The first generation is the first run through of a Tamagotchi's life cycle.
Function
When a Tamagotchi which uses the Generation function is set up, an egg hatches and the first generation starts immediately. When a Tamagotchi dies, the unit must be restarted, and the generation counter will restart at 1. The Mesutchi and Osutchi is the first release to have this feature, and only the characters from TMP1 are available.
Significance in Virtual Pets
In the earlier virtual pets since the brand rebooted in 2004, certain characters were not obtainable in a first generation, although they could be obtained in later generations. For example, Kuribotchi could not be obtained in the first generation at all on the Keitai Kaitsuu Tamagotchi Plus, while it was obtainable in all and any generations afterwards if at least one parent was a tier-one adult (Violetchi or Mametchi).
This rule applied to teen stage characters as well. Ichigotchi can be obtained in any odd generation except the first. The methods to obtain it are unclear, but it is theorized to be obtained through Kuribotchi.
On the other hand, however, on the Tamagotchi Connection Version 1, Marutchi is able to become Ichigotchi, whereas it can only become Hinotamatchi in the third generation onwards, or Oniontchi in even generations.
Similarly, on the Tamagotchi Connection Version 2 and its immediate sequel in the first generation the only child characters available are later obtained through breeding two adult characters of tier three or worse. They can become any of the five teen characters obtainable in an odd generation at first, but from the third generation onwards they can only become Nikatchi or Oniontchi (Version 2) Pirorirotchi (Version 3). In even generations they can only become Hinotamatchi or Propellertchi (Version 2) Hashitamatchi (Version 3). This applies even if they received perfect care.
On the Tamagotchi Connection Version 5 and similar releases, all three babies will hatch from the eggs, which rarely happens in later generations. Later on in the Tamagotchi m!x onwards, only the first generation has pure characters, as subsequent generations have My Tama characters instead.