Final answer:
The Igf2m allele in the heterozygous dwarf female mouse likely came from the mother due to the process of genomic imprinting, where the maternal Igf2 allele is typically silenced.
Step-by-step explanation:
Considering the nature of the gene Igf2 in mouse genetics, which is involved in growth regulation, the allele notation Igf2m typically represents an imprinted allele. Genomic imprinting is an epigenetic phenomenon by which certain genes are expressed in a parent-of-origin-specific manner. When it comes to the Igf2 gene in mice, the paternal allele is usually the active one, while the maternal allele (Igf2m) is often silenced due to imprinting. Therefore, given that the dwarf female mouse is heterozygous for Igf2 (Igf2 Igf2m), we can conclude that the Igf2m allele likely came from the mother, because the maternal allele would have been imprinted (silenced), allowing the phenotype to be dictated by the paternal allele.