Extra chromosomal / Cytoplasmic inheritance

Certain characters are controlled by non- nuclear genomes found in chloroplast, mitochondria, infective agents and plasmids. These characters do not reveal Mendelian pattern of inheritance. The inheritance of the extra chromosomal genes are found to exhibit maternal influence. Maternal effect is due to the asymmetric contribution of the female parent to the development of zygote. Although both male and female parents contribute equally to the zygote in terms of chromosomal genes, the female parent usually contributes the zygote’s initial cytoplasm and organelles, since the sperms contain very little cytoplasm. If there are hereditary units in the cytoplasm, these will be transmitted to the offsprings through the egg, so the offsprings exhibit maternal effect.

The cytoplasmic extranuclear genes have a characteristic pattern of inheritance which do not resemble the genes of nuclear chromosomes and is known as extra chromosomal or extra nuclear or cytoplasmic inheritance and exhibit maternal influence. In extra nuclear inheritance, male and female parents contribute equally their nuclear genes to the progeny but do not make equal contribution of extra chromosomal genes hence, the crosses can yield different (or) non Mendelian results. Cytoplasmic inheritance in animals can be studied with reference to shell coiling in Limnaea and kappa particles in Paramecium.