All lifeforms have the Lineage Factor, and by manipulating it from a young age, the growth of a lifeform can be changed. Eating a Devil Fruit affects one's Lineage Factor, which allows them to gain a supernatural power.
While his wife was pregnant, Judge forced her to undergo the surgery which manipulated the Lineage Factor of the four fetuses she was carrying. However, due to Sora's interference, only three of their four sons were born as modified humans. After their birth, they were further experimented on, including being administered drugs, in order to increase their development. Later, these experiments were extended to their elder sister.
The successfully modified sons (Ichiji, Niji and Yonji) developed an Exoskeleton (外骨格 Kaikokkaku?), gained an increased healing ability, and, with training, rapidly increased in strength and resilience; soon, these qualities developed to a point equalling those of adult soldiers, while the children were in their early childhood. However, the modifications also took away their ability to feel emotions.
While Reiju received the same physical enhancements, she retained her emotions, but lost the ability to refuse Judge's orders.
Vinsmoke Judge conducted several researches and experiments with the Lineage Factor, becoming able to artificially manufacture human beings to fill the soldier ranks of Germa 66. Those soldiers are known as Clone Soldiers (複製 (クローン) 兵 Kurōn Hei?) and they are made without the knowledge of their origin.
- The Lineage Factor is a reference to DNA, the molecule that contains all the genetic information of the host organism. There doesn't seem to be any difference between the two other than their names.
- One Piece Manga — Vol. 84 Chapter 840 (p. 3-5), Yonji explains the Lineage Factor and cloning.
- One Piece Manga and Anime — Vol. 70 Chapter 698 and Episode 625, Law explains that Caesar's research is an application of Vegapunk's genetics research.
- One Piece Manga — Vol. 85 Chapter 852, Reiju tells Sanji about their mother's intention.
- One Piece Manga — Vol. 84 Chapter 841.