Question

Two phenotypically normal mice produce a single f1 male progeny that is also phenotypically normal. this f1 male is mated with an igf2/igf2 female and ½ of the progeny are dwarf. where did the igf2m allele in the f2 progeny originate?

Answer 1

IN the case of IGF2 gene, we can observe the process of genomic imprinting. This means, that in mice, only the allele inherited from the individual's father is expressed.
So, in the parent generation, there was a cross between a male that is normal and a female that has the IGF2m gene.
In the F1 generation, the male progeny had a genotype of IGF2/IGF2m , but the mutated allele was inherited from the mother, so it wasn't expressed.
In the F2 generation, half of the progeny inherited the IGF2m allele from their father, and only that allele was expressed due to genomic imprinting.
So, the IGF2m allele in the F2 progeny originates from the female in the parent generation (their ''grandmother'').