called 'cytogene' or 'plasmagene' or 'extra nuclear gene'.
• Total cytogene present in cytoplasm is called 'plasmon'.
• A gene which is located in the nucleus is called 'karyogene'.
• Inheritance of cytogene in organisms occurs only through the female. Because female gamete has karyoplasm, simultaneously it has cytogene because of more cytoplasm.
• The male gamete of higher plant is called male nucleus. It has very minute [equivelent to nil] cytoplasm. So male gamete only inherited karyogene.
• Thus, inheritance of cytogene occurs only through female. (Also called maternal inheritance).
• If there is a reciprocal cross in this condition, then results may be affected.
• Cytoplasmic inheritance involving essential organelles like, Chroloplast and mitochondria called as organellar inheritance.

Example of Organellar Inheritance: (true examples of cytoplasmic inheritance).

1. plastid inheritance in Mirabilia jalapa- cytoplasmic inheritance was first discovered by Correns in Mirabilia jalapa. In Mirabilis jalapa branch (leaf) colour is decided by type of plastid present in leaf cells. So it is an example of cytoplasmic inheritance.

2. Male sterility in maize plant- Gene of male sterelity is present in mitochondria. If normal male plant crossed with a female plant which has genes of male sterility then all the generation of male become sterile because a particular gene was present with female which has inherited by female.

3. Albinism in plant- Gene of albinism found in chloroplast. Gene of albinism in maize is lethal.

CYTOPLASMIC INHERITANCE- maternal effects in Limnea shell coiling.

Shell Coiling In fresh water Snail (Lymnaea peregra).

• In snails (gastropods)the shell is spirally coiled.
• Snails exhibit two types of coiling of their shell:

fig : A sinistral (left) and a dextral (right) shell

• Shell coiled to right is dextral.
• Shell coiled to left is sinistral.
• This direction of coiling is genetically controlled. The gene for dextral coiling is dominant 'D' and for sinistral coiling is recessive 'd'. So, that the dextral is 'DD' and sinistral is 'dd'.

• Boycott and Driver(1923) showed that the character of coiling is determined by the gene of the mother and not by the individual's own gene.

• Shell coiling in the hermaphroditic snail Limnaea peregra may be right-handed (dextral) or left-handed (sinistral).
• The coiling depends on the genotype of egg donor parent, regardless of the phenotype of that parent.
• Shell coiling in snail is controlled by D (dextral) and d (sinistral) alleles. The genotype of progeny depends on genotype of mother. If mother is "DD", the Dd offspring are dextral but the "Dd" progeny of "dd" mother has sinistral coiling.

CARBON DIOXIDE SENSITIVITY IN DROSOPHILA

Carbon dioxide is used as an anaesthetic for Drosophila and normal flies can with stand high CO2 concentrations without any adverse effect. But some strains of Drosophila are sensitive to CO2 so that within a few minutes they become paralysed and die. Reciprocal crosses between CO2 sensitive and resistance flies gave differing results.

If a CO2 sensitive female is crossed with a normal (resistant) male, almost all the offspring are sensitive. But the reciprocal cross of a sensitive male with a normal female yields almost all normal offspring indicating extra nuclear inheritance of this trait.

There is mostly maternal transmission although some transmission through sperm may also take place. Moreover, normal flies can be induced to become sensitive by injecting a cell free filtrate of haemolyph obtain from sensitive flies.

The casual agent for CO2 sensitivity has been observed in electron micrographs to be a virus-like particle called sigma. In flies that have been induced to become sensitive, sigma enters the egg cytoplasm and is transmitted maternally.

In the naturally occuring sensitive strains, sigma particles become incorporated into the nucleus (without associating with a specific chromosome) of the egg or some of the sperms and are transmitted in a non-mendelian manner. They are self-reproducing independent bodies and can mutate.

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