Question

In water snails, shell coiling displays maternal effect and right-handed shell coiling (D) is dominant to left-handed shell coiling (d).

A heterozygous left-handed female is crossed with a heterozygous right-handed male.

1. Their offspring will display what genotypic ratio of DD:Dd:dd?

2. What is the phenotypic ratio that will be right-handed and left-handed?

Answer 1

Maternal effect is the phenomenon in which phenotype of the offspring is NOT determined its own the genotype but detrmined by the genotype of the female parent. (by maternal genotype only)

There is NO role genotype of male parent (paternal genptype)

Right-handed shell coiling / dextral (D) is dominant to left-handed shell coiling / sinistral (d).

Given cross: A heterozygous left-handed female is crossed with a heterozygous right-handed male.

Parent	Left handed coiling; female	Right handed coiling, male
Genotype	Dd	Dd
Gametes	D , d	D , d

F1 generation:

	D	d
D	DD right-handed	Dd right-handed
d	Dd right-handed	dd right-handed

Genotypic ratio: 1 DD : 2 Dd : 1 dd (1:2:1)

Pheotypic ratio: All offsprings are with right handed coiling

Explaination:

All offspings are with right handed coiling because female parent is heterozygous (Dd). It has one of the allele as D. So, all offsprings whether DD, Dd or dd all will be right handed, as their phenotype is determined by the D in the female parent.