On use, the character, including stats and initial items, is reset and teleported to a unique Bedroom that acts like an I AM ERROR room. It has a Bed, two random Chests, two Sacks, and a random Trinket. The only way out is through the beam of light.
This includes resetting the character to the initial one even if you transformed due to items such as Ankh.
For every item removed using Genesis, there will be a set of three items from the removed item's pool to choose from, each set appearing one after the other.
If you have multiple copies of an item being removed, the amount of extra choices you get is how many copies of that item you have squared. (4 of the same item -> 16 choices in Genesis)
If used on one of the final floors of the game, such as Chest or Dark Room, the beam of light will be replaced with a Crawl Space that leads Isaac back to the starting room of the floor he used it on. In addition, the aforementioned chests will include items. These items will not disappear along the "choice" pedestals.
If used during The Beast's fight, it will merely teleport Isaac to the middle of the room, removing no items from him.
Genesis will remove all currently active curses, meaning Curse of the Blind will not affect the item pedestals present inside the room.
If used as Jacob and Esau or Tainted Lazarus, or in Co-op, both the characters will get all their items removed, but you'll be only presented with replacement pedestals for items removed from the character using Genesis, likely resulting in a net item loss.
Jacob and Esau cannot take two items at once from the choices offered by Genesis.
Tainted Bethany will lose all of her Lemegeton fires upon use, but not her red heart charges. This allows the player to quickly gather power again after the reset.
If used as Tainted Forgotten, the link between the two characters is broken. If an item is picked up as the Soul, it is added to the Soul; if the Forgotten is thrown at an item, it is added to the Forgotten. With this method, it is possible to give the Soul items such as orbitals or passive abilities that will help protect it; however, any stat upgrades are rendered useless when picked up as the Soul. Sleeping in the bed as the Soul will not grant any hearts; the Forgotten must be thrown at the bed. It should be noted that the run must be exited and reopened after entering the beam of light to avoid a glitch that kills the Soul in 1 hit.
If used as Eden, a new set of items and stats will be generated. The first random passive item Eden starts with is rerolled immediately with no choice of three.
If used as Keeper after getting the extra health from beating hush, the health cap is set back to 2 (bug?)
If you are on the alternate path, the trapdoor can deviate you from this path.
Using Genesis after defeating Mom's Heart on Mausoleum II will teleport the player to a room that is empty besides a trapdoor leading to Corpse I and the three item selections.
If items such as Teleport, Teleport 2.0, Telepills, etc, are obtained inside of the Genesis Room, the player may use them to teleport back to the main floor and avoid the necessary floor skip.
Players should be cautious when using unidentified pills or opening red chests inside the Genesis Room before collecting all offered items, as all teleportation effects will remove Isaac from the special room with no way back.
Since Genesis resets the player character's health, it is one way to remove Broken Hearts.
As Genesis cannot remove Damocles, it will work in the Genesis room as well, meaning that each set of choice pedestals will spawn an extra set.
It also does not remove the downside of Magic Skin; items can still be replaced by it even after Genesis is used.
Using Genesis after collecting Eden's Blessing during the run will spawn Isaac in the Genesis room already holding a random item. This consumes Eden's Blessing, and the next run the player starts will not start with a random item.
Take caution; choosing an active item while holding another one will not drop your previous item to the ground, but replace it.
Using Glowing Hour Glass works as intended and will give you back your stats, items, pickups, and health.
Using Crooked Penny has a 50% chance to cause item pedestals to duplicate, as normal. Otherwise, they will disappear and no new ones will spawn.
Using D7 will spawn three items, all of which can be taken.
Genesis refers to the first book in the Bible, and is an Ancient Greek term for "beginning". The item description refers to the very first words in the Book of Genesis.
In Challenges, using this item will restore you to your original loadout, but with duplicates of your Passive items. This is specifically because your starting items in Challenges are hardcoded to not be rerolled or removed.
If Isaac had multiple copies of any given item before using Genesis, many more sets of items will be presented than normal.
Using Genesis as Jacob and Esau causes the link between characters to break — Esau can die without killing Jacob, and speed does not get synced between them.
Additionally, after restarting you need to set up another controller to continue play.
Using Genesis as Tainted Forgotten causes the link between characters to break — Tainted Soul will now die in a single hit regardless of health remaining. Quitting and continuing will repair the link and fix the health issue, but objects taken before will be stuck on Tainted Soul and unusable.
Using Genesis as Dead Tainted Lazarus will sever the link between the characters; the other character will be removed forever, removing the downside of the character at the expense of half of the items.
After using this item in the run, the trapdoor on Womb II will lead to the Cathedral instead of Sheol.
Using Genesis immediately after taking Dad's Note will teleport you to the Genesis room, then to the start of the ascent with no items except those that cannot be removed (Dad's Note, Key Pieces, etc.).
If you exit the game in the Genesis Room, all three items in the current choice will become safe to pick and the next choice of three items will stack over them. The effect is stackable for every item you had.
When Tainted ??? uses Genesis, he will appear in the Genesis Room with a second Hold in his active item slot, in addition to the 'fixed' Hold in the card/pill/rune slot.
Using Genesis in the mirror world in Downpour/Dross II will result in a softlock, as the ray of light to get out of the room will not spawn.