a maternal effect is a situation where the phenotype of an organism is determined not only by the environment it experiences and its genotype, but also by the environment and genotype of its mother. in genetics, maternal effects occur when an organism shows the phenotype expected from the genotype of the mother, irrespective of its own genotype, often due to the mother supplying messenger rna or proteins to the egg. maternal effects can also be caused by the maternal environment independent of genotype, sometimes controlling the size, sex, or behaviour of the offspring. these adaptive maternal effects lead to phenotypes of offspring that increase their fitness. further, it introduces the concept of phenotypic plasticity, an important evolutionary concept. it has been proposed that maternal effects are important for the evolution of adaptive responses to environmental heterogeneity.