I'm not 100% sure, but I'd say the availability of vaccines and medicine.
You can do golden rice. It was genetically modified to produce beta carotene, which rice does not usually have. When metabolized by the body beta carotene is turned to vitamin A which is essential to human vision, immune systems, and healthier skin
Alright if this works the same way it does with humans (i'm almost totally sure it does but just in case) the answer is: the zygote has 56 chromosomes, 28 inherited from the mother and 28 from the father.