39. Gene ‘A’ in mouse is needed for its normal growth [normal size). Mouse strains with deletion in gene ‘A’ (A^del) have been developed.
    When the following cross is made

…..A^delA X AA……
All progeny (both males and females) are of normal size When the F1, progeny with the following genotypes are crossed:

…..A^delA X AA……

50% of the progeny obtained are small in size.
The above observation can be explained by:
(1) Maternal inheritance
(2) Maternal imprinting
(3) Paternal imprinting
(4) Maternal effect

Gene ‘A’ in mouse growth: Explanation of inheritance and imprinting effects

The observation that all F1 progeny from an AdelA x AA cross are normal size, but 50% of the next generation (from AdelA x AA F1 intercross) are small indicates a non-Mendelian inheritance pattern related to parental origin effects. This is best explained by maternal effect, where the mother’s genotype influences offspring phenotype regardless of the offspring’s genotype.

Explanation of each option:

1. Maternal inheritance refers to the transmission of genes exclusively or predominantly from the mother, often via mitochondria. This does not explain the observed Mendelian ratio of normal and small mice because mitochondrial inheritance affects all offspring similarly and is not involved in gene A’s nuclear inheritance pattern.

2. Maternal imprinting means that the allele inherited from the mother is epigenetically silenced, so only the paternal allele is expressed. Such imprinting would lead to defects only if the paternal allele was absent or mutated. However, here the effect seems linked to the mother’s genotype, not the silencing of the maternal allele itself.

3. Paternal imprinting is the epigenetic silencing of the paternal allele. This does not fit because the phenotype depends on the mother’s genotype controlling offspring size, not expression from the paternal allele.

4. Maternal effect occurs when the mother’s genotype affects the phenotype of her offspring directly, often through substances deposited in the egg like mRNAs or proteins prior to fertilization. In this case, Adel deletion in mothers leads to offspring being small-sized regardless of their own genotype if the mother lacks a functional gene A product.

Introduction: Gene ‘A’ in mice is crucial for normal growth. Crossing experiments with gene ‘A’ deletion strains reveal growth defects in offspring linked to the mother’s genotype. This article clarifies the concepts of maternal inheritance, imprinting, and maternal effect to explain the observed inheritance pattern and its impact on mouse size.

This question demonstrates how maternal effect genes influence offspring phenotypes based on the mother’s genotype rather than the offspring’s own gene status, differing fundamentally from maternal or paternal imprinting which involve allele silencing based on parental origin. These insights are vital in genetic studies involving growth and development in model organisms like mice.​