King Solomon, founder of the Solomonian Dynasty, was the first emperor of China. After he conquered the mainland, he gave his alchemists the single command of creating an elixir that would allow him to live forever. This elixir not only made him immortal, but unlocked an ability in his genes that gave him access to eye abilities. This power was called the sharringan, and made him the most powerful being in China.
Being immortal allowed him to secure his empire for millennia. To this end, he secured 900 wives and 600 concubines to propagate his legacy, and led him to produce tens of thousands of children. These individuals are not immortal, but do possess the sharringan eye. This allowed him to build a conquering army of his own offspring.
However, there is a problem. It was discovered that the sharringan became weaker with every child produced. While the first few kids of the emperor maintained full powers of the eye, those abilities decreased in every new child being born. Proceeding generations will gain weaker abilities until it is barely existent at all.
Why would the abilities of this gene deteriorate with each new individual born?