10. Maternal inheritance of coiling of shell in snail (limnaea peregra) is well established. The dextral coiling depends on dominant allele D and sinistral coiling depends upon recessive allele d. A female F1 progeny of dextral(Dd) type is crossed with a male sinistral snail. What will be the ratio of heterozygous: homozygous individuals in its F2 progeny?
(1) 3:1           (2) 1:1
(3) 1:3           (4) 1:2:1

The inheritance ratio of heterozygous to homozygous individuals in the F2 progeny of this snail shell coiling experiment is 1:1.

Explanation of Inheritance Pattern in Snails

The coiling of snail shells in Limnaea peregra exhibits maternal inheritance, where the phenotype of the offspring depends primarily on the genotype of the mother rather than the individual’s own genes. In this case, dextral (right-coiling) is dominant (allele D), and sinistral (left-coiling) is recessive (allele d).​​

Cross Details and F2 Ratio

Given that a female F1 snail with heterozygous genotype Dd (dextral) is crossed with a male sinistral (dd), the F1 genotypes are:

• Female: Dd

• Male: dd

The F2 progeny resulting from selfing or crossing these F1s produce genotypic ratios of 1:2:1:

• 1 DD (homozygous dextral)

• 2 Dd (heterozygous dextral)

• 1 dd (homozygous sinistral)

However, since maternal inheritance determines the phenotype—regardless of the genotype—the phenotype ratio in F2 becomes a 1:1 ratio of heterozygous (Dd) and homozygous (DD or dd) individuals for the genotype depending on the maternal genotype.​​

Ratio of Heterozygous to Homozygous

In the F2 generation, the ratio of heterozygous: homozygous individuals (considering maternal influence) will be:

• Dd (heterozygous): 1 part

• Homozygous (either DD or dd): 1 part combined

Thus, the ratio is 1:1.​

Explanation of options:

• Option 1 (3:1): Typical Mendelian phenotypic ratio for dominant-recessive inheritance in F2, but not relevant here because phenotype depends on maternal genotype, not individual genotype.

• Option 2 (1:1): Correct, because it reflects the ratio of heterozygous to homozygous genotypes under maternal inheritance.

• Option 3 (1:3): Opposite of the correct ratio.

• Option 4 (1:2:1): The genotypic ratio in F2 but not in phenotypic maternal inheritance scenario.

Introduction:
Maternal inheritance plays a crucial role in determining traits such as shell coiling in Limnaea peregra, where the phenotype depends on the mother’s genotype. Understanding this inheritance pattern, including the expected genotypic ratios in F2 progeny, is fundamental to studying genetic principles in developmental biology.