11. A male snail homozygous for dextral alleles is crossed with a female homozygous for sinistral alleles. All the F1 individuals showed sinistral phenotype. When F1 progeny snails were self-fertilized all individuals of F2 progeny had dextral coiling. This experiment demonstrated
(1) dominant epistasis as dextral allele is dominant over sinistral allele
(2) recessive epistasis as in F2 dextral allele appeared in homozygous condition
(3) maternal effect as the nuclear genotype of the F1 mother has governed the phenotype of the F2 individuals.
(4) maternal inheritance as the mitochondrial genes of the F1 mother has governed the phenotype of the F2 individuals.

Introduction:
Shell coiling direction in snails—dextral (right-handed) or sinistral (left-handed)—is a classic example of maternal effect inheritance. This phenomenon shows how the mother’s genotype, rather than the individual’s own genotype, determines the shell coiling phenotype of the offspring. Understanding this is crucial for geneticists studying non-Mendelian inheritance patterns, particularly in organisms such as the snail Limnaea.

Explanation of the Experiment and Options:

In the cross where a male snail homozygous for dextral alleles (DD) is crossed with a female snail homozygous for sinistral alleles (dd), all F1 progeny unexpectedly show the sinistral phenotype even though they carry a dominant dextral allele from the father. Upon self-fertilization of these F1 snails, all F2 progeny exhibit dextral coiling. This outcome highlights that the phenotype depends not on the individual’s genotype but on the maternal genotype.​​

Now, analyzing the options:

1. Dominant epistasis as dextral allele is dominant over sinistral allele: Although dextral (D) is dominant over sinistral (d) in allele terms, the maternal effect overrides this simple dominance in F1. The F1 phenotype showing all sinistral despite heterozygosity contradicts straightforward dominant epistasis, ruling out this as the main explanation.

2. Recessive epistasis as in F2 dextral allele appeared in homozygous condition: The appearance of dextral coiling in all F2 is due to maternal genotype influence rather than recessive epistasis where one gene masks another. Recessive epistasis typically involves gene interaction, not maternal control, so this is incorrect.

3. Maternal effect as the nuclear genotype of the F1 mother has governed the phenotype of the F2 individuals: This is the correct explanation. The phenotype of the offspring depends on the mother’s nuclear genotype. The F1 individuals have a sinistral phenotype because their mother was sinistral, and their F2 offspring have dextral phenotype because the F1 snail mothers carry the dextral allele genotype and produce the dextral phenotype in their progeny.

4. Maternal inheritance as the mitochondrial genes of the F1 mother has governed the phenotype of the F2 individuals: Mitochondrial (cytoplasmic) inheritance involves organelle DNA passed from the mother, but shell coiling is controlled by nuclear genes expressed via maternal effect, not mitochondrial genes, so this is incorrect.

Summary:
The experiment with snail shell coiling demonstrates the concept of maternal effect inheritance, where the nuclear genotype of the mother determines the phenotype of the offspring regardless of the offspring’s own genotype. This explains why F1 shows sinistral coiling despite carrying a dominant dextral allele and why all F2 progeny are dextral due to their mothers’ genotype controlling the phenotype.

This understanding is essential for recognizing complex inheritance mechanisms in genetics beyond Mendelian principles, especially maternal effect inheritance illustrated clearly by snail shell coiling.​​

Keywords: maternal effect shell coiling snails, snail genetics maternal effect, dextral sinistral inheritance, Limnaea snail coiling