In a game in which friendly or tameable creatures play an important role, sometimes a player will find themselves needing more or better critters than what they can find naturally. Reasons for this could vary based on the goals of the game, but the solution to the problem is always the same: you breed the beasts you have in order to get your desired result. Some games may instead opt for a "fusion" system that causes the two parent creatures to disappear, but the idea behind it is very much the same.
In-game breeding systems involve putting two compatible creatures together in order to obtain offspring that take after one or both parents. An extremely basic system would result in offspring that look just like one of the parents. However, more complex systems can include multiple traits that can be bred for, complicated rules for inheritance, and perhaps even methods for producing Mix-and-Match Critters.
Although this trope can fit comfortably into many different genres, it is most often found in RPGs and simulation games. In the former, it often plays out as a sort of Lamarckian Min-Maxing, in which the player carefully picks breeding pairs, possibly over several generations, in order to obtain a highly optimized creature at the end. Simulation games are more likely to have highly complex breeding systems, to the point that "breeding sim" has become a recognizable subgenre in and of itself, usually found in the form of browser games.
Hot Skitty-on-Wailord Action is a separate trope, but overlap with this one is not uncommon. For tropes that may result from a breeding system, see Gender Equals Breed, Patchwork Kids, All Genes Are Codominant, Super Breeding Program and Hybrid Power. Fusion-based versions bear some relation to Item Crafting.
- Sonic Adventure and its sequel both include Chao gardens. Chao may be trained and any two may be crossed, with the usual Lamarckian mechanics coming into play. This doesn't affect anything outside of the garden, though.
- BeastKeeper is a pet game and breeding sim. Both positive and negative traits can be inherited by offspring, so players must be careful to avoid breeding pairs that may produce weak, stupid, or otherwise undesirable children.
- Celestial Vale and its sister games have/had breeding systems with differing levels of complexity.
- Celestial Vale in particular had three breeding systems during its lifespan, with an extremely complicated genetics system, which only the site's programmer fully understood. 1.0's DNA system was short-lived due to its flawed execution resulting in some colors being disproportionately common as breeding results. For the rest of 1.0's lifespan, breeding results could be influenced by the colors of a creature's ancestors, with more recent ancestors having greater chances of their color being inherited. 2.0 brought DNA back, but with so much added complexity that only the site's programmer fully understood how it worked.
- Valley of Unicorns: Breeding results are usually semi-randomized, but some rare colors have unique inheritance behaviors that can encourage purebred lineages or breeding at specific times of the month.
- Dragon Cave is a game that simply involves collecting dragons. Most of these dragons can breed, and most of the time baby dragons will grow up to be identical to one of the parents. To spice things up, there are a few hybrids and alternate sprites that can only be obtained through breeding, and some egg varieties can only be laid at certain times of year. A dragon's pedigree is viewable, and the value of a dragon has a lot to do with how visually appealing its pedigree is.
- Flight Rising is another game about breeding dragons. Dragons can come in several breeds and colors, and "genes" can be added to a dragon in order to give it special markings and features. All of these traits can potentially be passed down, with inheritance calculated differently for each.
- Khimeros is a virtual pet game that features breeding.
- Strangelings involves the breeding of fox-like creatures.
- Wajas is an online pet game that revolves entirely around breeding the eponymous creatures. Breed, color, markings, and mutations are all traits that can be bred for. There's an enormous range of options and no predefined goal, so it's rare to find two players who are trying to breed for the exact same result.
- Star Trek Online has tribble breeding, done by either leaving the tribble in inventory with particular foods, or by duty officer assignments found in your ship interior. The former creates a new tribble, the latter transforms your old one. Different tribbles grant different buffs: for example feeding a tribble ketracel-white will produce a tribble that buffs your damage against Jem'Hadar.
- The first several games in the Dragon Quest Monsters series have a breeding system in which offspring may not have a similar appearance to their parents, but they will inherit their parents' powers. Joker and its sequel changed this to a fusion system.
