Question

Which of the following crosses could produce a right-hand coiled shell heterozygote in the snail Limnaea peregra, if the gene D, which codes for right-hand coiled shells, is dominant to the gene d, which codes for left-hand coiled shells?

1) Right-hand coiled dd female × left-hand coiled Dd male
2) Left-hand coiled dd female × right-hand coiled Dd male
3) Left-hand coiled dd female × right-hand coiled DD male
4) Left-hand coiled Dd female × left-hand coiled dd male

Answer 1

The cross that can produce a right-hand coiled shell heterozygote is a cross between a left-hand coiled dd female and a right-hand coiled Dd male, as this would result in offspring with genotypes Dd or dd.

Step-by-step explanation:

The cross that could produce a right-hand coiled shell heterozygote in the snail Limnaea peregra is the second option: Left-hand coiled dd female × right-hand coiled Dd male. To understand this, we need to consider the genotype of the potential parents and use a Punnett square to determine the possible genotypes of their offspring. Since the right-hand coiled shell gene (D) is dominant to the left-hand coiled shell gene (d), a snail with right-hand coiled shells can have a genotype of either DD or Dd, while a snail with left-hand coiled shells will have a genotype of dd.

For the second option, crossing a dd (left-hand coiled) female with a Dd (right-hand coiled) male would result in the following potential offspring genotypes: Dd (right-hand coiled heterozygote) and dd (left-hand coiled). The resultant offspring would thus have a 50% chance of being heterozygous for the right-hand coiled shell (Dd), fitting the requirement of the question.