Final answer:
The expected phenotypic ratio in the F2 generation of the self-crossed F1 snails is 3 sinistral : 1 dextral. The F2 phenotypic ratio of snail shell coiling in this example of maternal effect would be 100% sinistral since all F1 progeny are sinistral due to the phenotype being determined by the mother's genotype, not by typical dominant-recessive inheritance.
Step-by-step explanation:
In snails, dextral shell coiling is dominant to sinistral. However, when a female sinistral is crossed to a male dextral, all progeny in the F1 generation are sinistral due to maternal effect. When the F1 snails are allowed to self-cross, the expected phenotypic ratio in the F2 generation is 3 sinistral : 1 dextral.
The F2 phenotypic ratio of snail shell coiling in this example of maternal effect would be 100% sinistral since all F1 progeny are sinistral due to the phenotype being determined by the mother's genotype, not by typical dominant-recessive inheritance.
In the case of snail shell coiling, we see an example of maternal effect, where the phenotype of the offspring is determined not by its own genotype but by the genotype of the mother. The phenotypic ratio in the F2 generation of snails from a cross between a sinistral female and a dextral male, where all F1 offspring are sinistral, can be understood by examining the underlying genetics.
Assuming the sinistral (recessive) mother carries two recessive alleles (ss) and the dextral (dominant) father carries at least one dominant allele, the F1 progeny will all display the mother's phenotype due to the maternal effect, despite carrying a heterozygous genotype (Ds).
When these F1 snails self-cross, the genotypes that can be produced are DD, Ds, and ss. However, due to the maternal effect, the phenotype of the progeny will be determined solely by the genotype of the F1 mother. Thus, if the F1 snails are all phenotypically sinistral, they must carry the ss genotype, as maternal effect supersedes the typical dominant-recessive relationship in this case. As a result, all F2 progeny will be sinistral, leading to a phenotypic ratio of 100% sinistral snails.