Humans are usually monoovulators, and for identical twins to occur, a single sperm fertilizes a single egg to create a zygote, and that zygote then splits to form two separate embryos; however since the two embryos were originally a single zygote, they are genetically identical (thus "monozygotic" twins).

In contrast, mice and other rodents are polyovulators, where female mice release several eggs with each estrus cycle, each with different genetics (assuming they are outbred mouse stocks). If there is successful mating by a male during estrus, several eggs are each fertilized by a different spermatozoan (which are also not genetically identical), resulting in multiple zygotes each with different genetics and therefore different genotypes from the start. If you are working with an inbred mouse strain (such as BALB/c or C57BL/6), each mouse is genetically identical except for the sex chromosomes, so any resulting offspring are going to have the same genotype as the parents and as their littermates unless a mutation occurs.

Thanks ! Victoria, you answer is quite clear.

Dear Hao, in mouse, many ovums release from ovary. So, different genetically offspring produce. But in human, one zygote with determined genotype may divide into two masses and therefore the offspring have the identical genotype and phenotype.