Q.What is extra chromosomal inheritance? Explain with an example.

Step 1. Define the phenomenon: certain hereditary characters are controlled by genomes located outside the nucleus — in the chloroplast, mitochondrion, an infective cytoplasmic agent, or a plasmid — and such characters do not follow ordinary Mendelian segregation ratios; this is called extra-chromosomal, extra-nuclear or cytoplasmic inheritance.

Step 2. Explain the maternal bias: although sperm and egg contribute equally to a zygote's nuclear (chromosomal) genes, only the egg supplies the zygote's initial cytoplasm and organelles in any real quantity, since a sperm carries very little cytoplasm of its own; any hereditary unit riding in that cytoplasm is therefore transmitted essentially only through the mother, producing what is called maternal effect and often giving reciprocal crosses (swapping which parent is male/female) different results — something ordinary nuclear Mendelian inheritance never does. …

Define extra-chromosomal inheritance, explain why it shows maternal bias, and walk thro …

• Explaining maternal effect as the mother's genes being 'dominant', rather than correctly attributing it to her being the sole supplier of the zygote's initial cytoplasm. …