In a cross of Limnaea, the snail contributing the eggs was dextral but of unknown genotype. Both the genotype and the phenotype of the other snail are unknown. All F1 offspring exhibited dextral coiling. Ten of the F1 snails were allowed to undergo self-fertilization. One-half produced only dextrally coiled offspring, whereas the other half produced only sinistrally coiled offspring. What were the genotypes of the original parents?

Maternal inheritance refers to those cases in which certain phenotypic traits depends on the maternal genotype, independently of the individual genotype. In a cross, the genotype follows the Mendelian model, but the phenotype is influenced by the maternal genotype. The phenotypic traits of generations F1, F2, F3, and so on, do not correspond to the expression of their own genes, but ti the expression of the maternal genes.

Maternal inheritance has been studied in the Lymnaea species. In gasteropods, the dextral coil is clockwise, while the sinistral coil is counter-clockwise. Dextral is expressed by the dominant allele (D), while sinistral is expressed by the recessive allele (d).

In the exposed example:

The snail contributing the eggs (females) was dextral but of unknown genotype --> D-
Both the genotype and the phenotype of the other snail are unknown.
All F1 offspring exhibited dextral coiling
In the F2, One-half produced only dextrally coiled offspring, whereas the other half produced only sinistrally coiled offspring.

Cross)

Parental) D- x --

F1) 100% dextral (The whole progeny exhibited the dextral trait because the

individual that contribute the eggs was dextrally coiled)

F2) 50% dextral

50% sinistral

The fact that the F2 expresses 50% sinistral and 50% dextral means that one of the progenitors that contributed with the eggs (female) must have had at least one of the dominant allele D for the dextral trait, while the other female must have had the recessive genotype dd for the sinistral trait.

So the genotypes for the original parents must have been Dd x dd

1st Cross:

Parental) Dd x dd

The snail contributing the eggs Dd

the snail contributing the sperm dd

F1) 2/4 = 1/2 Dd

2/4 = 1/2 dd

100% expressing dextral coiling

2nd Cross:

Parental) Dd x Dd dd x dd

F2) 1/4 DD 100% dd

2/4 Dd 100% Sinistral coiling

1/4 dd

100% Dextral coiling

50% of the F2 exhibited dextral coiling, while the other 50% exhibited

sinistral coiling