[Discussion] why do uchiha need senju dna

[ReLax -]

The Senju have the body of So6P and the Uchiha have the eyes, so there is a difference between their DNA. It's a split DNA, among both clans and most importantly as someone already stated getting both DNA enchants their abilities.

Simple explanation: It's an anime don't overthink it with real world logic. We will find out soon enough what it takes to obtain rinnegan.

^This pretty much.

 

[Modaro]

they got only x or y mind telling how u divide x into half or y into half :sy: and you can't even control which sperm is going to come

The Chromosome from the father contains genes that contains recessive and Dominant traits, which are to be matched with the mothers Chromosomes. So

Let the allele for Spiritual Energy be S and Life Energy be Y

So maybe the Sage's gametes were SY and the Mother's gametes were sy

If you don't know what this means, then it means that there would be a Fiffty percent chance of having a child with Yin or Yang Energies.

in Other words the Uchiha ancesestor's gametes could of been Ss or Sy and the Senju ancestor could of been Yy or Ys.

Sorry for the bio lesson but you asked