What in the reproductive process is so successful in causing 50% of each parents DNA to turn into a normal human?
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Every person carries a double complement of DNA. During reproduction a person’s double complement is first re-shuffled for better random results (random genes are moved from one complement to another and back), and then divided into two single (randomized) complements. Every parent gives one single complement of DNA to a child, who thus gets a normal double complement of DNA again.
I’m not exactly sure what your question even means.
The success of sexual reproduction is that it enables more jumbling of DNA from parent to offspring since it combines the DNA of 2 parents. This gives more variation to offspring than asexual reproduction does. More variation in offspring means that if something, say a disease, comes along then not all the offspring may die from it due to their slightly different DNA. However, I don’t think this is exactly what you’re asking.
DNA’s information is compromised of 4 characters. ACTG. Normally A <-> T & C <-> G are linked. But lets make it simpler. Think of the information in DNA as
Mac + CheesePB + J
Now we know mac is always with Che and PB is always with J. So when the Mom bits of DNA show up, it is half the info along with half the dad bits of info. (Ova and sperm).
So the Mom bits can be PB + CHE + MAC + J + PB
And the Dad bits can be CHE + CHE + MAC + PB + PB
Every bit of the person has the same amount of information (excluding the X&Y portion that determines sex, but lets forget it for now). SO the mom and dad bits shuffle their information just a little bit. The new sequence can be PB + CHE + MAC + PB + PB , not a dramatic change, just little bits here and there. After the exchange of information, the sequence becomes the new person.
I it is important to remember that even if half the information is sent, the half you send can quickly make the other set.
Are you asking what force guides a fertilized egg to grow into a baby or are you asking why is everyone made up of 50% of each parents DNA
Human chromosomes are Paired. You’re not getting a random half of each parent’s DNA but a predetermined set from each. You could plausibly get away with just one set but human genetics is designed to work in pairs.
As others have said, the genes of most animals come in pairs, and there is an incredibly intricate process called meiosis which ensures that offspring get one member of each pair of genes from each parents, so you aren’t just randomly splitting the DNA in half and hoping for the best.
However! Randomly splitting the DNA in half and hoping for the best *is* what a lot of microbes do! This is for asexual reproduction, not sexual, but essentially what they do is gather all their DNA in a big clump in the middle of the cell, and then just pull the clump in two with no organization. The reason this works is that instead of 2 copies of each gene, ciliate eykaryotes that use this process can have as many as several thousand copies, so just by sheer chance they’re bound to have at least a few copies of each gene in both daughter cells. Over time though, imbalances do accumulate, and the microbe will actually transition to using sexual reproduction to get itself organized and replenish its genes before going back to the asexual process. Ciliate eykaryotes are super common and include really well-known organisms like paramecia, so while definitely not the norm, this kind of random, hope-for-the-best approach to DNA and reproduction is definitely an important process!
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