Inheritance of most of the characters in eukaryotic organisms shows the following characteristic features.
Non Mendelian Inheritance
•But some characters in several organisms do not show Mendelian inheritance or they show a non Mendelian inheritance pattern.
In such cases, the following characteristic features are observed.
•In addition to these cases of non Mendelian inheritance, some characters in several organisms exhibit a Mendelian inheritance
pattern but the development of these characters in an individual is markedly affected by the genotype of the maternal parent of the concerned individual; such cases are classified as maternal effects.
The evidence for cytoplasmic inheritance was first presented by Correns in Mirabilis jalapa and by Baur in Pelargonium zonale in 1908.
•In case of cytoplasmic inheritance generally the character of only one of the two parents (usually the female parent) is transmitted to the progeny.
•As a result, reciprocal crosses exhibit consistent differences for such characters and there is a lack of segregation in the F2 and the subsequent generations.
•Such inheritance is also referred as extra nuclear inheritance, extrachromosomal inheritance and maternal inheritance.
•Genes governing the traits showing cytoplasmic inheritance are located outside the nucleus and in the cytoplasm; hence they are referred to as plasma genes, cytoplasmic genes, cytogenes, extranuclear genes or extra chromosomal genes.
•The sum total of all the genes present in the cytoplasm of a cell is known as Plasmon, while all the genes present in a plastid constitute a plastron.
Characteristics of cytoplasmic inheritance:
•The known cases of true cytoplasmic inheritance are concerned with either choloroplast or mitochondrial traits and are usually associated with their DNA.
•Such cases are therefore often referred to as organellar inheritance, plastid inheritance and mitochondrial inheritance.
•The inheritance pattern of plastid characters due to plasma genes located in plastid is known as plastid inheritance.
•Plastid inheritance was first case of cytoplasmic inheritance to be discovered independently by Correns and Baur in 1908.
•Variegation refers to the presence of white or yellow spots of variable size on the green back ground of leaves.
•Variegation may be produced by some environmental factors, some nuclear genes and in some cases, plasma genes.
Inheritance of plastids in Mirabilis jalapa:
•The inheritance of plastids in Four ‘O’ clock plant Meiabilis jalapa
was first described by Correns (1908).
•In M. Jalapa, some of the branches may have normal green leaves, while in the same plant, some other branches may have only pale green or white leaves and still others may have variegated leaves.
•Flowers on branches with normal green leaves produce seeds that grow into plants with normal green leaves irrespective of whether they are pollinated by pollen from branches with normal green variegated or pale green leaves.
Progeny of a variegated four ‘O’ clock plant
•It is clear that variegation is determined by agencies transmitted through the female and that it is not influenced by the type of pollen used.
•These agencies are the chloroplast.
•They are capable of self-duplication and are transmitted from generation to generation through the cytoplasm of the egg.
•Seeds borne on a green branch have three gene only green plastids, seeds borne on a pale green branch have three gene only pale green plastids and seeds borne on a variegated branch have green or pale green or a mixture of the two types of plastids.
•Variegation is thus a heredity character determined by stable, self-duplicating, extra nuclear particles called plastids.
•Neither the nucleus of the female gamete nor the male gamete is involved in the control of this type of heredity character.
Maternal inheritance by ‘iojap’ gene in maize
•The egg regularly contributes much more cytoplasm to the next generation than does the sperm.
•It should therefore be expected that in cases of cytoplasmic inheritance, differences between reciprocal crosses would result.
•Rhoades (1946) identified the ‘iojap’ gene (ijij) in maize located in
chromosome VII controlling plastid inheritance in the plant.
•The gene ‘Ij’ is responsible for the normal green colour of the plant.
•When normal green plants with IjIj are used as female and pollinated by pollen from stripped with ijij, F1 plants are wholly green.
Cytoplasmic male sterility in Maize
•In case of male sterility in maize, pollen grains of such male sterile are aborted.
•This male sterility is transmitted only through the female and never by the pollen.
•When all of the chromosomes of the male terile line were replaced with chromosomes of normal plants, the line still remained male sterile, showing thereby that male sterility in controlled by some agency in the cytoplasm.
•It was later recognized that cytoplasmic male sterility in maize results from alterations in the heredity units in the mitochondria (mitochondrial DNA).
Inheritance of Kappa particles in Paramecium
•In Paramecium aurelia, two strains of individuals have been reported.
•One is called as ‘Killer’ which secretes a toxic substance
‘paramecin’ and the other strain in known as ‘sensitive’ and is killed if comes in contact with the ‘paramecin’.
•In the cytoplasm of the killer strain the kappa particles (cytoplasmic – DNA) are present kappa particles are absent in sensitive strains.
•The transmission of kappa particles is through cytoplasm but maintenance of kappa particles and production of paramecin is
controlled by ‘k’ we assume that the killer strains carry dominant allele ‘kk; and that sensitive ‘kk’.
•Conjugation Rare conjugation (cytoplasmic exchange) On conjugation, congugents exchange their nuclear material so that ex-conjugants ‘kk’ resulted from conjugants ‘kk’ and ‘kk’ when conjugation is for normal time, then only nuclear material is exchanged and therefore killer will produce killer daughters and sensitive will produce sensitive daughters.
•But if the conjugation is in longer period, there will be exchange of cytoplasm resulting in the inheritance of kappa particles by both the ex-conjugants so that all the daughter paramecia
produced are killers because all in herit the kappa particles through the mixing of cytoplasm.
•Therefore this trait is transmitted through cytoplasmic heredity.
•The trait is only stable is killer strains.
Inheritance through mitochondria
•Mitochondria can self-replicate and represent another genetic system in the cell.
•Of course, the amount of mitochondrial DNA is so small, representing less than 1% of the nuclear DNA is mammalian cells and it can code for a part of the protein in the mitochondria.
•The synthesis of the cytochrome found in mitochondria for example, is known to be present in minute amount in cytoplasm under the control of nuclear genes.
•Therefore, it is suggested that both mitochondria and chloroplast seem to have a semiautonomous existence and their DNA forms the basis for genetic systems separate from that in the nucleus.
•Some hereditary particles have been found to exist in two states, either in an autonomous state in the cytoplasm, where they replicate in dependently, of the chromosomes, or in an integrated state incorporated into the chromosome.
•Particles with such properties are known as episomes and include such things as the sex factor.
•The episomes are apparently not essential to the life of the bacteria, because they may or may not be present.
•If they are absent, they can be acquired only from an external source.
•In bacteria, E coli, sex is determined by the presence or absence of the sex factor (F).
•Male bacterial cells (donor) have the sex factor and this factor is responsible for the transfer of DNA from male to female bacterial cells (Recipient).
•This sex factor is the cytoplasmic particle.
Significance of Cytoplasmic Inheritance
1.Development of cytoplasmic male sterility several crop plants like maize. Pearl millet, sorghum, cotton etc.
2.Role of mitochondria in the manifestation of heterosis.
3.Mutation of chloroplast DNA and mitochondrial DNA leads to generation of new variation.