Cytoplasmic Inheritance and Maternal Effect

DR. ASHISH PATELAssistant professor

Dept. AGB, Veterinary College, AAU, Anand

In mendelian inheritance

The contribution of both male and female is equal, so, the reciprocal crosses are identical.

The segregation produces 3:1 ratio in F1 monohybrid and 9:3:3:1 in F2 dihybrid.

The gene showing the mendelian inheritance are located on chromosomes of nuclei.

In non mendelian inheritance The reciprocal cross gives different results. The trait onlyfrom the female parent is transmitted

in to next generation (Because the cytoplasm is usually contributed entirely by one parent).

Nuclear genes can be easily mapped on chromosomes, but it is difficult to map cytoplasmic genes or prepare linkage map for such genes.

There is no segregation F2 generation. So, the non mendelian inheritance are divided in to two categories:

Maternal EffectsExtra nuclear inheritance / Cytoplasmic inheritance

The evidence of cytoplasmic inheritance was first presented by Correns in mirablis jalapa.

In 1943, Sonnenborn discovered kappa particles in paramecium and they are inherited through cytoplasm.

In extra nuclear inheritance the trait or character of female parent is only transmitted to the progeny. The reciprocal crosses exhibit difference in phenotypes of progeny and there is no segregation of gene in F2 generation. Such type of inheritance is called as cytoplasmic inheritance, extra chromosomal inheritance and maternal inheritance.

The genes governing the characters showing non mendelian inheritance are located outside of nucleus and found in cytoplasm, these genes are called as plasma genes or cytoplasmic genes or cytogenes or extranuclear genes or extrachromosomal genes.

The total gene present in the cytoplasm of a cell or an individual is known as Plasmon, while all genes in plastid called as plastome.

The genes present in the mitochondria called as chondriome.

The mitochondrial genes are abbreviated as mt DNA and the chloroplast genes are abbreviated as cp DNA.

Sr. Mendelian inheritance Non mendelian inheritance1 Governed by nuclear genes. Governed by plasma genes.2 Distinct segregation pattern. No distinct segregation.

3 Reciprocal differences not observed.

Reciprocal differences observed.

4 Does not show maternal effects.

Shows maternal effects.

5 Genes can be easily mapped on chromosomes.

Mapping of plasma genes is very difficult.

Examples for Non mendelian inheritance Plastid inheritance in Mirabilis Shell coiling in snail Kappa partcles in Paramecium Cytoplasmic male sterility in maize Sigma virus in Drosophila melanogaster Milk factor in mice Classes of Non mendelian Inheritance There are three different classes: Maternal Effects Inheritance Involving Infective Particles Cytoplasmic Inheritance

Maternal Effects When the expression of a character is influenced

by the genotype of female parent, it is referred to as maternal effect.

Such characters exhibit clear cut differences in F1 for reciprocal crosses.

Example: Coiling Pattern of Shell in Snail (Lymnea Peregra)

Two types of coiling pattern - right handed (dextral) (clockwise) and left handed (sinistral) (anticlockwise).

The dextral coiling: dominant allele D and sinistral by recessive allele dDextral coiling female X Sinistral coiling male All F1 and F2 progenies are of dextral type coiling. However, in F3 generation 3 dextral and 1 sinistral types of coiling observed.

In reciprocal cross, Sinistral coiling female X Dextral coiling male, All F1 progenies have sinistral coiling pattern but in F2 generation all progenies have dextral coiling pattern. However, in F3 3 dextral and 1 sinistral types of coiling observed.

This indicates that the inheritance of coiling direction in water snail depends on the genotype of female parent and not on its own genotype. 

The F1 progeny from both the crosses had the same genotype, Dd but they showed the different phenotypes. The phenotype of offspring by the mother’s genotype for coiling.

When F1 undergo for self fertilization the F2 offspring from both the crosses, irrespective of their own genotypes, showed the same phenotype (Dextral).

The true nature of inheritance of coiling is indicated when each F2 individual undergoes for self fertilization to produce F3. In F3, 3 dextral and 1 sinistral types of coiling observed and therefore this pattern of inheritance is also called as delayed mendelian inheritance.

“Phenotype of progeny exclusively depends on the genotype of mother irrespective of the offspring’s own genotype. Thus the ‘DD’ and ‘Dd’ mothers produce dextral progeny while, ‘dd’ mother always produce sinistral progeny”.

Inheritance Involving Infective Particles

In some cases, Non mendelian inheritance is associated with infective particles like parasite, symbiont or viruses which are present in the cytoplasm of an organism. However, such cases are not considered as true examples of cytoplasmic inheritance.

Examples: Kappa Particles in Paramecium and Sigma Particle in Drosophila

T. M. Sonneborn described the inheritance of some cytoplasmic particles known as kappa in Paramecium aurelia.

There are two strains of Paramecium: killer and sensitive.

Killer strain produces a toxic substance called paramecin that is lethal to other individuals called "sensitives" . The production of paramecin in killer type is controlled by certain cytoplasmic particles known as kappa particles. The sensitive strains lack these particles.

The kappa particles are transmitted through the cytoplasm. The existence, production and maintenance of kappa particles are controlled by a dominant gene ‘K’ present in the nucleus. However, ‘K’ cannot initiate the production of kappa in the total absence of kappa in the cytoplasm.

When a Paramecium of killer strain is having the genotype “KK” or (K+) conjugates with the Paramecium of non-killer strain having the genotype “kk”, the exconjugants are all heterozygous for “Kk” genes.

The development of a particular type depends upon the duration of cytoplasmic exchange If conjugation is normal, i.e., lasts only for a short time, and no exchange of cytoplasm takes place between the two, both killers and non-killers (sensitive) are produced.

However in rare or prolonged conjugation in addition to the nuclear material, the cytoplasmic materials are also exchanged.

During this cytoplasmic exchange, the kappa particles present in the cytoplasm of the killer type enter the non-killer type and convert it into a killer type. So all the offspring produced by the exconjugants are killer type.

Cytoplasmic Inheritance

There are two types of cytoplasmic inheritance:(1) plastid inheritance (2) mitochondrial inheritance. The first example of cytoplasmic inheritance was

reported by Correns (1909) in a variegated variety of the four-o'clock plant Mirabilis jalapa.

Variegated plants have some branches which carry normal green leaves, some branches with variegated leaves (mosaic of green and white patches) and some branches which have all white leaves.

Flowers on wholly green branches produce seeds that grow into normal plants.

Flowers on variegated branches yield offspring of three kinds- green, white and variegated in variable proportions.

Flowers from branches wholly white produce seeds that grow into white plants that is without chlorophyll.

The phenotype of the progeny always resembled the female parent and the male made no contribution at all to the character. So cytoplasm of the egg influences the type of leaf in Mirabilis.

The explanation for this unusual pattern of inheritance is that the genes concerned are located in the plastids within the cytoplasm, not in the nucleus and are therefore transmitted only through the female parent.

Plastids are of two types, namely green chloroplasts and colourless leucoplasts.

Green branches contain Green plastids in their leaves . Variegated branches contain Green plastids and

Colourless plastids.

Colourless branches are due to the presence of Colourless plastids.