The maternal effect is a fascinating non-Mendelian pattern of inheritance where the phenotype of an offspring is determined not by its own genotype, but by the genotype of its mother. This means that even if an offspring has a genotype that would typically lead to a specific phenotype, its actual phenotype can be influenced by the mother's genetic makeup. This phenomenon occurs in various organisms and is particularly significant in understanding developmental biology.
To grasp the maternal effect, it's essential to consider the process of meiosis. In a heterozygous cell, which contains both a dominant allele (represented as D) and a recessive allele (represented as d), DNA replication occurs before meiosis begins. This replication results in identical copies of each allele. During meiosis, specifically in the first division (meiosis I), these alleles segregate into different cells. However, some gene products from the dominant and recessive alleles may remain in the cells, influencing the phenotype of the offspring.
For instance, if the mother's egg contains leftover mRNA or proteins from her genotype, these gene products can affect the offspring's development. A classic example of this is seen in the shell coiling of the snail species Limnaea. In this case, right-handed coiling is associated with the dominant allele, while left-handed coiling is linked to the homozygous recessive genotype. However, due to the maternal effect, even if an offspring has a homozygous recessive genotype, its phenotype will reflect the mother's genotype. Thus, if the mother is homozygous dominant or heterozygous, the offspring will exhibit right-handed coiling, regardless of its own genetic makeup.
This illustrates the core concept of the maternal effect: the phenotype of the offspring is dictated by the mother's genotype, showcasing a unique aspect of genetic inheritance that diverges from traditional Mendelian principles.