How two males can have a baby.
Having read an obscene amount of Gravy fanfics, I noticed that one of the many Gravy fans' dreams (right there with a 'mushy Yuki' and a 'strawberry-and-chocolate-covered-Shuichi') is a 'real-Yuki-and-Shu' baby. Sadly enough, this is something Mother Nature doesn't normally allow, and we are on a regular basis presented with various Alternate Universes. For the sake of those of us, who want to see a 'real-Yuki-and-Shu' baby, but do not like the idea of AUs and like to stick to some semblance of reality, a naturally (if only theoretically) possible way for two males to have a baby is presented. Being an undergraduate genetics student, I came up with this idea in my free time. Now to the technicalities.
We have all heard of the procedure of cloning. In this process, an oocyte (egg, gamete) from a female donor is taken, the nucleus (the place where all DNA and therefore genetic information is stored) is removed from it and is then replaced with a nucleus from a normal body cell. The oocyte is then induced to divide and we get a full organism with the genetic makeup of the organism from which the body cell is taken. In normal reproduction, a spermatozoid enters an oocyte, their nuclei are fused and the fertilized cell now starts to divide. I am sure most of you people know it already, but I repeat it just to be sure. Now to the interesting part. We can theoretically join the processes of normal fertilization and cloning to obtain a baby for two males. To this end we take sperm samples from the males in question (even one male is actually enough, though it's risky for the health of the baby), and an oocyte from any random female. The nucleus of the oocyte is removed and replaced with the fused nuclei of the two spermatozoa from our males. (The technically most difficult part is fusing these nuclei, but it should be possible, I think.) We now have a fertilized diploid oocyte for our two happy fathers. It can be a girl or a boy (two females can also theoretically have a baby, but only a girl… sniff sniff). There is also a chance of getting a 'YY' baby, but it wouldn't survive anyway (gosh, I love technicalities, did you notice?).
Now that we have a healthy diploid (full number of chromosomes) oocyte, we can induce it to divide and it's development begins. No, males cannot carry the baby for 9 months (no needed organ systems, sorry), which means that the baby will have to develop in either a female body, or an artificial womb. After nine months we get a baby with Shu's hair and Yuki's eyes or whatever you want!
Now some practical ideas for your stories. Techniques like this one are not performed these days, but you can write a contemporary story without stretching the reality too much, just note that this is a novel technique or something. If the story is set in the future you can also use an artificial womb for the baby, the project for this artificial organ is actually being developed these days and will possibly become reality in the next decade or two. Yay for scientific revolution!
If you want to add some drama (I know you people! ) you can have Shu have a child secretly from Yuki (or other way around, whatever works). All you need is a sperm sample after all, though I don't know how he would obtain it… ; Maybe from some medical test?
So here it is, a natural way for a 'Yuichi baby' or 'Hiro-Suguru' or 'Yuki-Tohma'… That's disturbing… Use it wisely kids. If anybody has any questions, don't hesitate to write and ask! Always happy to help!
Sincerely yours, Ray.