Q.14 The direction of shell coiling in the snail Limnaea peregra is a classic example of
(A) chromosomal inheritance (B) extra-chromosomal inheritance
(C) chromosomal translocation (D) homologous recombination
Shell coiling in the snail Limnaea peregra follows maternal effect inheritance, where offspring phenotype depends on the mother’s nuclear genotype via egg cytoplasm factors. Dextral (right-handed) coiling dominates (D allele) over sinistral (left-handed, d allele), but cleavage orientation during early embryogenesis dictates shell direction. This non-Mendelian pattern appears extra-chromosomal since maternal cytoplasm controls it.
Correct Answer
Extra-chromosomal inheritance (B) best describes this phenomenon. The mother’s genotype establishes egg polarity before fertilization, producing a cytoplasmic determinant that orients the first mitotic spindle and determines coiling—independent of offspring’s nuclear genes. F1 from sinistral dd mothers (Dd genotype) show sinistral shells, while F2 from those F1 (Dd mothers) exhibit dextral coiling despite mixed genotypes.
Option Explanations
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Chromosomal inheritance (A): Follows nuclear Mendelian segregation from both parents; shell coiling delays phenotypic expression one generation, violating direct genotype-phenotype linkage.
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Extra-chromosomal inheritance (B): Captures maternal cytoplasmic control over developmental asymmetry, distinguishing it from true cytoplasmic (mitochondrial) inheritance by nuclear gene mediation.
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Chromosomal translocation (C): Involves DNA segment movement causing gene disruption or fusion; irrelevant to coiling’s cytoplasmic determination mechanism.
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Homologous recombination (D): Exchanges genetic material between similar chromosomes during meiosis; does not explain maternal-specific phenotypic control.
Research Relevance
In biotechnology, this model informs maternal effect genes in microbial genetics and bioengineering, aiding enzyme expression studies in asymmetric cell divisions. Jaipur researchers apply it to developmental biology, enhancing SEO-optimized publications on non-Mendelian inheritance patterns.
