I am working on transgenic mouse model in which the homozygote of my protein mutation is lethal in the mouse. Therefore I went back to E3.5 to see whether I can isolate the homozygote of my ES cell or not. I could isolate the ES clls with homozygote genotype. the interesting thing is that the ES cells with homozygote genotype are derived from smaller blastocysts in size. I would like to know what can be the reasons behind the smaller size of blastocysts with homozygote genotype