In a cross of Lymnaea , the snail contributing the eggs was dextral but of unkno
ID: 168718 • Letter: I
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 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?
d) One of the original parents must have been DD, while the other must have been dd.
a) One of the original parents must have been dd and the other must have been dd too. b) One of the original parents must have been Dd and the other must have been Dd too. c) One of the original parents must have been Dd, while the other must have been dd.d) One of the original parents must have been DD, while the other must have been dd.
Explanation / Answer
DD and Dd are indistinguishable because they both produce dextral offspring. But DD snails should not exist, except as rare aberration, because the mother would have carried the dominant dextral allele.