Chromosomal aberrations, both structural and numerical leading to phenotypic abnormalities provided further proof for chromosomes albeit nucleus to be the only organelle concerned with the transmission of characters.
The discovery of sex linked characters gave further impetus for the chromosome theory of inheritance as it was firmly established that a particular trait followed a particular chromosome.
An additional proof for chromosomes being the only link between one generation and the other is the fact the gametes, at least the male gamete consists of mainly the nucleus and very little amount of cytoplasm.
With all these overwhelming evidences in favor of nuclear inheritance it was but natural for geneticists to believe that nucleus is the only seat where all the genetic potentialities are located
But there are several instances of a peculiar type of transmission of characters both in the animal and plant kingdom which seemed to defy every known genetic law assuming the whole of inheritance to be through the nucleus.
The inheritance of flower color in 4° Clock plants, Kappa particles in Paramecium etc, cannot be explained on the basis of nuclear inheritance. What then is the secret of their inheritance pattern?
Scientists started looking for other cell organelles which could perhaps play a role in inheritance. The gametes have some amount of cytoplasm surrounding the nucleus. The amount of cytoplasm is more in the case of female gametes, than in the male gametes.
Geneticists started investigating, whether the cytoplasm also could help in the transmission of hereditary characters.
If inheritance could be brought about by cytoplasm, there should be more chances of female or maternal inheritance as the egg contributes more amount of cytoplasm than the male gamete to the offspring.
Further, one of the important requirements of a transmission system (if it exists in cytoplasm) is the capacity of self reproduction and it should have the basic molecules of inheritance viz. nucleic acids – particularly DNA.
Further investigations did provide enough evidence of the existence of transmission systems in the cytoplasm (other than the nucleus).
Plastids, mitochondria, ribosome’s etc., have DNA of their own and many evidences have indicated that they replicate on their own, independent of the replication of nuclear DNA.
These evidences clearly indicated nuclear independent transmission machinery in cytoplasm and geneticists were able to explain the mechanism of inheritance in some apparently paradoxical instances in both plant and animal kingdom.
Some of these are inheritance of flower color in 4° clock plants, inheritance of killer particles (Kappa) in Paramecium etc. These were explained on the basis of cytoplasmic inheritance. Even though there are many organelles in cytoplasm possessing DNA.
Only the role of plastids in inheritance has been proved in plants. In animals, kappa particles and other traits seem to rest in cytoplasm but not in any definite organelle.
The salient features of cytoplasmic inheritance are:
1. Certain somatic characters do not follow the Mendelian pattern, nor are they following the chromosomes in inheritance.
2. The genes or hereditary particles often called plasma genes are located in cytoplasm.
3. In some instances, plasma genes may be traced to definite organelles like plastids.
4. the most salient feature of cytoplasmic inheritance is female dominance. Since the egg contributes more amount of cytoplasm, offspring are more likely to get the maternal characters than the paternal. This has been established by reciprocal crosses.
5. Characters dependent on cytoplasm for transmission are never located on the chromosome. Hence the inheritance is also known as extra chromosomal inheritance.