What is maternal-effect inheritance? Using recent research on snail shell chirality, explain how it differs from Mendelian inheritance.

Maternal-effect inheritance is a non-Mendelian pattern in which an offspring's phenotype is set by its mother's nuclear genotype, not by its own. Snail shell coiling is the classic case. A UK–Japan study in Current Biology (October 2026) has revisited it [1].

Maternal-effect inheritance: the mechanism

• The mother loads gene products (mRNA, proteins) into the egg. These steer the earliest cell divisions before the zygote's own genes switch on [2].
• The controlling gene is nuclear. This makes it different from cytoplasmic (mitochondrial) inheritance.

Evidence from snail shell chirality

• Shells are either dextral (clockwise) or sinistral (counter-clockwise), and the two are mirror images [1].
• Lymnaea (pond snail): the gene is a maternal formin, Lsdia1. A frame-shift mutation blocks the full-length protein in sinistral strains, and genome editing confirmed that it causes the trait [2].
• How the cells are arranged at the 8-cell stage sets handedness. This step acts before (upstream of) Nodal signalling, and pushing these cells by hand reverses it [3].
• Euhadra (land snail, 2026): the gene reported is an unconventional myosin. This is a new finding that still needs independent confirmation [1].
• Both proteins work on the actin cytoskeleton, so handedness at the level of the cell shapes the whole body.

How it differs from Mendelian inheritance

Basis	Mendelian	Maternal effect (snail coiling)
Phenotype decided by	Offspring's own genotype	Mother's genotype [1]
Timing	Shows in the same generation	Shows one generation late
Reciprocal crosses	Same result	Result depends on which parent supplies the egg
Genotype–phenotype match	Direct	A sinistral snail may carry dextral alleles
F2 ratio	3:1	F2 is uniform; 3:1 appears only in F3

• Snails are hermaphrodites, so the "mother" is whichever parent supplies the egg [1].
• Evolutionary result: in a flipped snail the genital pore lies on the opposite side, which creates a mating barrier [4]. A mixed population in north Japan, found in 2023 through hikers' online photos, shows that this barrier is leaky [1].

Snail chirality shows that a gene can act across generations through what the mother puts into the egg. Editing the Euhadra myosin gene, as was done for Lymnaea, would confirm the new finding. It would also deepen our understanding of non-Mendelian inheritance, body asymmetry and how new species form.