Question

In a cross of Lymnaea, the snail contributing the eggs was dextral but of unknown genotype. Both the genotype and the phenotype of the other snail are unknown. All \(\mathrm{F}_{1}\) offspring exhibited dextral coiling. Ten of the \(\mathrm{F}_{1}\) 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?

Short answer

Answer: Both original parents have the genotype Dd (heterozygous), with the dextral coiling phenotype.

Step-by-step solution

Understand the information provided

Here's the given information: - The snail contributing the eggs is dextral but has unknown genotype. - Both genotype and phenotype of the other snail are unknown. - All the F1 offspring have dextral coiling. - When F1 snails undergo self-fertilization, half of them produce only dextrally coiled offspring, and the other half produce only sinistrally coiled offspring.

Determine the possible genotypes of the dextrally coiled snail

The dextrally coiled snail either has a homozygous dominant genotype (DD) or a heterozygous genotype (Dd). The gene for dextral coiling (D) is dominant, and the gene for sinistral coiling (d) is recessive.

Analyze the F1 offspring

Since the F1 offspring are all dextrally coiled, it means that at least one of their alleles must be the dominant D allele. The possible combinations are DD (homozygous dominant) or Dd (heterozygous).

Determine the genotype of the unknown snail based on the F1 offspring

All of the F1 offspring have the dominant D allele, which means that the unknown snail parent (the one that is not contributing the eggs) must also have a D allele. Since the F1 offspring are either DD or Dd, and all of them have the dextral coiling phenotype, we can deduce that the unknown snail parent is either DD or Dd.

Analyze the offspring from self-fertilization

When the F1 snails undergo self-fertilization, half produce only dextrally coiled offspring, and half produce only sinistrally coiled offspring. This result indicates that half of the F1 snails are homozygous dominant (DD), and half are heterozygous (Dd).

Determine the genotypes of the original parents

We can deduce the genotypes of the original parents as follows: - The snail contributing the eggs must be heterozygous (Dd) because if it were homozygous dominant (DD), none of the F1 offspring would produce sinistrally coiled offspring during self-fertilization, and that's not the case. - The unknown snail parent must also be heterozygous (Dd) because it produces both DD and Dd F1 offspring based on the results of self-fertilization. In conclusion, both original parents have the genotype Dd (heterozygous), with the dextral coiling phenotype.