Maternal effect


 * Do not confuse with the pseudoscientific theory of Maternal impression 

A maternal effect, in genetics, is the phenomenon where the genotype of a mother is expressed in the phenotype of its offspring, unaltered by paternal genetic influence. The phenotype of an individual therefore reflects the genotype of its mother, rather than the genotype of the individual.

This maternal effect is usually attributed to maternally-produced molecules, such as mRNAs, that are deposited in the egg cell. Maternal effect genes often affect early developmental processes. An example in Drosophila melanogaster morphogenesis is axis formation, in which mRNA such as Bicoid and nanos is of maternal origin and loaded into the egg prior to fertilisation. Another mechanism for the specific expression of genes from one parent is stable epigenetic modification of germ line genes in one of the sexes. This form of a parental effect is termed genomic imprinting.

"Maternal effect" should not be confused with maternal inheritance, in which some aspect of an offspring's genotype is inherited solely from the mother. This is often attributed to maternal inheritance of mitochondria or plastids, each of which contains its own genome. Maternal inheritance is distinct from maternal effect inheritance because in maternal inheritance the individual's phenotype reflects its own genotype, rather than the genotype of a parent.

In contrast, a paternal effect is when a phenotype results from the genotype of the father, rather than the genotype of the individual. The genes responsible for these effects are components of sperm that are involved in fertilisation and early development. An example of a paternal-effect gene is the ms(3)sneaky in Drosophila, males with a mutant allele of this gene produce sperm that are able to fertilise an egg, but the snky-inseminated eggs do not develop normally. However, females with this mutation produce eggs that undergo normal development when fertilised.