The direction of shell coiling in the snail Lymnaea peregra...

The direction of shell coiling in the snail Lymnaea peregra (discussed in the introduction to this chapter) results from genetic maternal effect. An autosomal allele for a right-handed, or dextral, shell $\left(s^{+}\right)$ is dominant over the allele for a left-handed, or sinistral, shell (s). A pet snail called Martha is sinistral and reproduces only as a female (the snails are hermaphroditic). Indicate which of the following statements are true and which are false. Explain your reasoning in each case. a. Martha's genotype must be ss. b. Martha's genotype cannot be $s^{+} s^{+}$ c. All the offspring produced by Martha must be sinistral. d. At least some of the offspring produced by Martha must be sinistral. e. Martha's mother must have been sinistral. f. All of Martha's brothers must be sinistral.

The direction of shell coiling in the snail Lymnaea peregra (discussed in the introduction to this chapter) results from genetic maternal effect. An autosomal allele for a right-handed, or dextral, shell $\left(s^{+}\right)$ is dominant over the allele for a left-handed, or sinistral, shell (s). A pet snail called Martha is sinistral and reproduces only as a female (the snails are hermaphroditic). Indicate which of the following statements are true and which are false. Explain your reasoning in each case.
a. Martha's genotype must be ss.
b. Martha's genotype cannot be $s^{+} s^{+}$
c. All the offspring produced by Martha must be sinistral.
d. At least some of the offspring produced by Martha must be sinistral.
e. Martha's mother must have been sinistral.
f. All of Martha's brothers must be sinistral.

Key Concepts

Maternal Effect

This concept refers to situations where the phenotype of the offspring is determined not solely by its own genes but also by the genotype of the mother. In traits governed by maternal effects, gene products deposited in the egg can dictate the developmental outcome, independent of the offspring's genotype.

Autosomal Dominance and Recessiveness

This principle explains that alleles located on non-sex chromosomes (autosomes) can show dominance, where a single copy of the dominant allele is sufficient to mask the phenotype of the recessive allele. In this context, the dominant allele produces a trait even when present in only one copy, while the recessive trait requires two copies to be expressed.

Genotype versus Phenotype Relationships

Understanding the difference between genotype (the genetic makeup) and phenotype (the observable traits) is crucial, especially in scenarios where external factors like maternal contributions influence the final phenotype. The genotype provides the potential for a trait, but the maternal effect can alter its expression in the offspring.

Inheritance Patterns in Hermaphrodites

Even though the organism is hermaphroditic, standard Mendelian inheritance applies for autosomal genes. This means that alleles are transmitted following the same rules as in single-sex organisms, with dominant and recessive interactions dictating the trait outcomes regardless of the individual's sex.

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