AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

CYTOPLASMIC INHERITANCE

Cytoplasmic inheritance

• There are many exceptions to the rule in genetics.
• One of them is that not all inherited characters are determined by genes located in the nucleus.
• Some self replicating genes (DNA) are present in the cytoplasm (mitochondrial DNA and chloroplast DNA) also. These are called plasmagenes or cytogenes or plasmids or plasmons etc. The inheritance of characters by plasmagenes is called Non-mendelian or Extra-chromosomal or Cytoplasmic or Extra-nuclear inheritance.
• Since they are extrachromosomal (i.e. outside the chromosomes), such genes are not subject to the normal rules of Mendelian heredity.
• In most organisms, the organelles pass through the egg and not the sperm, giving a strict maternal pattern of inheritance for any mutations that may be present in the organelle DNA.
• In cytoplasmic inheritance, the results of reciprocal crosses are not the same.

Maternal inheritance

• The determination of the phenotype of offspring by the genotype of female parent is called maternal inheritance or uniparental inheritance or maternal effect.
• Example: Pattern of shell coiling in snail.
  • Here the shell coiling is determined by the genotype of the mother and not by the individual’s own genotype. 

Examples for cytoplasmic inheritance

• Plastid inheritance in Mirabilis
• Shell-coiling in snail
• Kappa particles in Paramecium
• Cytoplasmic male sterility in maize
• Sigma virus in Drosophila melanogaster
• Milk factor in mice