12th Class Biology Principle Of Inheritance And Variation Cytoplasmic / Extrachromosomal Inheritance

The fact that nucleus contains the units of inheritance was proposed by Oscar Hertwig in 1870. The mechanism was clearly understood with the development of Mendel's laws of inheritance. Further researchers proposed that cytoplasm also contains the hereditary material. The evidence for cytoplasmic inheritance was first presented by Correns in Mirabilis Jalapa and by Baur in Pelargonium zonale in 1908. The cytoplasm in such cases contain self perpetuating hereditary particles formed of DNA. These may be mitochondria, plastids or foreign organism, etc. The total self duplicating hereditary material of cytoplasm is called plasmon and the cytoplasmic units of inheritance are described as plasmagenes.

Criteria for cytoplasmic inheritance : The cases of cytoplasmic inheritance are found to exhibit maternal influence. The reason is very simple. Very little cytoplasm is contained in the sperm cell of an animal. Most of the cytoplasm is contributed to the zygote by the ovum or egg. Hence if there are hereditary units in the cytoplasm, these will be transmitted to the offsprings through the egg. The offspring, therefore will exhibit maternal influence. This could be explained further by following example :

(i) Maternal influence on shell coiling in snail.

(ii) Inheritance of sigma particles in Drosophila.

(iii) Breast tumour in mice.

(iv) Plastid inheritance in Mirabilis (4 O' clock plant).     

(v) Plastid inheritance in Oenothera.

(vi) Male sterility in plants – e.q. maize.

(vii) Inheritance of kappa particles in Paramecium.

(viii) Mitochondrial genetics – Sacromyces cerevieacae, Neurospora – crassa, Aspergillus nidulens.