Genetic inheritance explains how characteristics or traits are passed from one generation to the next. Genetic material is passed from the parents to the offspring following the Mendelian pattern of inheritance. But this stands true only for the nuclear genetic material.
But the extra chromosomal material found in the cytoplasm follow the non Mendelian pattern of inheritance, also known as cytoplasmic or maternal inheritance. Extra chromosomal inheritance was first reported by Boris Ephrussi in yeast during 1949.
Cytoplasmic DNA or extrachromosomal DNA are significantly found in organelles such as mitochondria and chloroplast. These organelles play a very important role in the inheritance of certain characteristics. However ,it is still not clear how these organelles acquired their genetic material. One theory suggests that these organelles were once free living bacteria which then entered into a symbiotic relationship with cells and ended up in state they are now, evolving as a major organelle in a living eukaryotic cell.
The mitochondrial and chloroplast genome is made up of few genes and several thousand base pairs, still it has their own rRNA, tRNA and DNA for replication, translation and transcription. Additionally, chloroplast has antibiotic resistance genes indicating that it was derived from bacteria, previously.
DEFINITION– The extrachromosomal DNA present in the cytoplasm which follows the non mendelian pattern of inheritance is known as extrachromosomal or cytoplasmic inheritance.
- The genes in extrachromosomal inheritance are also known as cytoplasmic genes, extrachromosomal genes or extranuclear genes.
- Generally only one parent contributes to such type of inheritance, mostly female parent.
- Chloroplast and mitochondria are two organelles which have shown such an inheritance pattern.
- Such type of inheritance are also known as maternal inheritance as mostly it is the female parent that contributes to the cytoplasm of the offspring.
- Mendelian principles are not valid for cytoplasmic inheritance.
EXAMPLE OF CYTOPLASMIC INHERITANCE
- Chloroplast inheritance-Variegation in Four O’clock Plant:
Mirabilis jalaba (4’o’ clock plant) is usually seen with three kinds of leaves. Some are fully green, some pale green or white and certain others a mixture of green and white (variegated). The inheritance pattern of these leaves have were found to follow the extra chromosomal model of inheritance.
Microscopic analysis of these leaves showed green leaves and the green areas of the variegated leaves containing normal chloroplastids and chlorophyll pigment, whereas the pale- leaves and pale patches lacking normal chloroplastids and pigment. On studying the inheritance of the leaf color, it was found that phenotype of the progeny will depend on phenotype of the parent.
Ovules derived from fully green portions of the plant, regardless of the source of pollen, will result only fully green plants and variegated character will not reappear in the subsequent generations.
When ovules are derived from variegated branches, three types of seeds are produced in variable numbers, again regardless of the male parent; some give rise to (sure green, some to pure white, and the majority to variegated offspring’s. Ovules derived from pale branches, will result only pale plants.
2. Mitochondrial inheritance- Cytoplasmic male sterility
Cytoplasmic male sterility is an example for mitochondrial inheritance and is associated with pollen failure in plants. In certain plants fertility is atleast partly controlled by the cytoplasmic genes. Absence of these genes may result in cytoplasmic male sterility.
In such cases, it has been noted that if the female parent is male sterile the progeny will also be male sterile as the cytoplasm of the progeny is mainly derived from the egg produced by the male sterile female parent.
Rhoades (1933) reported the analysis of first cytoplasmic male sterile plants in maize and demonstrated that male sterility was contributed by female parent and nuclear genes had no influence. A particular male sterile plant produced only male sterile plant progeny when fertilized with pollen from normal maize plants. It was also noticed that repeated backcrossing of the male sterile plant with male fertile plant did not restore male fertility confirming the fact that the genes are present in the mitochondria (cytoplasm) and not in the nuclear region.
When the expression of a character if influenced by the genotype of the mother, it is referred to as maternal effect. Maternal effects are seen in both plants and animals. The best studied example is coiling pattern in snail shells. It is a classic example of the intricate relationship between the maternal genotype and egg cytoplasm. The effect of maternal genotype on the coiling behavior in snail was studied by Sturtevant.
He showed that there are two strains of water snails (Limnaea peregra) that differ each other in the direction of coiling of shell.
Looking into the opening of the shell it can be seen that in one strain the shell always coils to the left (sinistral) whereas in the other strain the shell always coils to the right (dextral).
In the cross dextral x sinistral all the F, progeny have dextial coils implying that dextral is dominant over sinistral. However, in the F1, x F1, cross (i.e., inbreeding), all the F1 snails are also dextral. The reciprocal cross (dextral x sinistral ) produces F1, progeny that are all left coiler. In this case F1, x F1, cross also yields only dextral coils.
From these experiments it becomes clear that coiling of snails is not determined by individuals’ own genes but by those of mother.
The offspring whose mothers are either homozygous or heterozygous for right coiling are right coilers even if they are homozygous for sinistrality (left coiling). In the same way offspring of left coiling mother are left coilers even if they carry dominant genes for right coiling.