He could also just cum in a cup, genderbend, inseminate himself and carry the child for 9 month and turn back
Wouldn’t that make Luffy‘s dna 100% dragons and therefore just a clone of him?
no idea if it works like that, any doctor in the room?
Not really.
We are all the result of the fusion between a male pronucleus (located in the spermatozoid) and a female pronucleus (in the ovocyte). A pronucleus is half a nucleus (in a nutshell).
All the nuclei of your cells have 23 pairs of chromosomes. A pronucleus has only 23 chromosomes. 2 pronuclei fuse together to form a nucleus of 23 pairs of chromosomes.
However, the formation of spermatozoids (males) and ovocytes (females) is the result of meiosis.
Meiosis is what allowed your dad and your mom to form the gametes (spermatozoids and ovocytes) that fused later to give rise to you.
However, your dad and mom don't produce identical gametes, in fact we all generate variability during gametogenesis. The spermatozoids that your dad produces are all different (this sentence sounds weird...)
Your dad and mom have each one of them 23 chromosomes from their respective dad and 23 from their respective mom so 23 pairs of chromosomes.
During meiosis, you separate these chromosomes so each gamete has 23 chromosomes (and not 46) as you generate a pronucleus (half a nucleus). This separation is randomly so you can't tell what you have in each gamete and as a result, you generate different gametes.
Additionally, this is more complex because you have crossing over during meiosis which fuses random fragments of these chromosome pairs between them so you don't simply separate these chromosome pairs in half, in fact you separate newly fused chromosomes into two, which generates more variability.
So, you have tons of possibilities even if the two gametes would come from the same person as you would still create variability, though the variability originates from the same individual.
I'm a biomedical scientist (in other words, a biologist in medicine).