In the first generation cross from otherwise purebred strains, the offspring will always be 50% one lineage and 50% the other.
AFTER that, all bets are off. on a population scale and across multiple children, you'd get approximately 50% genomic remix, so its roughly accurate to call Pan 25% saiyajin, but in reality, she could be anywhere in the range from 0% to 50% depending on which chromosomes she inherited from her father, and it gets more complex even than that, as we don't really know which parts of the genome are responsible for saiyajin traits. its possible that every saiyajin in the show is effectively 100% saiyajin (even if the genomes are technically not) if all of those traits are dominant and have been fully passed onto their offspring.
We'll never get a full answer to this, nor do I actually want the author to try, Toriyama has no expertise in genetics and development and so whatever response he gives is likely to give me a migraine.