In this article we will discuss about the characteristics and examples of Cytoplasmic Inheritance.
Characteristics of Cytoplasmic Inheritance:
Information concerning cytoplasmic inheritance is meagre but whatever is known about the occurrence and action that is highly confusing. Some of the cytoplasm characteristics appear to be under the ultimate control of chromosomes but in other cases there is no evidence of such a relationship. Only a few cases of cytoplasmic inheritance, completely independent of nuclear genes, are known.
The cytoplasmic inheritance can be detected by special methods. Two rules are used for their detection; one is negative and the other positive. Genes in diploid organisms exist in pair and two members or alternative forms of a single gene are called alleles. The alleles are located on homologous chromosomes at the same position.
In the process of hybridization, one allele is contributed by the female parent and the other is contributed by male parent. The outcome of the cross in terms of phenotype of F, hybrid is same irrespective of which of the two alleles, recessive or dominant was contributed by female parent.
Thus in Mendelian inheritance phenotypic expression of F1 hybrids in reciprocal crosses is identical as illustrated below:
In certain cases, however, it has been found that phenotypes of F1 hybrids of reciprocal crosses are different. If it is so, it is the first indication that the inheritance of that particular character may not follow Mendelian pattern. Such phenotype is influenced by maternal parent (mother). This type of inheritance is referred to as Non-Mendelian inharitance.
The negative rule states that hereditary traits which are transmitted by cytoplasm must not show Mendelian segregation in crosses. The reciprocal crosses with respect to a particular set of characters governed by a set of extra-chromosomal genes yield dissimilar hybrids.
The positive rule is based on the fact that most of the cytoplasmically inherited characters would follow the matemal (mother) lines, i.e., uniparental mode of transmission.
In higher plants and animals, large ova (microgametes) contain large amount of cytoplasm but male gametes or sperms (microgametes) have essentially negligible amount of cytoplasm, so under such conditions most of cytoplasmic factors are transmitted to the offsprings through the ova of mother.
It is known as maternal inheritance or trans-ovarian transmission. In this mode of transmission, all the progenies of the parents have maternal features and only female parent can transmit the cytoplasmic characters to the succeeding generation.
Maternal inheritance may be, of two kinds:
I. On one hand, some treatments if applied to the female parent may affect its egg’s cytoplasm in such a way that her subsequent offsprings are modified in some way. Effects of this kind are called dauer-modifications or persisting modifications. These characteristically persist for only a small number of generations unless the same treatment is repeated on the progeny.
The dauer- modification was first discovered experimentally by V. Jollos. He treated protozoa with chemical poisons and heat shocks and induced several morphological abnormalities in them.
These could have been simple environmental modifications but they persisted in the progenies for several generations after the environmental stimulus was removed. Such abnormalities go on decreasing generation after generation and eventually disappear completely, as the original treated cytoplasm decreases gradually through cell division.
Fruit-flies subjected to heat treatment and bacteria treated with chemicals have provided further evidences.
II. On the other hand, some maternal inheritance are also known which do not depend upon the repeated application of an external stimulus to the cytoplasm. True cytoplasmic inheritance is controlled by independent cytoplasmic genes.
Maternal effects reflect the influence of mother’s genes on developing tissues and dauer modifications reflect the temporary influence of cytoplasmic structures induced temporarily by environmental conditions.
Some Examples of Cytoplasmic Inheritance:
The following are the two major groups of cytoplasmic factors which constitute extra-nuclear genetic systems:
1. The first group of cytoplasmic factors involves intracellular organelles. In eukaryotic cells, two cytoplasmic organelles, mitochondria and the plastids of green plants contain their own genetic materials (extra-nuclear genes or extra chromosomal genes. The term extra-chromosomal gene is potentially confusing because the genes found in the mitochondria and chloroplasts do in fact comprise extra-nuclear DNA.
2. The second group of cytoplasmic factors involves intracellular symbionts such as bacteria and viruses that live symbiotically within cells of host organisms. They replicate along with host’s genetic material and their presence causes a special modification of phenotype of host organisms.