Question

In Limnea peregra, snail coiling is determined by maternal effect. Two snails with sinistral coils were crossed (only one of the reciprocal crosses was performed). All progeny have dextral coiling. When these progeny are self-fertilized, half of them produce all dextral progeny, and the other half produce all sinistral progeny. What are the genotypes of the original male and female parents?

Answer 1

snail coiling is determined by maternal effect, maternal effect means phenotype of the progenies is determined by the genotype of the mother.

let the alleles be D-dextral and d-sinistral.

both snails are sinistral so they got d mRNA in the egg cell, so their mother's genotype is dd, so they have inherited d allele from their mother, but they have dextral progenies, which means the females have D allele so that it could transfer D mRNA to the egg cell.

so the genotype of female parents is Dd.

When F1 progeny are self-fertilized, half of them produce all dextral progeny, and the other half produce all sinistral progeny, that 50% which produced progeny with dextral coiling has D allele and the 50% of the progeny which produced sinistral coiling is dd.

so among F1 progeny 50% are Dd and 50% are dd, so the father's genotype is dd

Dd * dd

	D	d
d	Dd ( dextral)	dd ( sinistral)

here half of the progeny are Dd and half are dd. so genotype of original male and female parents are dd and Dd respectively.