“the genetic material from the sperm hasn’t completed combination with that from the mother until the 16th day”
Sorry, but that simply is not true. A zygote, at the single cell stage has unique DNA, a full compliment of 46 chromosomes that idenitify it as a unique individual. Combination of DNA is complete before the first division begins. The zygote is a totipotent cell and its unique DNA signature is carried forth in every division from that point forward.
Do feel free to provide some credible science that states explicitly that the offspring of two human beings is ever anything but a human being.
Gastrulation and the formation of the three germinal layers is the beginning of the subdivision of the mass of embryonic cells produced by cleavage. The cells then begin to change and diversify under the direction of the genes. The genes brought in by the sperm exert control for the first time; during cleavage all processes seem to be under control of the maternal genes. In cases of hybridization, in which individuals from different species produce offspring, the influence of the sperm is first apparent at gastrulation: paternal characteristics may appear at this stage; or the embryo may stop developing and die if the paternal genes are incompatible with the egg (as is the case in hybridization between species distantly related). Encyclopædia Britannica, 2009.
Search under 'Gastrulation' in the article on animal development.