- Final Fantasy:
- Acquiring colored chocobos in Final Fantasy VII requires you to breed chocobos you've captured and leveled up. Some players might be put off by the fact that related chocobos can be bred together, but this isn't too unlike animal breeding in real life. VG Cats examined this in its usual manner.
- Final Fantasy XIII-2 lets the players infuse one of their summoned creatures (which fill out the third party slot in battles) with another. This destroys the latter but transfers some of its special abilities to the former, effectively creating a stronger breed.
- Jade Cocoon has a Merging mechanic that allows you to fuse two minions together. The resulting minion has the body-shape of the first 'parent', the color scheme of the second, and the special attacks and elemental attributes of both.
- Pokémon has a breeding system that has grown in importance and complexity over the years. It can be used to obtain rare species and palette swaps, but it's also essential for producing high-quality teams for competitive play.
- Demons in the Shin Megami Tensei games can be fused with one another, or in some games simply sacrificed, to create new demons. As, after a while, the leveling requirements increase far too sharply for demons to remain useful, unlike the human characters, this is the only alternative to negotiating with more powerful creatures.
- Hybridizing flowers in Animal Crossing fits the spirit of this trope, if not the letter. If two differently-colored flowers of the same type are planted close enough to each other, a new flower of an unusual color may grow nearby. The catch is, you can't use just any two different colors; each hybrid has two or three possible combos that can produce it, and some hybrids are prerequisites for others.
- Harvest Moon is a farming sim so it always features cow pregnancies and chickens laying eggs. Harvest Moon 3 and Harvest Moon: A Wonderful Life have male animals, unlike most titles, and thus it's possible to breed your own animals instead of using a "Miracle Potion".
- Mew-Genics, an upcoming game that places the player in the role of a crazy cat lady, will have this as one of its main game mechanics.
- Certain models of Tamagotchi feature infrared connection. If two adult Tamagotchis of opposite genders are connected, they can "marry" and lay eggs that will eventually replace them. This allows players to begin the Tamagotchi life cycle over again.
- One of the main points of Viva Piņata is its "romancing" system. Outside of one very specific case, romancing is only useful for producing more piņatas of species you already own. However, romancing is essential for meeting the population requirements that some wild piņatas have.
- In Massive Chalice, you must partake in a hero breeding mechanic, where you, as the immortal king of the realm, must decide which heroes and heroines defending the lands have to retire to their castles, intermarry and produce a new lineup of young heroes who inherit some of their old folks' powers and generally become stronger with each generation.
- In Fire Emblem: Genealogy of the Holy War and Fire Emblem Awakening you have the option to pair off the first generation to produce a second generation. What parents pair up influence the children's stat growths, skills and abilities, and in Awakening what classes they can reclass to. However, unlike most examples on this page, there's at least a plot relevant reason to engage in this: Genealogy spans decades and the second half of the game centers around the children (although it should be noted that should certain people fail to be paired up, there will be replacement characters), and the children in Awakening are time travelers sent to avert a Bad Future (again, note that technically you're only required to get Lucina, the others are recruited in optional paralogues).
- Unlike the previous games, Fire Emblem Fates's Support system and children basically exist as a gratuitous case of this as they have no real relevance to the story outside the Heirs of Fate DLC, which is completely optional.
- In War Dragons, you get new dragons by breeding the ones you already have. The frustrating part is that Epic and Rare dragons need multiple egg fragments to be obtained before they can be hatched, and you need 20 egg tokens per egg or fragment bred.
- Minecraft has all sorts of friendly and passive mobs that provide utilities or materials that players may find useful. Since natural mob spawning can't entirely be controlled or relied on, there exists a breeding system that allows most animal-based mobs to produce babies. Breeding for most of these animals is very bare-bones, but horses have a more complicated system that involves colors, markings, and hidden stats.
- Spore, with the twist that the player controls a single creature directly. Evolution is the goal in that stage of the game: you hunt around for bones that contain instructions for new body parts, then use acquired evolution points to append selected parts in the next generation of creature. In the space stage, there's also a "creature tweaker" that allows manipulation of any creature found in the game, but this is purely cosmetic with no influence on play.