37. A mouse carrying two alleles of insulin-like growth factor II (igf2) is normal in size; whereas a mouse that carries two mutant alleles lacking the growth factor is dwarf. The size of a heterozygous mouse carrying one normal and one mutant allele depends on the parental origin of the wild type allele. Such pattern of inheritance is known as
(1) Sex-linked inheritance
(2) Genomic imprinting
(3) Gene-envr interaction
(4) Cytoplasm inheritance

Genomic imprinting is the inheritance pattern where the expression of a gene depends on whether the allele is inherited from the mother or the father. This explains why a mouse carrying two normal Igf2 alleles is normal in size, while one with two mutant alleles is dwarf. Importantly, a heterozygous mouse’s size depends on which parent contributed the functional Igf2 allele, illustrating genomic imprinting.​

Explanation of Options in the Question

• Sex-linked inheritance: This involves genes located on sex chromosomes (X or Y). It typically affects males and females differently but does not involve parental-origin-specific expression of alleles on autosomes like Igf2. Therefore, this option doesn’t fit the Igf2 pattern.

• Genomic imprinting: This is an epigenetic phenomenon where genes are expressed in a parent-of-origin-specific manner. For Igf2 in mice, only the paternal allele is expressed, while the maternal allele is silenced (imprinted). This leads to phenotypic differences based on the parental origin of the wild-type allele, matching the question’s description.

• Gene-environment interaction: This refers to the effect of environmental factors on gene expression or phenotype. It does not explain differential expression based on parental allele origin.

• Cytoplasmic inheritance: This concerns genes inherited through cytoplasm (such as mitochondrial DNA), which are generally maternally inherited. It does not explain nuclear gene expression patterns like those seen for Igf2.

Genomic imprinting detailed

Genomic imprinting involves epigenetic marks such as DNA methylation that silence one allele depending on the parent it comes from. For Igf2, the paternal allele is active, and the maternal allele is silenced, meaning the mouse’s growth phenotype depends on whether the functional allele is inherited from the father or mother. If the paternal allele is mutant, the mouse is dwarf even if the maternal allele is wild type because the maternal allele is imprinted (silenced).​

This epigenetic regulation ensures that only one copy of certain genes is expressed, which differs from typical Mendelian inheritance where both alleles contribute equally. The mechanism resets in the germline for each generation, maintaining the imprinting pattern.​

This inheritance pattern described in the question is best identified as genomic imprinting.